<HTML> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <!-- Created on October, 3 2005 by texi2html 1.64 --> <!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others. Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de> Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --> <HEAD> <TITLE>Bash Reference Manual: </TITLE> <META NAME="description" CONTENT="Bash Reference Manual: "> <META NAME="keywords" CONTENT="Bash Reference Manual: "> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META NAME="Generator" CONTENT="texi2html 1.64"> </HEAD> <BODY LANG="" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000"> <A NAME="SEC_Top"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H1>Bash Reference Manual</H1></P><P> This text is a brief description of the features that are present in the Bash shell (version 3.1-beta1, 5 September 2005).. </P><P> This is Edition 3.1-beta1, last updated 5 September 2005, of <CITE>The GNU Bash Reference Manual</CITE>, for <CODE>Bash</CODE>, Version 3.1-beta1. </P><P> Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (<TT>`sh'</TT>), the Korn Shell (<TT>`ksh'</TT>), and the C-shell (<TT>`csh'</TT> and its successor, <TT>`tcsh'</TT>). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature. </P><P> This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior. </P><P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC1">1. Introduction</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">An introduction to the shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC4">2. Definitions</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Some definitions used in the rest of this manual.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC5">3. Basic Shell Features</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The shell "building blocks".</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC56">4. Shell Builtin Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Commands that are a part of the shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC61">5. Shell Variables</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Variables used or set by Bash.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC64">6. Bash Features</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Features found only in Bash.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC87">7. Job Control</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">What job control is and how Bash allows you to use it.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC115">9. Using History Interactively</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Command History Expansion</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC91">8. Command Line Editing</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Chapter describing the command line editing features.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC122">10. Installing Bash</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to build and install Bash on your system.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC131">A. Reporting Bugs</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to report bugs in Bash.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC132">B. Major Differences From The Bourne Shell</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A terse list of the differences between Bash and historical versions of /bin/sh.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC134">C. Copying This Manual</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Copying this manual.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC137">Index of Shell Builtin Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Index of Bash builtin commands.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC138">Index of Shell Reserved Words</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Index of Bash reserved words.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC139">Parameter and Variable Index</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Quick reference helps you find the variable you want.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC140">Function Index</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Index of bindable Readline functions.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC141">Concept Index</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">General index for concepts described in this manual.</TD></TR> </TABLE></BLOCKQUOTE> <P> <HR SIZE=1> <A NAME="SEC1"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC2"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[ << ]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC4"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <A NAME="Introduction"></A> <H1> 1. Introduction </H1> <!--docid::SEC1::--> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC2">1.1 What is Bash?</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A short description of Bash.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC3">1.2 What is a shell?</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A brief introduction to shells.</TD></TR> </TABLE></BLOCKQUOTE> <P> <A NAME="What is Bash?"></A> <HR SIZE="6"> <A NAME="SEC2"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC1"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC3"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[ << ]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC1"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC4"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 1.1 What is Bash? </H2> <!--docid::SEC2::--> <P> Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the <SAMP>`Bourne-Again SHell'</SAMP>, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell <CODE>sh</CODE>, which appeared in the Seventh Edition Bell Labs Research version of Unix. </P><P> Bash is largely compatible with <CODE>sh</CODE> and incorporates useful features from the Korn shell <CODE>ksh</CODE> and the C shell <CODE>csh</CODE>. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). It offers functional improvements over <CODE>sh</CODE> for both interactive and programming use. </P><P> While the GNU operating system provides other shells, including a version of <CODE>csh</CODE>, Bash is the default shell. Like other GNU software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems - independently-supported ports exist for MS-DOS, OS/2, and Windows platforms. </P><P> <A NAME="What is a shell?"></A> <HR SIZE="6"> <A NAME="SEC3"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC2"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC4"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[ << ]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC1"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC4"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 1.2 What is a shell? </H2> <!--docid::SEC3::--> <P> At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. </P><P> A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as <TT>`/bin'</TT>, allowing users or groups to establish custom environments to automate their common tasks. </P><P> Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file. </P><P> A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The <EM>redirection</EM> constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments. </P><P> Shells also provide a small set of built-in commands (<EM>builtins</EM>) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, <CODE>cd</CODE>, <CODE>break</CODE>, <CODE>continue</CODE>, and <CODE>exec</CODE>) cannot be implemented outside of the shell because they directly manipulate the shell itself. The <CODE>history</CODE>, <CODE>getopts</CODE>, <CODE>kill</CODE>, or <CODE>pwd</CODE> builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections. </P><P> While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions. </P><P> Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual. </P><P> <A NAME="Definitions"></A> <HR SIZE="6"> <A NAME="SEC4"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC3"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H1> 2. Definitions </H1> <!--docid::SEC4::--> These definitions are used throughout the remainder of this manual. <P> <DL COMPACT> <DT><CODE>POSIX</CODE> <DD><A NAME="IDX1"></A> A family of open system standards based on Unix. Bash is concerned with POSIX 1003.2, the Shell and Tools Standard. <P> <DT><CODE>blank</CODE> <DD>A space or tab character. <P> <DT><CODE>builtin</CODE> <DD><A NAME="IDX2"></A> A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system. <P> <DT><CODE>control operator</CODE> <DD><A NAME="IDX3"></A> A <CODE>word</CODE> that performs a control function. It is a <CODE>newline</CODE> or one of the following: <SAMP>`||'</SAMP>, <SAMP>`&&'</SAMP>, <SAMP>`&'</SAMP>, <SAMP>`;'</SAMP>, <SAMP>`;;'</SAMP>, <SAMP>`|'</SAMP>, <SAMP>`('</SAMP>, or <SAMP>`)'</SAMP>. <P> <DT><CODE>exit status</CODE> <DD><A NAME="IDX4"></A> The value returned by a command to its caller. The value is restricted to eight bits, so the maximum value is 255. <P> <DT><CODE>field</CODE> <DD><A NAME="IDX5"></A> A unit of text that is the result of one of the shell expansions. After expansion, when executing a command, the resulting fields are used as the command name and arguments. <P> <DT><CODE>filename</CODE> <DD><A NAME="IDX6"></A> A string of characters used to identify a file. <P> <DT><CODE>job</CODE> <DD><A NAME="IDX7"></A> A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group. <P> <DT><CODE>job control</CODE> <DD><A NAME="IDX8"></A> A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes. <P> <DT><CODE>metacharacter</CODE> <DD><A NAME="IDX9"></A> A character that, when unquoted, separates words. A metacharacter is a <CODE>blank</CODE> or one of the following characters: <SAMP>`|'</SAMP>, <SAMP>`&'</SAMP>, <SAMP>`;'</SAMP>, <SAMP>`('</SAMP>, <SAMP>`)'</SAMP>, <SAMP>`<'</SAMP>, or <SAMP>`>'</SAMP>. <P> <DT><CODE>name</CODE> <DD><A NAME="IDX10"></A> <A NAME="IDX11"></A> A <CODE>word</CODE> consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. <CODE>Name</CODE>s are used as shell variable and function names. Also referred to as an <CODE>identifier</CODE>. <P> <DT><CODE>operator</CODE> <DD><A NAME="IDX12"></A> A <CODE>control operator</CODE> or a <CODE>redirection operator</CODE>. See section <A HREF="bashref.html#SEC38">3.6 Redirections</A>, for a list of redirection operators. <P> <DT><CODE>process group</CODE> <DD><A NAME="IDX13"></A> A collection of related processes each having the same process group ID. <P> <DT><CODE>process group ID</CODE> <DD><A NAME="IDX14"></A> A unique identifer that represents a <CODE>process group</CODE> during its lifetime. <P> <DT><CODE>reserved word</CODE> <DD><A NAME="IDX15"></A> A <CODE>word</CODE> that has a special meaning to the shell. Most reserved words introduce shell flow control constructs, such as <CODE>for</CODE> and <CODE>while</CODE>. <P> <DT><CODE>return status</CODE> <DD><A NAME="IDX16"></A> A synonym for <CODE>exit status</CODE>. <P> <DT><CODE>signal</CODE> <DD><A NAME="IDX17"></A> A mechanism by which a process may be notified by the kernel of an event occurring in the system. <P> <DT><CODE>special builtin</CODE> <DD><A NAME="IDX18"></A> A shell builtin command that has been classified as special by the POSIX 1003.2 standard. <P> <DT><CODE>token</CODE> <DD><A NAME="IDX19"></A> A sequence of characters considered a single unit by the shell. It is either a <CODE>word</CODE> or an <CODE>operator</CODE>. <P> <DT><CODE>word</CODE> <DD><A NAME="IDX20"></A> A <CODE>token</CODE> that is not an <CODE>operator</CODE>. </DL> <P> <A NAME="Basic Shell Features"></A> <HR SIZE="6"> <A NAME="SEC5"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC4"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC6"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H1> 3. Basic Shell Features </H1> <!--docid::SEC5::--> <P> Bash is an acronym for <SAMP>`Bourne-Again SHell'</SAMP>. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the POSIX specification for the `standard' Unix shell. </P><P> This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell <I>parameters</I>, shell expansions, <I>redirections</I>, which are a way to direct input and output from and to named files, and how the shell executes commands. </P><P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC6">3.1 Shell Syntax</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">What your input means to the shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC15">3.2 Shell Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The types of commands you can use.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC23">3.3 Shell Functions</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Grouping commands by name.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC24">3.4 Shell Parameters</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How the shell stores values.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC27">3.5 Shell Expansions</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How Bash expands parameters and the various expansions available.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC38">3.6 Redirections</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A way to control where input and output go.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC48">3.7 Executing Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">What happens when you run a command.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC55">3.8 Shell Scripts</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Executing files of shell commands.</TD></TR> </TABLE></BLOCKQUOTE> <P> <A NAME="Shell Syntax"></A> <HR SIZE="6"> <A NAME="SEC6"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC7"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.1 Shell Syntax </H2> <!--docid::SEC6::--> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC7">3.1.1 Shell Operation</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The basic operation of the shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC8">3.1.2 Quoting</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to remove the special meaning from characters.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC14">3.1.3 Comments</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to specify comments.</TD></TR> </TABLE></BLOCKQUOTE> <P> When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (<SAMP>`#'</SAMP>), and the rest of that line. Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters. </P><P> The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing. </P><P> <A NAME="Shell Operation"></A> <HR SIZE="6"> <A NAME="SEC7"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC6"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC6"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.1.1 Shell Operation </H3> <!--docid::SEC7::--> <P> The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following: </P><P> <OL> <LI> Reads its input from a file (see section <A HREF="bashref.html#SEC55">3.8 Shell Scripts</A>), from a string supplied as an argument to the <SAMP>`-c'</SAMP> invocation option (see section <A HREF="bashref.html#SEC65">6.1 Invoking Bash</A>), or from the user's terminal. <P> <LI> Breaks the input into words and operators, obeying the quoting rules described in <A HREF="bashref.html#SEC8">3.1.2 Quoting</A>. These tokens are separated by <CODE>metacharacters</CODE>. Alias expansion is performed by this step (see section <A HREF="bashref.html#SEC80">6.6 Aliases</A>). <P> <LI> Parses the tokens into simple and compound commands (see section <A HREF="bashref.html#SEC15">3.2 Shell Commands</A>). <P> <LI> Performs the various shell expansions (see section <A HREF="bashref.html#SEC27">3.5 Shell Expansions</A>), breaking the expanded tokens into lists of filenames (see section <A HREF="bashref.html#SEC35">3.5.8 Filename Expansion</A>) and commands and arguments. <P> <LI> Performs any necessary redirections (see section <A HREF="bashref.html#SEC38">3.6 Redirections</A>) and removes the redirection operators and their operands from the argument list. <P> <LI> Executes the command (see section <A HREF="bashref.html#SEC48">3.7 Executing Commands</A>). <P> <LI> Optionally waits for the command to complete and collects its exit status (see section <A HREF="bashref.html#SEC53">3.7.5 Exit Status</A>). <P> </OL> <P> <A NAME="Quoting"></A> <HR SIZE="6"> <A NAME="SEC8"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC7"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC9"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC6"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.1.2 Quoting </H3> <!--docid::SEC8::--> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC9">3.1.2.1 Escape Character</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to remove the special meaning from a single character.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC10">3.1.2.2 Single Quotes</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to inhibit all interpretation of a sequence of characters.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC11">3.1.2.3 Double Quotes</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to suppress most of the interpretation of a sequence of characters.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC12">3.1.2.4 ANSI-C Quoting</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to expand ANSI-C sequences in quoted strings.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC13">3.1.2.5 Locale-Specific Translation</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to translate strings into different languages.</TD></TR> </TABLE></BLOCKQUOTE> <P> Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. </P><P> Each of the shell metacharacters (see section <A HREF="bashref.html#SEC4">2. Definitions</A>) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see section <A HREF="bashref.html#SEC118">9.3 History Expansion</A>), the <VAR>history expansion</VAR> character, usually <SAMP>`!'</SAMP>, must be quoted to prevent history expansion. See section <A HREF="bashref.html#SEC116">9.1 Bash History Facilities</A>, for more details concerning history expansion. </P><P> There are three quoting mechanisms: the <VAR>escape character</VAR>, single quotes, and double quotes. </P><P> <A NAME="Escape Character"></A> <HR SIZE="6"> <A NAME="SEC9"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC10"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.1.2.1 Escape Character </H4> <!--docid::SEC9::--> A non-quoted backslash <SAMP>`\'</SAMP> is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of <CODE>newline</CODE>. If a <CODE>\newline</CODE> pair appears, and the backslash itself is not quoted, the <CODE>\newline</CODE> is treated as a line continuation (that is, it is removed from the input stream and effectively ignored). <P> <A NAME="Single Quotes"></A> <HR SIZE="6"> <A NAME="SEC10"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC9"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC11"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC11"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.1.2.2 Single Quotes </H4> <!--docid::SEC10::--> <P> Enclosing characters in single quotes (<SAMP>`''</SAMP>) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. </P><P> <A NAME="Double Quotes"></A> <HR SIZE="6"> <A NAME="SEC11"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC10"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC12"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC12"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.1.2.3 Double Quotes </H4> <!--docid::SEC11::--> <P> Enclosing characters in double quotes (<SAMP>`"'</SAMP>) preserves the literal value of all characters within the quotes, with the exception of <SAMP>`$'</SAMP>, <SAMP>``'</SAMP>, <SAMP>`\'</SAMP>, and, when history expansion is enabled, <SAMP>`!'</SAMP>. The characters <SAMP>`$'</SAMP> and <SAMP>``'</SAMP> retain their special meaning within double quotes (see section <A HREF="bashref.html#SEC27">3.5 Shell Expansions</A>). The backslash retains its special meaning only when followed by one of the following characters: <SAMP>`$'</SAMP>, <SAMP>``'</SAMP>, <SAMP>`"'</SAMP>, <SAMP>`\'</SAMP>, or <CODE>newline</CODE>. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an <SAMP>`!'</SAMP> appearing in double quotes is escaped using a backslash. The backslash preceding the <SAMP>`!'</SAMP> is not removed. </P><P> The special parameters <SAMP>`*'</SAMP> and <SAMP>`@'</SAMP> have special meaning when in double quotes (see section <A HREF="bashref.html#SEC30">3.5.3 Shell Parameter Expansion</A>). </P><P> <A NAME="ANSI-C Quoting"></A> <HR SIZE="6"> <A NAME="SEC12"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC11"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC13"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC13"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.1.2.4 ANSI-C Quoting </H4> <!--docid::SEC12::--> <P> Words of the form <CODE>$'<VAR>string</VAR>'</CODE> are treated specially. The word expands to <VAR>string</VAR>, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows: </P><P> <DL COMPACT> <DT><CODE>\a</CODE> <DD>alert (bell) <DT><CODE>\b</CODE> <DD>backspace <DT><CODE>\e</CODE> <DD>an escape character (not ANSI C) <DT><CODE>\f</CODE> <DD>form feed <DT><CODE>\n</CODE> <DD>newline <DT><CODE>\r</CODE> <DD>carriage return <DT><CODE>\t</CODE> <DD>horizontal tab <DT><CODE>\v</CODE> <DD>vertical tab <DT><CODE>\\</CODE> <DD>backslash <DT><CODE>\'</CODE> <DD>single quote <DT><CODE>\<VAR>nnn</VAR></CODE> <DD>the eight-bit character whose value is the octal value <VAR>nnn</VAR> (one to three digits) <DT><CODE>\x<VAR>HH</VAR></CODE> <DD>the eight-bit character whose value is the hexadecimal value <VAR>HH</VAR> (one or two hex digits) <DT><CODE>\c<VAR>x</VAR></CODE> <DD>a control-<VAR>x</VAR> character </DL> <P> The expanded result is single-quoted, as if the dollar sign had not been present. </P><P> <A NAME="Locale Translation"></A> <HR SIZE="6"> <A NAME="SEC13"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC12"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC8"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.1.2.5 Locale-Specific Translation </H4> <!--docid::SEC13::--> <P> A double-quoted string preceded by a dollar sign (<SAMP>`$'</SAMP>) will cause the string to be translated according to the current locale. If the current locale is <CODE>C</CODE> or <CODE>POSIX</CODE>, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted. </P><P> <A NAME="IDX21"></A> <A NAME="IDX22"></A> <A NAME="IDX23"></A> Some systems use the message catalog selected by the <CODE>LC_MESSAGES</CODE> shell variable. Others create the name of the message catalog from the value of the <CODE>TEXTDOMAIN</CODE> shell variable, possibly adding a suffix of <SAMP>`.mo'</SAMP>. If you use the <CODE>TEXTDOMAIN</CODE> variable, you may need to set the <CODE>TEXTDOMAINDIR</CODE> variable to the location of the message catalog files. Still others use both variables in this fashion: <CODE>TEXTDOMAINDIR</CODE>/<CODE>LC_MESSAGES</CODE>/LC_MESSAGES/<CODE>TEXTDOMAIN</CODE>.mo. </P><P> <A NAME="Comments"></A> <HR SIZE="6"> <A NAME="SEC14"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC13"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC6"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.1.3 Comments </H3> <!--docid::SEC14::--> <P> In a non-interactive shell, or an interactive shell in which the <CODE>interactive_comments</CODE> option to the <CODE>shopt</CODE> builtin is enabled (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>), a word beginning with <SAMP>`#'</SAMP> causes that word and all remaining characters on that line to be ignored. An interactive shell without the <CODE>interactive_comments</CODE> option enabled does not allow comments. The <CODE>interactive_comments</CODE> option is on by default in interactive shells. See section <A HREF="bashref.html#SEC74">6.3 Interactive Shells</A>, for a description of what makes a shell interactive. </P><P> <A NAME="Shell Commands"></A> <HR SIZE="6"> <A NAME="SEC15"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC14"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC16"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.2 Shell Commands </H2> <!--docid::SEC15::--> <P> A simple shell command such as <CODE>echo a b c</CODE> consists of the command itself followed by arguments, separated by spaces. </P><P> More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping. </P><P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC16">3.2.1 Simple Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The most common type of command.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC17">3.2.2 Pipelines</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Connecting the input and output of several commands.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to execute commands sequentially.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC19">3.2.4 Compound Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Shell commands for control flow.</TD></TR> </TABLE></BLOCKQUOTE> <P> <A NAME="Simple Commands"></A> <HR SIZE="6"> <A NAME="SEC16"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC17"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.2.1 Simple Commands </H3> <!--docid::SEC16::--> <P> A simple command is the kind of command encountered most often. It's just a sequence of words separated by <CODE>blank</CODE>s, terminated by one of the shell's control operators (see section <A HREF="bashref.html#SEC4">2. Definitions</A>). The first word generally specifies a command to be executed, with the rest of the words being that command's arguments. </P><P> The return status (see section <A HREF="bashref.html#SEC53">3.7.5 Exit Status</A>) of a simple command is its exit status as provided by the POSIX 1003.1 <CODE>waitpid</CODE> function, or 128+<VAR>n</VAR> if the command was terminated by signal <VAR>n</VAR>. </P><P> <A NAME="Pipelines"></A> <HR SIZE="6"> <A NAME="SEC17"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC16"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC18"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC18"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.2.2 Pipelines </H3> <!--docid::SEC17::--> <P> A <CODE>pipeline</CODE> is a sequence of simple commands separated by <SAMP>`|'</SAMP>. </P><P> <A NAME="IDX24"></A> <A NAME="IDX25"></A> <A NAME="IDX26"></A> The format for a pipeline is <TABLE><tr><td> </td><td class=example><pre>[<CODE>time</CODE> [<CODE>-p</CODE>]] [<CODE>!</CODE>] <VAR>command1</VAR> [<CODE>|</CODE> <VAR>command2</VAR> <small>...</small>] </pre></td></tr></table></P><P> The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output. </P><P> The reserved word <CODE>time</CODE> causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The <SAMP>`-p'</SAMP> option changes the output format to that specified by POSIX. The <CODE>TIMEFORMAT</CODE> variable may be set to a format string that specifies how the timing information should be displayed. See section <A HREF="bashref.html#SEC63">5.2 Bash Variables</A>, for a description of the available formats. The use of <CODE>time</CODE> as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external <CODE>time</CODE> command cannot time these easily. </P><P> If the pipeline is not executed asynchronously (see section <A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A>), the shell waits for all commands in the pipeline to complete. </P><P> Each command in a pipeline is executed in its own subshell (see section <A HREF="bashref.html#SEC51">3.7.3 Command Execution Environment</A>). The exit status of a pipeline is the exit status of the last command in the pipeline, unless the <CODE>pipefail</CODE> option is enabled (see section <A HREF="bashref.html#SEC59">4.3 The Set Builtin</A>). If <CODE>pipefail</CODE> is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word <SAMP>`!'</SAMP> precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value. </P><P> <A NAME="Lists"></A> <HR SIZE="6"> <A NAME="SEC18"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC17"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.2.3 Lists of Commands </H3> <!--docid::SEC18::--> <P> A <CODE>list</CODE> is a sequence of one or more pipelines separated by one of the operators <SAMP>`;'</SAMP>, <SAMP>`&'</SAMP>, <SAMP>`&&'</SAMP>, or <SAMP>`||'</SAMP>, and optionally terminated by one of <SAMP>`;'</SAMP>, <SAMP>`&'</SAMP>, or a <CODE>newline</CODE>. </P><P> Of these list operators, <SAMP>`&&'</SAMP> and <SAMP>`||'</SAMP> have equal precedence, followed by <SAMP>`;'</SAMP> and <SAMP>`&'</SAMP>, which have equal precedence. </P><P> A sequence of one or more newlines may appear in a <CODE>list</CODE> to delimit commands, equivalent to a semicolon. </P><P> If a command is terminated by the control operator <SAMP>`&'</SAMP>, the shell executes the command asynchronously in a subshell. This is known as executing the command in the <VAR>background</VAR>. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see section <A HREF="bashref.html#SEC87">7. Job Control</A>), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from <CODE>/dev/null</CODE>. </P><P> Commands separated by a <SAMP>`;'</SAMP> are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed. </P><P> The control operators <SAMP>`&&'</SAMP> and <SAMP>`||'</SAMP> denote AND lists and OR lists, respectively. An AND list has the form <TABLE><tr><td> </td><td class=example><pre><VAR>command1</VAR> && <VAR>command2</VAR> </pre></td></tr></table></P><P> <VAR>command2</VAR> is executed if, and only if, <VAR>command1</VAR> returns an exit status of zero. </P><P> An OR list has the form <TABLE><tr><td> </td><td class=example><pre><VAR>command1</VAR> || <VAR>command2</VAR> </pre></td></tr></table></P><P> <VAR>command2</VAR> is executed if, and only if, <VAR>command1</VAR> returns a non-zero exit status. </P><P> The return status of AND and OR lists is the exit status of the last command executed in the list. </P><P> <A NAME="Compound Commands"></A> <HR SIZE="6"> <A NAME="SEC19"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC18"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC20"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC15"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.2.4 Compound Commands </H3> <!--docid::SEC19::--> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC20">3.2.4.1 Looping Constructs</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Shell commands for iterative action.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC21">3.2.4.2 Conditional Constructs</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Shell commands for conditional execution.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC22">3.2.4.3 Grouping Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Ways to group commands.</TD></TR> </TABLE></BLOCKQUOTE> <P> Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see section <A HREF="bashref.html#SEC38">3.6 Redirections</A>) associated with a compound command apply to all commands within that compound command unless explicitly overridden. </P><P> Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit. </P><P> <A NAME="Looping Constructs"></A> <HR SIZE="6"> <A NAME="SEC20"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC21"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.2.4.1 Looping Constructs </H4> <!--docid::SEC20::--> <P> Bash supports the following looping constructs. </P><P> Note that wherever a <SAMP>`;'</SAMP> appears in the description of a command's syntax, it may be replaced with one or more newlines. </P><P> <DL COMPACT> <DT><CODE>until</CODE> <DD><A NAME="IDX27"></A> <A NAME="IDX28"></A> <A NAME="IDX29"></A> The syntax of the <CODE>until</CODE> command is: <TABLE><tr><td> </td><td class=example><pre>until <VAR>test-commands</VAR>; do <VAR>consequent-commands</VAR>; done </pre></td></tr></table>Execute <VAR>consequent-commands</VAR> as long as <VAR>test-commands</VAR> has an exit status which is not zero. The return status is the exit status of the last command executed in <VAR>consequent-commands</VAR>, or zero if none was executed. <P> <DT><CODE>while</CODE> <DD><A NAME="IDX30"></A> The syntax of the <CODE>while</CODE> command is: <TABLE><tr><td> </td><td class=example><pre>while <VAR>test-commands</VAR>; do <VAR>consequent-commands</VAR>; done </pre></td></tr></table><P> Execute <VAR>consequent-commands</VAR> as long as <VAR>test-commands</VAR> has an exit status of zero. The return status is the exit status of the last command executed in <VAR>consequent-commands</VAR>, or zero if none was executed. </P><P> <DT><CODE>for</CODE> <DD><A NAME="IDX31"></A> The syntax of the <CODE>for</CODE> command is: <P> <TABLE><tr><td> </td><td class=example><pre>for <VAR>name</VAR> [in <VAR>words</VAR> <small>...</small>]; do <VAR>commands</VAR>; done </pre></td></tr></table>Expand <VAR>words</VAR>, and execute <VAR>commands</VAR> once for each member in the resultant list, with <VAR>name</VAR> bound to the current member. If <SAMP>`in <VAR>words</VAR>'</SAMP> is not present, the <CODE>for</CODE> command executes the <VAR>commands</VAR> once for each positional parameter that is set, as if <SAMP>`in "$@"'</SAMP> had been specified (see section <A HREF="bashref.html#SEC26">3.4.2 Special Parameters</A>). The return status is the exit status of the last command that executes. If there are no items in the expansion of <VAR>words</VAR>, no commands are executed, and the return status is zero. </P><P> An alternate form of the <CODE>for</CODE> command is also supported: </P><P> <TABLE><tr><td> </td><td class=example><pre>for (( <VAR>expr1</VAR> ; <VAR>expr2</VAR> ; <VAR>expr3</VAR> )) ; do <VAR>commands</VAR> ; done </pre></td></tr></table>First, the arithmetic expression <VAR>expr1</VAR> is evaluated according to the rules described below (see section <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>). The arithmetic expression <VAR>expr2</VAR> is then evaluated repeatedly until it evaluates to zero. Each time <VAR>expr2</VAR> evaluates to a non-zero value, <VAR>commands</VAR> are executed and the arithmetic expression <VAR>expr3</VAR> is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in <VAR>list</VAR> that is executed, or false if any of the expressions is invalid. </P><P> </DL> <P> The <CODE>break</CODE> and <CODE>continue</CODE> builtins (see section <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>) may be used to control loop execution. </P><P> <A NAME="Conditional Constructs"></A> <HR SIZE="6"> <A NAME="SEC21"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC20"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC22"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC22"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.2.4.2 Conditional Constructs </H4> <!--docid::SEC21::--> <P> <DL COMPACT> <DT><CODE>if</CODE> <DD><A NAME="IDX32"></A> <A NAME="IDX33"></A> <A NAME="IDX34"></A> <A NAME="IDX35"></A> <A NAME="IDX36"></A> The syntax of the <CODE>if</CODE> command is: <P> <TABLE><tr><td> </td><td class=example><pre>if <VAR>test-commands</VAR>; then <VAR>consequent-commands</VAR>; [elif <VAR>more-test-commands</VAR>; then <VAR>more-consequents</VAR>;] [else <VAR>alternate-consequents</VAR>;] fi </pre></td></tr></table></P><P> The <VAR>test-commands</VAR> list is executed, and if its return status is zero, the <VAR>consequent-commands</VAR> list is executed. If <VAR>test-commands</VAR> returns a non-zero status, each <CODE>elif</CODE> list is executed in turn, and if its exit status is zero, the corresponding <VAR>more-consequents</VAR> is executed and the command completes. If <SAMP>`else <VAR>alternate-consequents</VAR>'</SAMP> is present, and the final command in the final <CODE>if</CODE> or <CODE>elif</CODE> clause has a non-zero exit status, then <VAR>alternate-consequents</VAR> is executed. The return status is the exit status of the last command executed, or zero if no condition tested true. </P><P> <DT><CODE>case</CODE> <DD><A NAME="IDX37"></A> <A NAME="IDX38"></A> <A NAME="IDX39"></A> The syntax of the <CODE>case</CODE> command is: <P> <TABLE><tr><td> </td><td class=example><pre><CODE>case <VAR>word</VAR> in [ [(] <VAR>pattern</VAR> [| <VAR>pattern</VAR>]<small>...</small>) <VAR>command-list</VAR> ;;]<small>...</small> esac</CODE> </pre></td></tr></table></P><P> <CODE>case</CODE> will selectively execute the <VAR>command-list</VAR> corresponding to the first <VAR>pattern</VAR> that matches <VAR>word</VAR>. If the shell option <CODE>nocasematch</CODE> (see the description of <CODE>shopt</CODE> in <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>) is enabled, the match is performed without regard to the case of alphabetic characters. The <SAMP>`|'</SAMP> is used to separate multiple patterns, and the <SAMP>`)'</SAMP> operator terminates a pattern list. A list of patterns and an associated command-list is known as a <VAR>clause</VAR>. Each clause must be terminated with <SAMP>`;;'</SAMP>. The <VAR>word</VAR> undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before matching is attempted. Each <VAR>pattern</VAR> undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion. </P><P> There may be an arbitrary number of <CODE>case</CODE> clauses, each terminated by a <SAMP>`;;'</SAMP>. The first pattern that matches determines the command-list that is executed. </P><P> Here is an example using <CODE>case</CODE> in a script that could be used to describe one interesting feature of an animal: </P><P> <TABLE><tr><td> </td><td class=example><pre>echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs." </pre></td></tr></table></P><P> The return status is zero if no <VAR>pattern</VAR> is matched. Otherwise, the return status is the exit status of the <VAR>command-list</VAR> executed. </P><P> <DT><CODE>select</CODE> <DD><A NAME="IDX40"></A> <P> The <CODE>select</CODE> construct allows the easy generation of menus. It has almost the same syntax as the <CODE>for</CODE> command: </P><P> <TABLE><tr><td> </td><td class=example><pre>select <VAR>name</VAR> [in <VAR>words</VAR> <small>...</small>]; do <VAR>commands</VAR>; done </pre></td></tr></table></P><P> The list of words following <CODE>in</CODE> is expanded, generating a list of items. The set of expanded words is printed on the standard error output stream, each preceded by a number. If the <SAMP>`in <VAR>words</VAR>'</SAMP> is omitted, the positional parameters are printed, as if <SAMP>`in "$@"'</SAMP> had been specifed. The <CODE>PS3</CODE> prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of <VAR>name</VAR> is set to that word. If the line is empty, the words and prompt are displayed again. If <CODE>EOF</CODE> is read, the <CODE>select</CODE> command completes. Any other value read causes <VAR>name</VAR> to be set to null. The line read is saved in the variable <CODE>REPLY</CODE>. </P><P> The <VAR>commands</VAR> are executed after each selection until a <CODE>break</CODE> command is executed, at which point the <CODE>select</CODE> command completes. </P><P> Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected. </P><P> <TABLE><tr><td> </td><td class=example><pre>select fname in *; do echo you picked $fname \($REPLY\) break; done </pre></td></tr></table></P><P> <DT><CODE>((<small>...</small>))</CODE> <DD><TABLE><tr><td> </td><td class=example><pre>(( <VAR>expression</VAR> )) </pre></td></tr></table><P> The arithmetic <VAR>expression</VAR> is evaluated according to the rules described below (see section <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to <TABLE><tr><td> </td><td class=example><pre>let "<VAR>expression</VAR>" </pre></td></tr></table>See section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>, for a full description of the <CODE>let</CODE> builtin. </P><P> <DT><CODE>[[<small>...</small>]]</CODE> <DD><A NAME="IDX41"></A> <A NAME="IDX42"></A> <TABLE><tr><td> </td><td class=example><pre>[[ <VAR>expression</VAR> ]] </pre></td></tr></table><P> Return a status of 0 or 1 depending on the evaluation of the conditional expression <VAR>expression</VAR>. Expressions are composed of the primaries described below in <A HREF="bashref.html#SEC78">6.4 Bash Conditional Expressions</A>. Word splitting and filename expansion are not performed on the words between the <SAMP>`[['</SAMP> and <SAMP>`]]'</SAMP>; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as <SAMP>`-f'</SAMP> must be unquoted to be recognized as primaries. </P><P> When the <SAMP>`=='</SAMP> and <SAMP>`!='</SAMP> operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in <A HREF="bashref.html#SEC36">3.5.8.1 Pattern Matching</A>. If the shell option <CODE>nocasematch</CODE> (see the description of <CODE>shopt</CODE> in <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string. </P><P> An additional binary operator, <SAMP>`=~'</SAMP>, is available, with the same precedence as <SAMP>`=='</SAMP> and <SAMP>`!='</SAMP>. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in <I>regex</I>3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the shell option <CODE>nocasematch</CODE> (see the description of <CODE>shopt</CODE> in <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>) is enabled, the match is performed without regard to the case of alphabetic characters. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable <CODE>BASH_REMATCH</CODE>. The element of <CODE>BASH_REMATCH</CODE> with index 0 is the portion of the string matching the entire regular expression. The element of <CODE>BASH_REMATCH</CODE> with index <VAR>n</VAR> is the portion of the string matching the <VAR>n</VAR>th parenthesized subexpression. </P><P> Expressions may be combined using the following operators, listed in decreasing order of precedence: </P><P> <DL COMPACT> <DT><CODE>( <VAR>expression</VAR> )</CODE> <DD>Returns the value of <VAR>expression</VAR>. This may be used to override the normal precedence of operators. <P> <DT><CODE>! <VAR>expression</VAR></CODE> <DD>True if <VAR>expression</VAR> is false. <P> <DT><CODE><VAR>expression1</VAR> && <VAR>expression2</VAR></CODE> <DD>True if both <VAR>expression1</VAR> and <VAR>expression2</VAR> are true. <P> <DT><CODE><VAR>expression1</VAR> || <VAR>expression2</VAR></CODE> <DD>True if either <VAR>expression1</VAR> or <VAR>expression2</VAR> is true. </DL> The <CODE>&&</CODE> and <CODE>||</CODE> operators do not evaluate <VAR>expression2</VAR> if the value of <VAR>expression1</VAR> is sufficient to determine the return value of the entire conditional expression. <P> </DL> <P> <A NAME="Command Grouping"></A> <HR SIZE="6"> <A NAME="SEC22"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC21"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC19"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.2.4.3 Grouping Commands </H4> <!--docid::SEC22::--> <P> Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream. </P><P> <DL COMPACT> <DT><CODE>()</CODE> <DD><TABLE><tr><td> </td><td class=example><pre>( <VAR>list</VAR> ) </pre></td></tr></table><P> Placing a list of commands between parentheses causes a subshell environment to be created (see section <A HREF="bashref.html#SEC51">3.7.3 Command Execution Environment</A>), and each of the commands in <VAR>list</VAR> to be executed in that subshell. Since the <VAR>list</VAR> is executed in a subshell, variable assignments do not remain in effect after the subshell completes. </P><P> <DT><CODE>{}</CODE> <DD><A NAME="IDX43"></A> <A NAME="IDX44"></A> <TABLE><tr><td> </td><td class=example><pre>{ <VAR>list</VAR>; } </pre></td></tr></table><P> Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following <VAR>list</VAR> is required. </DL> <P> In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are <CODE>reserved words</CODE>, so they must be separated from the <VAR>list</VAR> by <CODE>blank</CODE>s. The parentheses are <CODE>operators</CODE>, and are recognized as separate tokens by the shell even if they are not separated from the <VAR>list</VAR> by whitespace. </P><P> The exit status of both of these constructs is the exit status of <VAR>list</VAR>. </P><P> <A NAME="Shell Functions"></A> <HR SIZE="6"> <A NAME="SEC23"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC22"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC24"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC24"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.3 Shell Functions </H2> <!--docid::SEC23::--> <P> Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them. </P><P> Functions are declared using this syntax: <A NAME="IDX45"></A> <TABLE><tr><td> </td><td class=example><pre>[ <CODE>function</CODE> ] <VAR>name</VAR> () <VAR>compound-command</VAR> [ <VAR>redirections</VAR> ] </pre></td></tr></table></P><P> This defines a shell function named <VAR>name</VAR>. The reserved word <CODE>function</CODE> is optional. If the <CODE>function</CODE> reserved word is supplied, the parentheses are optional. The <VAR>body</VAR> of the function is the compound command <VAR>compound-command</VAR> (see section <A HREF="bashref.html#SEC19">3.2.4 Compound Commands</A>). That command is usually a <VAR>list</VAR> enclosed between { and }, but may be any compound command listed above. <VAR>compound-command</VAR> is executed whenever <VAR>name</VAR> is specified as the name of a command. Any redirections (see section <A HREF="bashref.html#SEC38">3.6 Redirections</A>) associated with the shell function are performed when the function is executed. </P><P> The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. </P><P> Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by <CODE>blank</CODE>s or newlines. This is because the braces are reserved words and are only recognized as such when they are separated by whitespace. Also, when using the braces, the <VAR>list</VAR> must be terminated by a semicolon, a <SAMP>`&'</SAMP>, or a newline. </P><P> When a function is executed, the arguments to the function become the positional parameters during its execution (see section <A HREF="bashref.html#SEC25">3.4.1 Positional Parameters</A>). The special parameter <SAMP>`#'</SAMP> that expands to the number of positional parameters is updated to reflect the change. Special parameter <CODE>0</CODE> is unchanged. The first element of the <CODE>FUNCNAME</CODE> variable is set to the name of the function while the function is executing. All other aspects of the shell execution environment are identical between a function and its caller with the exception that the <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> traps are not inherited unless the function has been given the <CODE>trace</CODE> attribute using the <CODE>declare</CODE> builtin or the <CODE>-o functrace</CODE> option has been enabled with the <CODE>set</CODE> builtin, (in which case all functions inherit the <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> traps). See section <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>, for the description of the <CODE>trap</CODE> builtin. </P><P> If the builtin command <CODE>return</CODE> is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the <CODE>RETURN</CODE> trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter <SAMP>`#'</SAMP> are restored to the values they had prior to the function's execution. If a numeric argument is given to <CODE>return</CODE>, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the <CODE>return</CODE>. </P><P> Variables local to the function may be declared with the <CODE>local</CODE> builtin. These variables are visible only to the function and the commands it invokes. </P><P> Function names and definitions may be listed with the <SAMP>`-f'</SAMP> option to the <CODE>declare</CODE> or <CODE>typeset</CODE> builtin commands (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>). The <SAMP>`-F'</SAMP> option to <CODE>declare</CODE> or <CODE>typeset</CODE> will list the function names only (and optionally the source file and line number, if the <CODE>extdebug</CODE> shell option is enabled). Functions may be exported so that subshells automatically have them defined with the <SAMP>`-f'</SAMP> option to the <CODE>export</CODE> builtin (see section <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>). Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell's children. Care should be taken in cases where this may cause a problem. </P><P> Functions may be recursive. No limit is placed on the number of recursive calls. </P><P> <A NAME="Shell Parameters"></A> <HR SIZE="6"> <A NAME="SEC24"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC23"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC25"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.4 Shell Parameters </H2> <!--docid::SEC24::--> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC25">3.4.1 Positional Parameters</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The shell's command-line arguments.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC26">3.4.2 Special Parameters</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Parameters denoted by special characters.</TD></TR> </TABLE></BLOCKQUOTE> <P> A <VAR>parameter</VAR> is an entity that stores values. It can be a <CODE>name</CODE>, a number, or one of the special characters listed below. A <VAR>variable</VAR> is a parameter denoted by a <CODE>name</CODE>. A variable has a <VAR>value</VAR> and zero or more <VAR>attributes</VAR>. Attributes are assigned using the <CODE>declare</CODE> builtin command (see the description of the <CODE>declare</CODE> builtin in <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>). </P><P> A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the <CODE>unset</CODE> builtin command. </P><P> A variable may be assigned to by a statement of the form <TABLE><tr><td> </td><td class=example><pre><VAR>name</VAR>=[<VAR>value</VAR>] </pre></td></tr></table>If <VAR>value</VAR> is not given, the variable is assigned the null string. All <VAR>value</VAR>s undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below). If the variable has its <CODE>integer</CODE> attribute set, then <VAR>value</VAR> is evaluated as an arithmetic expression even if the <CODE>$((<small>...</small>))</CODE> expansion is not used (see section <A HREF="bashref.html#SEC32">3.5.5 Arithmetic Expansion</A>). Word splitting is not performed, with the exception of <CODE>"$@"</CODE> as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the <CODE>alias</CODE>, <CODE>declare</CODE>, <CODE>typeset</CODE>, <CODE>export</CODE>, <CODE>readonly</CODE>, and <CODE>local</CODE> builtin commands. </P><P> In the context where an assignment statement is assigning a value to a shell variable or array index (see section <A HREF="bashref.html#SEC81">6.7 Arrays</A>), the <SAMP>`+='</SAMP> operator can be used to append to or add to the variable's previous value. When <SAMP>`+='</SAMP> is applied to a variable for which the integer attribute has been set, <VAR>value</VAR> is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When <SAMP>`+='</SAMP> is applied to an array variable using compound assignment (see section <A HREF="bashref.html#SEC81">6.7 Arrays</A>), the variable's value is not unset (as it is when using <SAMP>`='</SAMP>), and new values are appended to the array beginning at one greater than the array's maximum index. When applied to a string-valued variable, <VAR>value</VAR> is expanded and appended to the variable's value. </P><P> <A NAME="Positional Parameters"></A> <HR SIZE="6"> <A NAME="SEC25"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC24"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC26"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC24"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.4.1 Positional Parameters </H3> <!--docid::SEC25::--> <P> A <VAR>positional parameter</VAR> is a parameter denoted by one or more digits, other than the single digit <CODE>0</CODE>. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the <CODE>set</CODE> builtin command. Positional parameter <CODE>N</CODE> may be referenced as <CODE>${N}</CODE>, or as <CODE>$N</CODE> when <CODE>N</CODE> consists of a single digit. Positional parameters may not be assigned to with assignment statements. The <CODE>set</CODE> and <CODE>shift</CODE> builtins are used to set and unset them (see section <A HREF="bashref.html#SEC56">4. Shell Builtin Commands</A>). The positional parameters are temporarily replaced when a shell function is executed (see section <A HREF="bashref.html#SEC23">3.3 Shell Functions</A>). </P><P> When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces. </P><P> <A NAME="Special Parameters"></A> <HR SIZE="6"> <A NAME="SEC26"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC25"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC24"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.4.2 Special Parameters </H3> <!--docid::SEC26::--> <P> The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. </P><P> <DL COMPACT> <A NAME="IDX46"></A> <DT><CODE>*</CODE> <DD><A NAME="IDX47"></A> Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the <CODE>IFS</CODE> special variable. That is, <CODE>"$*"</CODE> is equivalent to <CODE>"$1<VAR>c</VAR>$2<VAR>c</VAR><small>...</small>"</CODE>, where <VAR>c</VAR> is the first character of the value of the <CODE>IFS</CODE> variable. If <CODE>IFS</CODE> is unset, the parameters are separated by spaces. If <CODE>IFS</CODE> is null, the parameters are joined without intervening separators. <P> <A NAME="IDX48"></A> <DT><CODE>@</CODE> <DD><A NAME="IDX49"></A> Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, <CODE>"$@"</CODE> is equivalent to <CODE>"$1" "$2" <small>...</small></CODE>. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, <CODE>"$@"</CODE> and <CODE>$@</CODE> expand to nothing (i.e., they are removed). <P> <A NAME="IDX50"></A> <DT><CODE>#</CODE> <DD><A NAME="IDX51"></A> Expands to the number of positional parameters in decimal. <P> <A NAME="IDX52"></A> <DT><CODE>?</CODE> <DD><A NAME="IDX53"></A> Expands to the exit status of the most recently executed foreground pipeline. <P> <A NAME="IDX54"></A> <DT><CODE>-</CODE> <DD><A NAME="IDX55"></A> (A hyphen.) Expands to the current option flags as specified upon invocation, by the <CODE>set</CODE> builtin command, or those set by the shell itself (such as the <SAMP>`-i'</SAMP> option). <P> <A NAME="IDX56"></A> <DT><CODE>$</CODE> <DD><A NAME="IDX57"></A> Expands to the process ID of the shell. In a <CODE>()</CODE> subshell, it expands to the process ID of the invoking shell, not the subshell. <P> <A NAME="IDX58"></A> <DT><CODE>!</CODE> <DD><A NAME="IDX59"></A> Expands to the process ID of the most recently executed background (asynchronous) command. <P> <A NAME="IDX60"></A> <DT><CODE>0</CODE> <DD><A NAME="IDX61"></A> Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (see section <A HREF="bashref.html#SEC55">3.8 Shell Scripts</A>), <CODE>$0</CODE> is set to the name of that file. If Bash is started with the <SAMP>`-c'</SAMP> option (see section <A HREF="bashref.html#SEC65">6.1 Invoking Bash</A>), then <CODE>$0</CODE> is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero. <P> <A NAME="IDX62"></A> <DT><CODE>_</CODE> <DD><A NAME="IDX63"></A> (An underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file. </DL> <P> <A NAME="Shell Expansions"></A> <HR SIZE="6"> <A NAME="SEC27"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC26"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC28"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.5 Shell Expansions </H2> <!--docid::SEC27::--> <P> Expansion is performed on the command line after it has been split into <CODE>token</CODE>s. There are seven kinds of expansion performed: <UL> <LI>brace expansion <LI>tilde expansion <LI>parameter and variable expansion <LI>command substitution <LI>arithmetic expansion <LI>word splitting <LI>filename expansion </UL> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC28">3.5.1 Brace Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Expansion of expressions within braces.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC29">3.5.2 Tilde Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Expansion of the ~ character.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC30">3.5.3 Shell Parameter Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How Bash expands variables to their values.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC31">3.5.4 Command Substitution</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Using the output of a command as an argument.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC32">3.5.5 Arithmetic Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How to use arithmetic in shell expansions.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC33">3.5.6 Process Substitution</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A way to write and read to and from a command.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC34">3.5.7 Word Splitting</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How the results of expansion are split into separate arguments.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC35">3.5.8 Filename Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">A shorthand for specifying filenames matching patterns.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC37">3.5.9 Quote Removal</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How and when quote characters are removed from words.</TD></TR> </TABLE></BLOCKQUOTE> <P> The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion. </P><P> On systems that can support it, there is an additional expansion available: <VAR>process substitution</VAR>. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution. </P><P> Only brace expansion, word splitting, and filename expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of <CODE>"$@"</CODE> (see section <A HREF="bashref.html#SEC26">3.4.2 Special Parameters</A>) and <CODE>"${<VAR>name</VAR>[@]}"</CODE> (see section <A HREF="bashref.html#SEC81">6.7 Arrays</A>). </P><P> After all expansions, <CODE>quote removal</CODE> (see section <A HREF="bashref.html#SEC37">3.5.9 Quote Removal</A>) is performed. </P><P> <A NAME="Brace Expansion"></A> <HR SIZE="6"> <A NAME="SEC28"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC29"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.1 Brace Expansion </H3> <!--docid::SEC28::--> <P> Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to <VAR>filename expansion</VAR> (see section <A HREF="bashref.html#SEC35">3.5.8 Filename Expansion</A>), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional <VAR>preamble</VAR>, followed by either a series of comma-separated strings or a sequnce expression between a pair of braces, followed by an optional <VAR>postscript</VAR>. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right. </P><P> Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, <TABLE><tr><td> </td><td class=example><pre>bash$ echo a{d,c,b}e ade ace abe </pre></td></tr></table></P><P> A sequence expression takes the form <CODE>{<VAR>x</VAR>..<VAR>y</VAR>}</CODE>, where <VAR>x</VAR> and <VAR>y</VAR> are either integers or single characters. When integers are supplied, the expression expands to each number between <VAR>x</VAR> and <VAR>y</VAR>, inclusive. When characters are supplied, the expression expands to each character lexicographically between <VAR>x</VAR> and <VAR>y</VAR>, inclusive. Note that both <VAR>x</VAR> and <VAR>y</VAR> must be of the same type. </P><P> Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string <SAMP>`${'</SAMP> is not considered eligible for brace expansion. </P><P> A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. </P><P> A { or <SAMP>`,'</SAMP> may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string <SAMP>`${'</SAMP> is not considered eligible for brace expansion. </P><P> This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example: <TABLE><tr><td> </td><td class=example><pre>mkdir /usr/local/src/bash/{old,new,dist,bugs} </pre></td></tr></table>or <TABLE><tr><td> </td><td class=example><pre>chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}} </pre></td></tr></table></P><P> <A NAME="Tilde Expansion"></A> <HR SIZE="6"> <A NAME="SEC29"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC28"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC30"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC30"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.2 Tilde Expansion </H3> <!--docid::SEC29::--> <P> If a word begins with an unquoted tilde character (<SAMP>`~'</SAMP>), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a <VAR>tilde-prefix</VAR>. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible <VAR>login name</VAR>. If this login name is the null string, the tilde is replaced with the value of the <CODE>HOME</CODE> shell variable. If <CODE>HOME</CODE> is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name. </P><P> If the tilde-prefix is <SAMP>`~+'</SAMP>, the value of the shell variable <CODE>PWD</CODE> replaces the tilde-prefix. If the tilde-prefix is <SAMP>`~-'</SAMP>, the value of the shell variable <CODE>OLDPWD</CODE>, if it is set, is substituted. </P><P> If the characters following the tilde in the tilde-prefix consist of a number <VAR>N</VAR>, optionally prefixed by a <SAMP>`+'</SAMP> or a <SAMP>`-'</SAMP>, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the <CODE>dirs</CODE> builtin invoked with the characters following tilde in the tilde-prefix as an argument (see section <A HREF="bashref.html#SEC82">6.8 The Directory Stack</A>). If the tilde-prefix, sans the tilde, consists of a number without a leading <SAMP>`+'</SAMP> or <SAMP>`-'</SAMP>, <SAMP>`+'</SAMP> is assumed. </P><P> If the login name is invalid, or the tilde expansion fails, the word is left unchanged. </P><P> Each variable assignment is checked for unquoted tilde-prefixes immediately following a <SAMP>`:'</SAMP> or the first <SAMP>`='</SAMP>. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to <CODE>PATH</CODE>, <CODE>MAILPATH</CODE>, and <CODE>CDPATH</CODE>, and the shell assigns the expanded value. </P><P> The following table shows how Bash treats unquoted tilde-prefixes: </P><P> <DL COMPACT> <DT><CODE>~</CODE> <DD>The value of <CODE>$HOME</CODE> <DT><CODE>~/foo</CODE> <DD><TT>`$HOME/foo'</TT> <P> <DT><CODE>~fred/foo</CODE> <DD>The subdirectory <CODE>foo</CODE> of the home directory of the user <CODE>fred</CODE> <P> <DT><CODE>~+/foo</CODE> <DD><TT>`$PWD/foo'</TT> <P> <DT><CODE>~-/foo</CODE> <DD><TT>`${OLDPWD-'~-'}/foo'</TT> <P> <DT><CODE>~<VAR>N</VAR></CODE> <DD>The string that would be displayed by <SAMP>`dirs +<VAR>N</VAR>'</SAMP> <P> <DT><CODE>~+<VAR>N</VAR></CODE> <DD>The string that would be displayed by <SAMP>`dirs +<VAR>N</VAR>'</SAMP> <P> <DT><CODE>~-<VAR>N</VAR></CODE> <DD>The string that would be displayed by <SAMP>`dirs -<VAR>N</VAR>'</SAMP> <P> </DL> <P> <A NAME="Shell Parameter Expansion"></A> <HR SIZE="6"> <A NAME="SEC30"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC29"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC31"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC31"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.3 Shell Parameter Expansion </H3> <!--docid::SEC30::--> <P> The <SAMP>`$'</SAMP> character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. </P><P> When braces are used, the matching ending brace is the first <SAMP>`}'</SAMP> not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion. </P><P> The basic form of parameter expansion is ${<VAR>parameter</VAR>}. The value of <VAR>parameter</VAR> is substituted. The braces are required when <VAR>parameter</VAR> is a positional parameter with more than one digit, or when <VAR>parameter</VAR> is followed by a character that is not to be interpreted as part of its name. </P><P> If the first character of <VAR>parameter</VAR> is an exclamation point, a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of <VAR>parameter</VAR> as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of <VAR>parameter</VAR> itself. This is known as <CODE>indirect expansion</CODE>. The exceptions to this are the expansions of ${!<VAR>prefix*</VAR>} and ${!<VAR>name</VAR>[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection. </P><P> In each of the cases below, <VAR>word</VAR> is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. </P><P> When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both existence and that the value is not null; if the colon is omitted, the operator tests only for existence. </P><P> <DL COMPACT> <DT><CODE>${<VAR>parameter</VAR>:-<VAR>word</VAR>}</CODE> <DD>If <VAR>parameter</VAR> is unset or null, the expansion of <VAR>word</VAR> is substituted. Otherwise, the value of <VAR>parameter</VAR> is substituted. <P> <DT><CODE>${<VAR>parameter</VAR>:=<VAR>word</VAR>}</CODE> <DD>If <VAR>parameter</VAR> is unset or null, the expansion of <VAR>word</VAR> is assigned to <VAR>parameter</VAR>. The value of <VAR>parameter</VAR> is then substituted. Positional parameters and special parameters may not be assigned to in this way. <P> <DT><CODE>${<VAR>parameter</VAR>:?<VAR>word</VAR>}</CODE> <DD>If <VAR>parameter</VAR> is null or unset, the expansion of <VAR>word</VAR> (or a message to that effect if <VAR>word</VAR> is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of <VAR>parameter</VAR> is substituted. <P> <DT><CODE>${<VAR>parameter</VAR>:+<VAR>word</VAR>}</CODE> <DD>If <VAR>parameter</VAR> is null or unset, nothing is substituted, otherwise the expansion of <VAR>word</VAR> is substituted. <P> <DT><CODE>${<VAR>parameter</VAR>:<VAR>offset</VAR>}</CODE> <DD><DT><CODE>${<VAR>parameter</VAR>:<VAR>offset</VAR>:<VAR>length</VAR>}</CODE> <DD>Expands to up to <VAR>length</VAR> characters of <VAR>parameter</VAR> starting at the character specified by <VAR>offset</VAR>. If <VAR>length</VAR> is omitted, expands to the substring of <VAR>parameter</VAR> starting at the character specified by <VAR>offset</VAR>. <VAR>length</VAR> and <VAR>offset</VAR> are arithmetic expressions (see section <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>). This is referred to as Substring Expansion. <P> <VAR>length</VAR> must evaluate to a number greater than or equal to zero. If <VAR>offset</VAR> evaluates to a number less than zero, the value is used as an offset from the end of the value of <VAR>parameter</VAR>. If <VAR>parameter</VAR> is <SAMP>`@'</SAMP>, the result is <VAR>length</VAR> positional parameters beginning at <VAR>offset</VAR>. If <VAR>parameter</VAR> is an array name indexed by <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the result is the <VAR>length</VAR> members of the array beginning with <CODE>${<VAR>parameter</VAR>[<VAR>offset</VAR>]}</CODE>. A negative <VAR>offset</VAR> is taken relative to one greater than the maximum index of the specified array. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the <SAMP>`:-'</SAMP> expansion. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1. </P><P> <DT><CODE>${!<VAR>prefix</VAR>*}</CODE> <DD><DT><CODE>${!<VAR>prefix</VAR>@}</CODE> <DD>Expands to the names of variables whose names begin with <VAR>prefix</VAR>, separated by the first character of the <CODE>IFS</CODE> special variable. <P> <DT><CODE>${!<VAR>name</VAR>[@]}</CODE> <DD><DT><CODE>${!<VAR>name</VAR>[*]}</CODE> <DD>If <VAR>name</VAR> is an array variable, expands to the list of array indices (keys) assigned in <VAR>name</VAR>. If <VAR>name</VAR> is not an array, expands to 0 if <VAR>name</VAR> is set and null otherwise. When <SAMP>`@'</SAMP> is used and the expansion appears within double quotes, each key expands to a separate word. <P> <DT><CODE>${#<VAR>parameter</VAR>}</CODE> <DD>The length in characters of the expanded value of <VAR>parameter</VAR> is substituted. If <VAR>parameter</VAR> is <SAMP>`*'</SAMP> or <SAMP>`@'</SAMP>, the value substituted is the number of positional parameters. If <VAR>parameter</VAR> is an array name subscripted by <SAMP>`*'</SAMP> or <SAMP>`@'</SAMP>, the value substituted is the number of elements in the array. <P> <DT><CODE>${<VAR>parameter</VAR>#<VAR>word</VAR>}</CODE> <DD><DT><CODE>${<VAR>parameter</VAR>##<VAR>word</VAR>}</CODE> <DD>The <VAR>word</VAR> is expanded to produce a pattern just as in filename expansion (see section <A HREF="bashref.html#SEC35">3.5.8 Filename Expansion</A>). If the pattern matches the beginning of the expanded value of <VAR>parameter</VAR>, then the result of the expansion is the expanded value of <VAR>parameter</VAR> with the shortest matching pattern (the <SAMP>`#'</SAMP> case) or the longest matching pattern (the <SAMP>`##'</SAMP> case) deleted. If <VAR>parameter</VAR> is <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If <VAR>parameter</VAR> is an array variable subscripted with <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. <P> <DT><CODE>${<VAR>parameter</VAR>%<VAR>word</VAR>}</CODE> <DD><DT><CODE>${<VAR>parameter</VAR>%%<VAR>word</VAR>}</CODE> <DD>The <VAR>word</VAR> is expanded to produce a pattern just as in filename expansion. If the pattern matches a trailing portion of the expanded value of <VAR>parameter</VAR>, then the result of the expansion is the value of <VAR>parameter</VAR> with the shortest matching pattern (the <SAMP>`%'</SAMP> case) or the longest matching pattern (the <SAMP>`%%'</SAMP> case) deleted. If <VAR>parameter</VAR> is <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If <VAR>parameter</VAR> is an array variable subscripted with <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. <P> <DT><CODE>${<VAR>parameter</VAR>/<VAR>pattern</VAR>/<VAR>string</VAR>}</CODE> <DD><DT><CODE>${<VAR>parameter</VAR>//<VAR>pattern</VAR>/<VAR>string</VAR>}</CODE> <DD><P> The <VAR>pattern</VAR> is expanded to produce a pattern just as in filename expansion. <VAR>Parameter</VAR> is expanded and the longest match of <VAR>pattern</VAR> against its value is replaced with <VAR>string</VAR>. In the first form, only the first match is replaced. The second form causes all matches of <VAR>pattern</VAR> to be replaced with <VAR>string</VAR>. If <VAR>pattern</VAR> begins with <SAMP>`#'</SAMP>, it must match at the beginning of the expanded value of <VAR>parameter</VAR>. If <VAR>pattern</VAR> begins with <SAMP>`%'</SAMP>, it must match at the end of the expanded value of <VAR>parameter</VAR>. If <VAR>string</VAR> is null, matches of <VAR>pattern</VAR> are deleted and the <CODE>/</CODE> following <VAR>pattern</VAR> may be omitted. If <VAR>parameter</VAR> is <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If <VAR>parameter</VAR> is an array variable subscripted with <SAMP>`@'</SAMP> or <SAMP>`*'</SAMP>, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list. </P><P> </DL> <P> <A NAME="Command Substitution"></A> <HR SIZE="6"> <A NAME="SEC31"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC30"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC32"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC32"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.4 Command Substitution </H3> <!--docid::SEC31::--> <P> Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows: <TABLE><tr><td> </td><td class=example><pre>$(<VAR>command</VAR>) </pre></td></tr></table>or <TABLE><tr><td> </td><td class=example><pre>`<VAR>command</VAR>` </pre></td></tr></table></P><P> Bash performs the expansion by executing <VAR>command</VAR> and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution <CODE>$(cat <VAR>file</VAR>)</CODE> can be replaced by the equivalent but faster <CODE>$(< <VAR>file</VAR>)</CODE>. </P><P> When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by <SAMP>`$'</SAMP>, <SAMP>``'</SAMP>, or <SAMP>`\'</SAMP>. The first backquote not preceded by a backslash terminates the command substitution. When using the <CODE>$(<VAR>command</VAR>)</CODE> form, all characters between the parentheses make up the command; none are treated specially. </P><P> Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. </P><P> If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results. </P><P> <A NAME="Arithmetic Expansion"></A> <HR SIZE="6"> <A NAME="SEC32"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC31"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC33"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC33"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.5 Arithmetic Expansion </H3> <!--docid::SEC32::--> <P> Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is: </P><P> <TABLE><tr><td> </td><td class=example><pre>$(( <VAR>expression</VAR> )) </pre></td></tr></table></P><P> The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic expansions may be nested. </P><P> The evaluation is performed according to the rules listed below (see section <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs. </P><P> <A NAME="Process Substitution"></A> <HR SIZE="6"> <A NAME="SEC33"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC32"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC34"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC34"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.6 Process Substitution </H3> <!--docid::SEC33::--> <P> Process substitution is supported on systems that support named pipes (FIFOs) or the <TT>`/dev/fd'</TT> method of naming open files. It takes the form of <TABLE><tr><td> </td><td class=example><pre><(<VAR>list</VAR>) </pre></td></tr></table>or <TABLE><tr><td> </td><td class=example><pre>>(<VAR>list</VAR>) </pre></td></tr></table>The process <VAR>list</VAR> is run with its input or output connected to a FIFO or some file in <TT>`/dev/fd'</TT>. The name of this file is passed as an argument to the current command as the result of the expansion. If the <CODE>>(<VAR>list</VAR>)</CODE> form is used, writing to the file will provide input for <VAR>list</VAR>. If the <CODE><(<VAR>list</VAR>)</CODE> form is used, the file passed as an argument should be read to obtain the output of <VAR>list</VAR>. Note that no space may appear between the <CODE><</CODE> or <CODE>></CODE> and the left parenthesis, otherwise the construct would be interpreted as a redirection. </P><P> When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion. </P><P> <A NAME="Word Splitting"></A> <HR SIZE="6"> <A NAME="SEC34"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC33"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC35"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC35"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.7 Word Splitting </H3> <!--docid::SEC34::--> <P> The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. </P><P> The shell treats each character of <CODE>$IFS</CODE> as a delimiter, and splits the results of the other expansions into words on these characters. If <CODE>IFS</CODE> is unset, or its value is exactly <CODE><space><tab><newline></CODE>, the default, then any sequence of <CODE>IFS</CODE> characters serves to delimit words. If <CODE>IFS</CODE> has a value other than the default, then sequences of the whitespace characters <CODE>space</CODE> and <CODE>tab</CODE> are ignored at the beginning and end of the word, as long as the whitespace character is in the value of <CODE>IFS</CODE> (an <CODE>IFS</CODE> whitespace character). Any character in <CODE>IFS</CODE> that is not <CODE>IFS</CODE> whitespace, along with any adjacent <CODE>IFS</CODE> whitespace characters, delimits a field. A sequence of <CODE>IFS</CODE> whitespace characters is also treated as a delimiter. If the value of <CODE>IFS</CODE> is null, no word splitting occurs. </P><P> Explicit null arguments (<CODE>""</CODE> or <CODE>"</CODE>) are retained. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained. </P><P> Note that if no expansion occurs, no splitting is performed. </P><P> <A NAME="Filename Expansion"></A> <HR SIZE="6"> <A NAME="SEC35"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC34"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC36"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.8 Filename Expansion </H3> <!--docid::SEC35::--> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC36">3.5.8.1 Pattern Matching</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How the shell matches patterns.</TD></TR> </TABLE></BLOCKQUOTE> <A NAME="IDX64"></A> <A NAME="IDX65"></A> <A NAME="IDX66"></A> <A NAME="IDX67"></A> <P> After word splitting, unless the <SAMP>`-f'</SAMP> option has been set (see section <A HREF="bashref.html#SEC59">4.3 The Set Builtin</A>), Bash scans each word for the characters <SAMP>`*'</SAMP>, <SAMP>`?'</SAMP>, and <SAMP>`['</SAMP>. If one of these characters appears, then the word is regarded as a <VAR>pattern</VAR>, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found, and the shell option <CODE>nullglob</CODE> is disabled, the word is left unchanged. If the <CODE>nullglob</CODE> option is set, and no matches are found, the word is removed. If the <CODE>failglob</CODE> shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option <CODE>nocaseglob</CODE> is enabled, the match is performed without regard to the case of alphabetic characters. </P><P> When a pattern is used for filename generation, the character <SAMP>`.'</SAMP> at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option <CODE>dotglob</CODE> is set. When matching a file name, the slash character must always be matched explicitly. In other cases, the <SAMP>`.'</SAMP> character is not treated specially. </P><P> See the description of <CODE>shopt</CODE> in <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>, for a description of the <CODE>nocaseglob</CODE>, <CODE>nullglob</CODE>, <CODE>failglob</CODE>, and <CODE>dotglob</CODE> options. </P><P> The <CODE>GLOBIGNORE</CODE> shell variable may be used to restrict the set of filenames matching a pattern. If <CODE>GLOBIGNORE</CODE> is set, each matching filename that also matches one of the patterns in <CODE>GLOBIGNORE</CODE> is removed from the list of matches. The filenames <TT>`.'</TT> and <TT>`..'</TT> are always ignored when <CODE>GLOBIGNORE</CODE> is set and not null. However, setting <CODE>GLOBIGNORE</CODE> to a non-null value has the effect of enabling the <CODE>dotglob</CODE> shell option, so all other filenames beginning with a <SAMP>`.'</SAMP> will match. To get the old behavior of ignoring filenames beginning with a <SAMP>`.'</SAMP>, make <SAMP>`.*'</SAMP> one of the patterns in <CODE>GLOBIGNORE</CODE>. The <CODE>dotglob</CODE> option is disabled when <CODE>GLOBIGNORE</CODE> is unset. </P><P> <A NAME="Pattern Matching"></A> <HR SIZE="6"> <A NAME="SEC36"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC35"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC35"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H4> 3.5.8.1 Pattern Matching </H4> <!--docid::SEC36::--> <P> Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally. </P><P> The special pattern characters have the following meanings: <DL COMPACT> <DT><CODE>*</CODE> <DD>Matches any string, including the null string. <DT><CODE>?</CODE> <DD>Matches any single character. <DT><CODE>[<small>...</small>]</CODE> <DD>Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a <VAR>range expression</VAR>; any character that sorts between those two characters, inclusive, using the current locale's collating sequence and character set, is matched. If the first character following the <SAMP>`['</SAMP> is a <SAMP>`!'</SAMP> or a <SAMP>`^'</SAMP> then any character not enclosed is matched. A <SAMP>`-'</SAMP> may be matched by including it as the first or last character in the set. A <SAMP>`]'</SAMP> may be matched by including it as the first character in the set. The sorting order of characters in range expressions is determined by the current locale and the value of the <CODE>LC_COLLATE</CODE> shell variable, if set. <P> For example, in the default C locale, <SAMP>`[a-dx-z]'</SAMP> is equivalent to <SAMP>`[abcdxyz]'</SAMP>. Many locales sort characters in dictionary order, and in these locales <SAMP>`[a-dx-z]'</SAMP> is typically not equivalent to <SAMP>`[abcdxyz]'</SAMP>; it might be equivalent to <SAMP>`[aBbCcDdxXyYz]'</SAMP>, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the <CODE>LC_COLLATE</CODE> or <CODE>LC_ALL</CODE> environment variable to the value <SAMP>`C'</SAMP>. </P><P> Within <SAMP>`['</SAMP> and <SAMP>`]'</SAMP>, <VAR>character classes</VAR> can be specified using the syntax <CODE>[:</CODE><VAR>class</VAR><CODE>:]</CODE>, where <VAR>class</VAR> is one of the following classes defined in the POSIX 1003.2 standard: <TABLE><tr><td> </td><td class=example><pre>alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit </pre></td></tr></table>A character class matches any character belonging to that class. The <CODE>word</CODE> character class matches letters, digits, and the character <SAMP>`_'</SAMP>. </P><P> Within <SAMP>`['</SAMP> and <SAMP>`]'</SAMP>, an <VAR>equivalence class</VAR> can be specified using the syntax <CODE>[=</CODE><VAR>c</VAR><CODE>=]</CODE>, which matches all characters with the same collation weight (as defined by the current locale) as the character <VAR>c</VAR>. </P><P> Within <SAMP>`['</SAMP> and <SAMP>`]'</SAMP>, the syntax <CODE>[.</CODE><VAR>symbol</VAR><CODE>.]</CODE> matches the collating symbol <VAR>symbol</VAR>. </DL> <P> If the <CODE>extglob</CODE> shell option is enabled using the <CODE>shopt</CODE> builtin, several extended pattern matching operators are recognized. In the following description, a <VAR>pattern-list</VAR> is a list of one or more patterns separated by a <SAMP>`|'</SAMP>. Composite patterns may be formed using one or more of the following sub-patterns: </P><P> <DL COMPACT> <DT><CODE>?(<VAR>pattern-list</VAR>)</CODE> <DD>Matches zero or one occurrence of the given patterns. <P> <DT><CODE>*(<VAR>pattern-list</VAR>)</CODE> <DD>Matches zero or more occurrences of the given patterns. <P> <DT><CODE>+(<VAR>pattern-list</VAR>)</CODE> <DD>Matches one or more occurrences of the given patterns. <P> <DT><CODE>@(<VAR>pattern-list</VAR>)</CODE> <DD>Matches one of the given patterns. <P> <DT><CODE>!(<VAR>pattern-list</VAR>)</CODE> <DD>Matches anything except one of the given patterns. </DL> <P> <A NAME="Quote Removal"></A> <HR SIZE="6"> <A NAME="SEC37"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC36"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC27"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.5.9 Quote Removal </H3> <!--docid::SEC37::--> <P> After the preceding expansions, all unquoted occurrences of the characters <SAMP>`\'</SAMP>, <SAMP>`''</SAMP>, and <SAMP>`"'</SAMP> that did not result from one of the above expansions are removed. </P><P> <A NAME="Redirections"></A> <HR SIZE="6"> <A NAME="SEC38"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC37"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC39"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.6 Redirections </H2> <!--docid::SEC38::--> <P> Before a command is executed, its input and output may be <VAR>redirected</VAR> using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right. </P><P> In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <SAMP>`<'</SAMP>, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is <SAMP>`>'</SAMP>, the redirection refers to the standard output (file descriptor 1). </P><P> The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error. </P><P> Note that the order of redirections is significant. For example, the command <TABLE><tr><td> </td><td class=example><pre>ls > <VAR>dirlist</VAR> 2>&1 </pre></td></tr></table>directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file <VAR>dirlist</VAR>, while the command <TABLE><tr><td> </td><td class=example><pre>ls 2>&1 > <VAR>dirlist</VAR> </pre></td></tr></table>directs only the standard output to file <VAR>dirlist</VAR>, because the standard error was duplicated as standard output before the standard output was redirected to <VAR>dirlist</VAR>. </P><P> Bash handles several filenames specially when they are used in redirections, as described in the following table: </P><P> <DL COMPACT> <DT><CODE>/dev/fd/<VAR>fd</VAR></CODE> <DD>If <VAR>fd</VAR> is a valid integer, file descriptor <VAR>fd</VAR> is duplicated. <P> <DT><CODE>/dev/stdin</CODE> <DD>File descriptor 0 is duplicated. <P> <DT><CODE>/dev/stdout</CODE> <DD>File descriptor 1 is duplicated. <P> <DT><CODE>/dev/stderr</CODE> <DD>File descriptor 2 is duplicated. <P> <DT><CODE>/dev/tcp/<VAR>host</VAR>/<VAR>port</VAR></CODE> <DD>If <VAR>host</VAR> is a valid hostname or Internet address, and <VAR>port</VAR> is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. <P> <DT><CODE>/dev/udp/<VAR>host</VAR>/<VAR>port</VAR></CODE> <DD>If <VAR>host</VAR> is a valid hostname or Internet address, and <VAR>port</VAR> is an integer port number or service name, Bash attempts to open a UDP connection to the corresponding socket. <P> </DL> <P> A failure to open or create a file causes the redirection to fail. </P><P> Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally. </P><P> <HR SIZE="6"> <A NAME="SEC39"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC40"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.1 Redirecting Input </H3> <!--docid::SEC39::--> Redirection of input causes the file whose name results from the expansion of <VAR>word</VAR> to be opened for reading on file descriptor <CODE>n</CODE>, or the standard input (file descriptor 0) if <CODE>n</CODE> is not specified. <P> The general format for redirecting input is: <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]<<VAR>word</VAR> </pre></td></tr></table></P><P> <HR SIZE="6"> <A NAME="SEC40"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC39"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC41"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC41"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.2 Redirecting Output </H3> <!--docid::SEC40::--> Redirection of output causes the file whose name results from the expansion of <VAR>word</VAR> to be opened for writing on file descriptor <VAR>n</VAR>, or the standard output (file descriptor 1) if <VAR>n</VAR> is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. <P> The general format for redirecting output is: <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]>[|]<VAR>word</VAR> </pre></td></tr></table></P><P> If the redirection operator is <SAMP>`>'</SAMP>, and the <CODE>noclobber</CODE> option to the <CODE>set</CODE> builtin has been enabled, the redirection will fail if the file whose name results from the expansion of <VAR>word</VAR> exists and is a regular file. If the redirection operator is <SAMP>`>|'</SAMP>, or the redirection operator is <SAMP>`>'</SAMP> and the <CODE>noclobber</CODE> option is not enabled, the redirection is attempted even if the file named by <VAR>word</VAR> exists. </P><P> <HR SIZE="6"> <A NAME="SEC41"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC40"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC42"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC42"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.3 Appending Redirected Output </H3> <!--docid::SEC41::--> Redirection of output in this fashion causes the file whose name results from the expansion of <VAR>word</VAR> to be opened for appending on file descriptor <VAR>n</VAR>, or the standard output (file descriptor 1) if <VAR>n</VAR> is not specified. If the file does not exist it is created. <P> The general format for appending output is: <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]>><VAR>word</VAR> </pre></td></tr></table></P><P> <HR SIZE="6"> <A NAME="SEC42"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC41"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC43"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC43"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.4 Redirecting Standard Output and Standard Error </H3> <!--docid::SEC42::--> Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of <VAR>word</VAR> with this construct. <P> There are two formats for redirecting standard output and standard error: <TABLE><tr><td> </td><td class=example><pre>&><VAR>word</VAR> </pre></td></tr></table>and <TABLE><tr><td> </td><td class=example><pre>>&<VAR>word</VAR> </pre></td></tr></table>Of the two forms, the first is preferred. This is semantically equivalent to <TABLE><tr><td> </td><td class=example><pre>><VAR>word</VAR> 2>&1 </pre></td></tr></table></P><P> <HR SIZE="6"> <A NAME="SEC43"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC42"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC44"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC44"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.5 Here Documents </H3> <!--docid::SEC43::--> This type of redirection instructs the shell to read input from the current source until a line containing only <VAR>word</VAR> (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. <P> The format of here-documents is: <TABLE><tr><td> </td><td class=example><pre><<[-]<VAR>word</VAR> <VAR>here-document</VAR> <VAR>delimiter</VAR> </pre></td></tr></table></P><P> No parameter expansion, command substitution, arithmetic expansion, or filename expansion is performed on <VAR>word</VAR>. If any characters in <VAR>word</VAR> are quoted, the <VAR>delimiter</VAR> is the result of quote removal on <VAR>word</VAR>, and the lines in the here-document are not expanded. If <VAR>word</VAR> is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence <CODE>\newline</CODE> is ignored, and <SAMP>`\'</SAMP> must be used to quote the characters <SAMP>`\'</SAMP>, <SAMP>`$'</SAMP>, and <SAMP>``'</SAMP>. </P><P> If the redirection operator is <SAMP>`<<-'</SAMP>, then all leading tab characters are stripped from input lines and the line containing <VAR>delimiter</VAR>. This allows here-documents within shell scripts to be indented in a natural fashion. </P><P> <HR SIZE="6"> <A NAME="SEC44"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC43"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC45"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC45"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.6 Here Strings </H3> <!--docid::SEC44::--> A variant of here documents, the format is: <TABLE><tr><td> </td><td class=example><pre><<< <VAR>word</VAR> </pre></td></tr></table><P> The <VAR>word</VAR> is expanded and supplied to the command on its standard input. </P><P> <HR SIZE="6"> <A NAME="SEC45"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC44"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC46"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC46"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.7 Duplicating File Descriptors </H3> <!--docid::SEC45::--> The redirection operator <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]<&<VAR>word</VAR> </pre></td></tr></table>is used to duplicate input file descriptors. If <VAR>word</VAR> expands to one or more digits, the file descriptor denoted by <VAR>n</VAR> is made to be a copy of that file descriptor. If the digits in <VAR>word</VAR> do not specify a file descriptor open for input, a redirection error occurs. If <VAR>word</VAR> evaluates to <SAMP>`-'</SAMP>, file descriptor <VAR>n</VAR> is closed. If <VAR>n</VAR> is not specified, the standard input (file descriptor 0) is used. <P> The operator <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]>&<VAR>word</VAR> </pre></td></tr></table>is used similarly to duplicate output file descriptors. If <VAR>n</VAR> is not specified, the standard output (file descriptor 1) is used. If the digits in <VAR>word</VAR> do not specify a file descriptor open for output, a redirection error occurs. As a special case, if <VAR>n</VAR> is omitted, and <VAR>word</VAR> does not expand to one or more digits, the standard output and standard error are redirected as described previously. </P><P> <HR SIZE="6"> <A NAME="SEC46"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC45"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC47"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC47"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.8 Moving File Descriptors </H3> <!--docid::SEC46::--> The redirection operator <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]<&<VAR>digit</VAR>- </pre></td></tr></table>moves the file descriptor <VAR>digit</VAR> to file descriptor <VAR>n</VAR>, or the standard input (file descriptor 0) if <VAR>n</VAR> is not specified. <VAR>digit</VAR> is closed after being duplicated to <VAR>n</VAR>. <P> Similarly, the redirection operator <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]>&<VAR>digit</VAR>- </pre></td></tr></table>moves the file descriptor <VAR>digit</VAR> to file descriptor <VAR>n</VAR>, or the standard output (file descriptor 1) if <VAR>n</VAR> is not specified. </P><P> <HR SIZE="6"> <A NAME="SEC47"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC46"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC38"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.6.9 Opening File Descriptors for Reading and Writing </H3> <!--docid::SEC47::--> The redirection operator <TABLE><tr><td> </td><td class=example><pre>[<VAR>n</VAR>]<><VAR>word</VAR> </pre></td></tr></table>causes the file whose name is the expansion of <VAR>word</VAR> to be opened for both reading and writing on file descriptor <VAR>n</VAR>, or on file descriptor 0 if <VAR>n</VAR> is not specified. If the file does not exist, it is created. <P> <A NAME="Executing Commands"></A> <HR SIZE="6"> <A NAME="SEC48"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC47"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC49"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.7 Executing Commands </H2> <!--docid::SEC48::--> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC49">3.7.1 Simple Command Expansion</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How Bash expands simple commands before executing them.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC50">3.7.2 Command Search and Execution</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">How Bash finds commands and runs them.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC51">3.7.3 Command Execution Environment</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The environment in which Bash executes commands that are not shell builtins.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC52">3.7.4 Environment</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The environment given to a command.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC53">3.7.5 Exit Status</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">The status returned by commands and how Bash interprets it.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC54">3.7.6 Signals</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">What happens when Bash or a command it runs receives a signal.</TD></TR> </TABLE></BLOCKQUOTE> <P> <A NAME="Simple Command Expansion"></A> <HR SIZE="6"> <A NAME="SEC49"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC50"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.1 Simple Command Expansion </H3> <!--docid::SEC49::--> <P> When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right. </P><P> <OL> <LI> The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. <P> <LI> The words that are not variable assignments or redirections are expanded (see section <A HREF="bashref.html#SEC27">3.5 Shell Expansions</A>). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments. <P> <LI> Redirections are performed as described above (see section <A HREF="bashref.html#SEC38">3.6 Redirections</A>). <P> <LI> The text after the <SAMP>`='</SAMP> in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable. </OL> <P> If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status. </P><P> If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status. </P><P> If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero. </P><P> <A NAME="Command Search and Execution"></A> <HR SIZE="6"> <A NAME="SEC50"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC49"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC51"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC51"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.2 Command Search and Execution </H3> <!--docid::SEC50::--> <P> After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken. </P><P> <OL> <LI> If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described in <A HREF="bashref.html#SEC23">3.3 Shell Functions</A>. <P> <LI> If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked. <P> <LI> If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of <CODE>$PATH</CODE> for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple <CODE>PATH</CODE> searches (see the description of <CODE>hash</CODE> in <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>). A full search of the directories in <CODE>$PATH</CODE> is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127. <P> <LI> If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any. <P> <LI> If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a <VAR>shell script</VAR> and the shell executes it as described in <A HREF="bashref.html#SEC55">3.8 Shell Scripts</A>. <P> <LI> If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status. <P> </OL> <P> <A NAME="Command Execution Environment"></A> <HR SIZE="6"> <A NAME="SEC51"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC50"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC52"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC52"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.3 Command Execution Environment </H3> <!--docid::SEC51::--> <P> The shell has an <VAR>execution environment</VAR>, which consists of the following: </P><P> <UL> <LI> open files inherited by the shell at invocation, as modified by redirections supplied to the <CODE>exec</CODE> builtin <P> <LI> the current working directory as set by <CODE>cd</CODE>, <CODE>pushd</CODE>, or <CODE>popd</CODE>, or inherited by the shell at invocation <P> <LI> the file creation mode mask as set by <CODE>umask</CODE> or inherited from the shell's parent <P> <LI> current traps set by <CODE>trap</CODE> <P> <LI> shell parameters that are set by variable assignment or with <CODE>set</CODE> or inherited from the shell's parent in the environment <P> <LI> shell functions defined during execution or inherited from the shell's parent in the environment <P> <LI> options enabled at invocation (either by default or with command-line arguments) or by <CODE>set</CODE> <P> <LI> options enabled by <CODE>shopt</CODE> <P> <LI> shell aliases defined with <CODE>alias</CODE> (see section <A HREF="bashref.html#SEC80">6.6 Aliases</A>) <P> <LI> various process IDs, including those of background jobs (see section <A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A>), the value of <CODE>$$</CODE>, and the value of <CODE>$PPID</CODE> <P> </UL> <P> When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell. </P><P> <UL> <LI> the shell's open files, plus any modifications and additions specified by redirections to the command <P> <LI> the current working directory <P> <LI> the file creation mode mask <P> <LI> shell variables and functions marked for export, along with variables exported for the command, passed in the environment (see section <A HREF="bashref.html#SEC52">3.7.4 Environment</A>) <P> <LI> traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored <P> </UL> <P> A command invoked in this separate environment cannot affect the shell's execution environment. </P><P> Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment. </P><P> If a command is followed by a <SAMP>`&'</SAMP> and job control is not active, the default standard input for the command is the empty file <TT>`/dev/null'</TT>. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections. </P><P> <A NAME="Environment"></A> <HR SIZE="6"> <A NAME="SEC52"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC51"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC53"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC53"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.4 Environment </H3> <!--docid::SEC52::--> <P> When a program is invoked it is given an array of strings called the <VAR>environment</VAR>. This is a list of name-value pairs, of the form <CODE>name=value</CODE>. </P><P> Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for <VAR>export</VAR> to child processes. Executed commands inherit the environment. The <CODE>export</CODE> and <SAMP>`declare -x'</SAMP> commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the <CODE>unset</CODE> and <SAMP>`export -n'</SAMP> commands, plus any additions via the <CODE>export</CODE> and <SAMP>`declare -x'</SAMP> commands. </P><P> The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in <A HREF="bashref.html#SEC24">3.4 Shell Parameters</A>. These assignment statements affect only the environment seen by that command. </P><P> If the <SAMP>`-k'</SAMP> option is set (see section <A HREF="bashref.html#SEC59">4.3 The Set Builtin</A>), then all parameter assignments are placed in the environment for a command, not just those that precede the command name. </P><P> When Bash invokes an external command, the variable <SAMP>`$_'</SAMP> is set to the full path name of the command and passed to that command in its environment. </P><P> <A NAME="Exit Status"></A> <HR SIZE="6"> <A NAME="SEC53"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC52"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC54"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC54"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.5 Exit Status </H3> <!--docid::SEC53::--> <P> For the shell's purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is <VAR>N</VAR>, Bash uses the value 128+<VAR>N</VAR> as the exit status. </P><P> If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126. </P><P> If a command fails because of an error during expansion or redirection, the exit status is greater than zero. </P><P> The exit status is used by the Bash conditional commands (see section <A HREF="bashref.html#SEC21">3.2.4.2 Conditional Constructs</A>) and some of the list constructs (see section <A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A>). </P><P> All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage. </P><P> <A NAME="Signals"></A> <HR SIZE="6"> <A NAME="SEC54"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC53"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC48"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H3> 3.7.6 Signals </H3> <!--docid::SEC54::--> <P> When Bash is interactive, in the absence of any traps, it ignores <CODE>SIGTERM</CODE> (so that <SAMP>`kill 0'</SAMP> does not kill an interactive shell), and <CODE>SIGINT</CODE> is caught and handled (so that the <CODE>wait</CODE> builtin is interruptible). When Bash receives a <CODE>SIGINT</CODE>, it breaks out of any executing loops. In all cases, Bash ignores <CODE>SIGQUIT</CODE>. If job control is in effect (see section <A HREF="bashref.html#SEC87">7. Job Control</A>), Bash ignores <CODE>SIGTTIN</CODE>, <CODE>SIGTTOU</CODE>, and <CODE>SIGTSTP</CODE>. </P><P> Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore <CODE>SIGINT</CODE> and <CODE>SIGQUIT</CODE> in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals <CODE>SIGTTIN</CODE>, <CODE>SIGTTOU</CODE>, and <CODE>SIGTSTP</CODE>. </P><P> The shell exits by default upon receipt of a <CODE>SIGHUP</CODE>. Before exiting, an interactive shell resends the <CODE>SIGHUP</CODE> to all jobs, running or stopped. Stopped jobs are sent <CODE>SIGCONT</CODE> to ensure that they receive the <CODE>SIGHUP</CODE>. To prevent the shell from sending the <CODE>SIGHUP</CODE> signal to a particular job, it should be removed from the jobs table with the <CODE>disown</CODE> builtin (see section <A HREF="bashref.html#SEC89">7.2 Job Control Builtins</A>) or marked to not receive <CODE>SIGHUP</CODE> using <CODE>disown -h</CODE>. </P><P> If the <CODE>huponexit</CODE> shell option has been set with <CODE>shopt</CODE> (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>), Bash sends a <CODE>SIGHUP</CODE> to all jobs when an interactive login shell exits. </P><P> If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the <CODE>wait</CODE> builtin, the reception of a signal for which a trap has been set will cause the <CODE>wait</CODE> builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed. </P><P> <A NAME="Shell Scripts"></A> <HR SIZE="6"> <A NAME="SEC55"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC54"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC5"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 3.8 Shell Scripts </H2> <!--docid::SEC55::--> <P> A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the <SAMP>`-c'</SAMP> nor <SAMP>`-s'</SAMP> option is supplied (see section <A HREF="bashref.html#SEC65">6.1 Invoking Bash</A>), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in <CODE>$PATH</CODE> if not found there. </P><P> When Bash runs a shell script, it sets the special parameter <CODE>0</CODE> to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset. </P><P> A shell script may be made executable by using the <CODE>chmod</CODE> command to turn on the execute bit. When Bash finds such a file while searching the <CODE>$PATH</CODE> for a command, it spawns a subshell to execute it. In other words, executing <TABLE><tr><td> </td><td class=example><pre>filename <VAR>arguments</VAR> </pre></td></tr></table>is equivalent to executing <TABLE><tr><td> </td><td class=example><pre>bash filename <VAR>arguments</VAR> </pre></td></tr></table></P><P> if <CODE>filename</CODE> is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of <CODE>hash</CODE> in <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>) are retained by the child. </P><P> Most versions of Unix make this a part of the operating system's command execution mechanism. If the first line of a script begins with the two characters <SAMP>`#!'</SAMP>, the remainder of the line specifies an interpreter for the program. Thus, you can specify Bash, <CODE>awk</CODE>, Perl, or some other interpreter and write the rest of the script file in that language. </P><P> The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters. </P><P> Bash scripts often begin with <CODE>#! /bin/bash</CODE> (assuming that Bash has been installed in <TT>`/bin'</TT>), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell. </P><P> <A NAME="Shell Builtin Commands"></A> <HR SIZE="6"> <A NAME="SEC56"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC55"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC57"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H1> 4. Shell Builtin Commands </H1> <!--docid::SEC56::--> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Builtin commands inherited from the Bourne Shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Table of builtins specific to Bash.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC59">4.3 The Set Builtin</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">This builtin is so overloaded it deserves its own section.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC60">4.4 Special Builtins</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Builtin commands classified specially by POSIX.2.</TD></TR> </TABLE></BLOCKQUOTE> <P> Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (see section <A HREF="bashref.html#SEC16">3.2.1 Simple Commands</A>), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities. </P><P> This section briefly the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash. </P><P> Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see section <A HREF="bashref.html#SEC89">7.2 Job Control Builtins</A>), the directory stack (see section <A HREF="bashref.html#SEC83">6.8.1 Directory Stack Builtins</A>), the command history (see section <A HREF="bashref.html#SEC117">9.2 Bash History Builtins</A>), and the programmable completion facilities (see section <A HREF="bashref.html#SEC114">8.7 Programmable Completion Builtins</A>). </P><P> Many of the builtins have been extended by POSIX or Bash. </P><P> Unless otherwise noted, each builtin command documented as accepting options preceded by <SAMP>`-'</SAMP> accepts <SAMP>`--'</SAMP> to signify the end of the options. For example, the <CODE>:</CODE>, <CODE>true</CODE>, <CODE>false</CODE>, and <CODE>test</CODE> builtins do not accept options. </P><P> <A NAME="Bourne Shell Builtins"></A> <HR SIZE="6"> <A NAME="SEC57"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC58"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 4.1 Bourne Shell Builtins </H2> <!--docid::SEC57::--> <P> The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard. </P><P> <DL COMPACT> <DT><CODE>: (a colon)</CODE> <DD><A NAME="IDX68"></A> <TABLE><tr><td> </td><td class=example><pre>: [<VAR>arguments</VAR>] </pre></td></tr></table>Do nothing beyond expanding <VAR>arguments</VAR> and performing redirections. The return status is zero. <P> <DT><CODE>. (a period)</CODE> <DD><A NAME="IDX69"></A> <TABLE><tr><td> </td><td class=example><pre>. <VAR>filename</VAR> [<VAR>arguments</VAR>] </pre></td></tr></table>Read and execute commands from the <VAR>filename</VAR> argument in the current shell context. If <VAR>filename</VAR> does not contain a slash, the <CODE>PATH</CODE> variable is used to find <VAR>filename</VAR>. When Bash is not in POSIX mode, the current directory is searched if <VAR>filename</VAR> is not found in <CODE>$PATH</CODE>. If any <VAR>arguments</VAR> are supplied, they become the positional parameters when <VAR>filename</VAR> is executed. Otherwise the positional parameters are unchanged. The return status is the exit status of the last command executed, or zero if no commands are executed. If <VAR>filename</VAR> is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to <CODE>source</CODE>. <P> <DT><CODE>break</CODE> <DD><A NAME="IDX70"></A> <TABLE><tr><td> </td><td class=example><pre>break [<VAR>n</VAR>] </pre></td></tr></table>Exit from a <CODE>for</CODE>, <CODE>while</CODE>, <CODE>until</CODE>, or <CODE>select</CODE> loop. If <VAR>n</VAR> is supplied, the <VAR>n</VAR>th enclosing loop is exited. <VAR>n</VAR> must be greater than or equal to 1. The return status is zero unless <VAR>n</VAR> is not greater than or equal to 1. <P> <DT><CODE>cd</CODE> <DD><A NAME="IDX71"></A> <TABLE><tr><td> </td><td class=example><pre>cd [-L|-P] [<VAR>directory</VAR>] </pre></td></tr></table>Change the current working directory to <VAR>directory</VAR>. If <VAR>directory</VAR> is not given, the value of the <CODE>HOME</CODE> shell variable is used. If the shell variable <CODE>CDPATH</CODE> exists, it is used as a search path. If <VAR>directory</VAR> begins with a slash, <CODE>CDPATH</CODE> is not used. <P> The <SAMP>`-P'</SAMP> option means to not follow symbolic links; symbolic links are followed by default or with the <SAMP>`-L'</SAMP> option. If <VAR>directory</VAR> is <SAMP>`-'</SAMP>, it is equivalent to <CODE>$OLDPWD</CODE>. </P><P> If a non-empty directory name from <CODE>CDPATH</CODE> is used, or if <SAMP>`-'</SAMP> is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. </P><P> The return status is zero if the directory is successfully changed, non-zero otherwise. </P><P> <DT><CODE>continue</CODE> <DD><A NAME="IDX72"></A> <TABLE><tr><td> </td><td class=example><pre>continue [<VAR>n</VAR>] </pre></td></tr></table>Resume the next iteration of an enclosing <CODE>for</CODE>, <CODE>while</CODE>, <CODE>until</CODE>, or <CODE>select</CODE> loop. If <VAR>n</VAR> is supplied, the execution of the <VAR>n</VAR>th enclosing loop is resumed. <VAR>n</VAR> must be greater than or equal to 1. The return status is zero unless <VAR>n</VAR> is not greater than or equal to 1. <P> <DT><CODE>eval</CODE> <DD><A NAME="IDX73"></A> <TABLE><tr><td> </td><td class=example><pre>eval [<VAR>arguments</VAR>] </pre></td></tr></table>The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of <CODE>eval</CODE>. If there are no arguments or only empty arguments, the return status is zero. <P> <DT><CODE>exec</CODE> <DD><A NAME="IDX74"></A> <TABLE><tr><td> </td><td class=example><pre>exec [-cl] [-a <VAR>name</VAR>] [<VAR>command</VAR> [<VAR>arguments</VAR>]] </pre></td></tr></table>If <VAR>command</VAR> is supplied, it replaces the shell without creating a new process. If the <SAMP>`-l'</SAMP> option is supplied, the shell places a dash at the beginning of the zeroth arg passed to <VAR>command</VAR>. This is what the <CODE>login</CODE> program does. The <SAMP>`-c'</SAMP> option causes <VAR>command</VAR> to be executed with an empty environment. If <SAMP>`-a'</SAMP> is supplied, the shell passes <VAR>name</VAR> as the zeroth argument to <VAR>command</VAR>. If no <VAR>command</VAR> is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero. <P> <DT><CODE>exit</CODE> <DD><A NAME="IDX75"></A> <TABLE><tr><td> </td><td class=example><pre>exit [<VAR>n</VAR>] </pre></td></tr></table>Exit the shell, returning a status of <VAR>n</VAR> to the shell's parent. If <VAR>n</VAR> is omitted, the exit status is that of the last command executed. Any trap on <CODE>EXIT</CODE> is executed before the shell terminates. <P> <DT><CODE>export</CODE> <DD><A NAME="IDX76"></A> <TABLE><tr><td> </td><td class=example><pre>export [-fn] [-p] [<VAR>name</VAR>[=<VAR>value</VAR>]] </pre></td></tr></table>Mark each <VAR>name</VAR> to be passed to child processes in the environment. If the <SAMP>`-f'</SAMP> option is supplied, the <VAR>name</VAR>s refer to shell functions; otherwise the names refer to shell variables. The <SAMP>`-n'</SAMP> option means to no longer mark each <VAR>name</VAR> for export. If no <VAR>names</VAR> are supplied, or if the <SAMP>`-p'</SAMP> option is given, a list of exported names is displayed. The <SAMP>`-p'</SAMP> option displays output in a form that may be reused as input. If a variable name is followed by =<VAR>value</VAR>, the value of the variable is set to <VAR>value</VAR>. <P> The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or <SAMP>`-f'</SAMP> is supplied with a name that is not a shell function. </P><P> <DT><CODE>getopts</CODE> <DD><A NAME="IDX77"></A> <TABLE><tr><td> </td><td class=example><pre>getopts <VAR>optstring</VAR> <VAR>name</VAR> [<VAR>args</VAR>] </pre></td></tr></table><CODE>getopts</CODE> is used by shell scripts to parse positional parameters. <VAR>optstring</VAR> contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. The colon (<SAMP>`:'</SAMP>) and question mark (<SAMP>`?'</SAMP>) may not be used as option characters. Each time it is invoked, <CODE>getopts</CODE> places the next option in the shell variable <VAR>name</VAR>, initializing <VAR>name</VAR> if it does not exist, and the index of the next argument to be processed into the variable <CODE>OPTIND</CODE>. <CODE>OPTIND</CODE> is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, <CODE>getopts</CODE> places that argument into the variable <CODE>OPTARG</CODE>. The shell does not reset <CODE>OPTIND</CODE> automatically; it must be manually reset between multiple calls to <CODE>getopts</CODE> within the same shell invocation if a new set of parameters is to be used. <P> When the end of options is encountered, <CODE>getopts</CODE> exits with a return value greater than zero. <CODE>OPTIND</CODE> is set to the index of the first non-option argument, and <CODE>name</CODE> is set to <SAMP>`?'</SAMP>. </P><P> <CODE>getopts</CODE> normally parses the positional parameters, but if more arguments are given in <VAR>args</VAR>, <CODE>getopts</CODE> parses those instead. </P><P> <CODE>getopts</CODE> can report errors in two ways. If the first character of <VAR>optstring</VAR> is a colon, <VAR>silent</VAR> error reporting is used. In normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable <CODE>OPTERR</CODE> is set to 0, no error messages will be displayed, even if the first character of <CODE>optstring</CODE> is not a colon. </P><P> If an invalid option is seen, <CODE>getopts</CODE> places <SAMP>`?'</SAMP> into <VAR>name</VAR> and, if not silent, prints an error message and unsets <CODE>OPTARG</CODE>. If <CODE>getopts</CODE> is silent, the option character found is placed in <CODE>OPTARG</CODE> and no diagnostic message is printed. </P><P> If a required argument is not found, and <CODE>getopts</CODE> is not silent, a question mark (<SAMP>`?'</SAMP>) is placed in <VAR>name</VAR>, <CODE>OPTARG</CODE> is unset, and a diagnostic message is printed. If <CODE>getopts</CODE> is silent, then a colon (<SAMP>`:'</SAMP>) is placed in <VAR>name</VAR> and <CODE>OPTARG</CODE> is set to the option character found. </P><P> <DT><CODE>hash</CODE> <DD><A NAME="IDX78"></A> <TABLE><tr><td> </td><td class=example><pre>hash [-'r] [-p <VAR>filename</VAR>] [-dt] [<VAR>name</VAR>] </pre></td></tr></table>Remember the full pathnames of commands specified as <VAR>name</VAR> arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in <CODE>$PATH</CODE>. The <SAMP>`-p'</SAMP> option inhibits the path search, and <VAR>filename</VAR> is used as the location of <VAR>name</VAR>. The <SAMP>`-r'</SAMP> option causes the shell to forget all remembered locations. The <SAMP>`-d'</SAMP> option causes the shell to forget the remembered location of each <VAR>name</VAR>. If the <SAMP>`-t'</SAMP> option is supplied, the full pathname to which each <VAR>name</VAR> corresponds is printed. If multiple <VAR>name</VAR> arguments are supplied with <SAMP>`-t'</SAMP> the <VAR>name</VAR> is printed before the hashed full pathname. The <SAMP>`-l'</SAMP> option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only <SAMP>`-l'</SAMP> is supplied, information about remembered commands is printed. The return status is zero unless a <VAR>name</VAR> is not found or an invalid option is supplied. <P> <DT><CODE>pwd</CODE> <DD><A NAME="IDX79"></A> <TABLE><tr><td> </td><td class=example><pre>pwd [-LP] </pre></td></tr></table>Print the absolute pathname of the current working directory. If the <SAMP>`-P'</SAMP> option is supplied, the pathname printed will not contain symbolic links. If the <SAMP>`-L'</SAMP> option is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied. <P> <DT><CODE>readonly</CODE> <DD><A NAME="IDX80"></A> <TABLE><tr><td> </td><td class=example><pre>readonly [-apf] [<VAR>name</VAR>[=<VAR>value</VAR>]] <small>...</small> </pre></td></tr></table>Mark each <VAR>name</VAR> as readonly. The values of these names may not be changed by subsequent assignment. If the <SAMP>`-f'</SAMP> option is supplied, each <VAR>name</VAR> refers to a shell function. The <SAMP>`-a'</SAMP> option means each <VAR>name</VAR> refers to an array variable. If no <VAR>name</VAR> arguments are given, or if the <SAMP>`-p'</SAMP> option is supplied, a list of all readonly names is printed. The <SAMP>`-p'</SAMP> option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =<VAR>value</VAR>, the value of the variable is set to <VAR>value</VAR>. The return status is zero unless an invalid option is supplied, one of the <VAR>name</VAR> arguments is not a valid shell variable or function name, or the <SAMP>`-f'</SAMP> option is supplied with a name that is not a shell function. <P> <DT><CODE>return</CODE> <DD><A NAME="IDX81"></A> <TABLE><tr><td> </td><td class=example><pre>return [<VAR>n</VAR>] </pre></td></tr></table>Cause a shell function to exit with the return value <VAR>n</VAR>. If <VAR>n</VAR> is not supplied, the return value is the exit status of the last command executed in the function. This may also be used to terminate execution of a script being executed with the <CODE>.</CODE> (or <CODE>source</CODE>) builtin, returning either <VAR>n</VAR> or the exit status of the last command executed within the script as the exit status of the script. Any command associated with the <CODE>RETURN</CODE> trap is executed before execution resumes after the function or script. The return status is non-zero if <CODE>return</CODE> is used outside a function and not during the execution of a script by <CODE>.</CODE> or <CODE>source</CODE>. <P> <DT><CODE>shift</CODE> <DD><A NAME="IDX82"></A> <TABLE><tr><td> </td><td class=example><pre>shift [<VAR>n</VAR>] </pre></td></tr></table>Shift the positional parameters to the left by <VAR>n</VAR>. The positional parameters from <VAR>n</VAR>+1 <small>...</small> <CODE>$#</CODE> are renamed to <CODE>$1</CODE> <small>...</small> <CODE>$#</CODE>-<VAR>n</VAR>+1. Parameters represented by the numbers <CODE>$#</CODE> to <VAR>n</VAR>+1 are unset. <VAR>n</VAR> must be a non-negative number less than or equal to <CODE>$#</CODE>. If <VAR>n</VAR> is zero or greater than <CODE>$#</CODE>, the positional parameters are not changed. If <VAR>n</VAR> is not supplied, it is assumed to be 1. The return status is zero unless <VAR>n</VAR> is greater than <CODE>$#</CODE> or less than zero, non-zero otherwise. <P> <DT><CODE>test</CODE> <DD><DT><CODE>[</CODE> <DD><A NAME="IDX83"></A> <A NAME="IDX84"></A> Evaluate a conditional expression <VAR>expr</VAR>. Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in <A HREF="bashref.html#SEC78">6.4 Bash Conditional Expressions</A>. <CODE>test</CODE> does not accept any options, nor does it accept and ignore an argument of <SAMP>`--'</SAMP> as signifying the end of options. <P> When the <CODE>[</CODE> form is used, the last argument to the command must be a <CODE>]</CODE>. </P><P> Expressions may be combined using the following operators, listed in decreasing order of precedence. </P><P> <DL COMPACT> <DT><CODE>! <VAR>expr</VAR></CODE> <DD>True if <VAR>expr</VAR> is false. <P> <DT><CODE>( <VAR>expr</VAR> )</CODE> <DD>Returns the value of <VAR>expr</VAR>. This may be used to override the normal precedence of operators. <P> <DT><CODE><VAR>expr1</VAR> -a <VAR>expr2</VAR></CODE> <DD>True if both <VAR>expr1</VAR> and <VAR>expr2</VAR> are true. <P> <DT><CODE><VAR>expr1</VAR> -o <VAR>expr2</VAR></CODE> <DD>True if either <VAR>expr1</VAR> or <VAR>expr2</VAR> is true. </DL> <P> The <CODE>test</CODE> and <CODE>[</CODE> builtins evaluate conditional expressions using a set of rules based on the number of arguments. </P><P> <DL COMPACT> <DT>0 arguments <DD>The expression is false. <P> <DT>1 argument <DD>The expression is true if and only if the argument is not null. <P> <DT>2 arguments <DD>If the first argument is <SAMP>`!'</SAMP>, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see section <A HREF="bashref.html#SEC78">6.4 Bash Conditional Expressions</A>), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false. <P> <DT>3 arguments <DD>If the second argument is one of the binary conditional operators (see section <A HREF="bashref.html#SEC78">6.4 Bash Conditional Expressions</A>), the result of the expression is the result of the binary test using the first and third arguments as operands. If the first argument is <SAMP>`!'</SAMP>, the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly <SAMP>`('</SAMP> and the third argument is exactly <SAMP>`)'</SAMP>, the result is the one-argument test of the second argument. Otherwise, the expression is false. The <SAMP>`-a'</SAMP> and <SAMP>`-o'</SAMP> operators are considered binary operators in this case. <P> <DT>4 arguments <DD>If the first argument is <SAMP>`!'</SAMP>, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above. <P> <DT>5 or more arguments <DD>The expression is parsed and evaluated according to precedence using the rules listed above. </DL> <P> <DT><CODE>times</CODE> <DD><A NAME="IDX85"></A> <TABLE><tr><td> </td><td class=example><pre>times </pre></td></tr></table>Print out the user and system times used by the shell and its children. The return status is zero. <P> <DT><CODE>trap</CODE> <DD><A NAME="IDX86"></A> <TABLE><tr><td> </td><td class=example><pre>trap [-lp] [<VAR>arg</VAR>] [<VAR>sigspec</VAR> <small>...</small>] </pre></td></tr></table>The commands in <VAR>arg</VAR> are to be read and executed when the shell receives signal <VAR>sigspec</VAR>. If <VAR>arg</VAR> is absent (and there is a single <VAR>sigspec</VAR>) or equal to <SAMP>`-'</SAMP>, each specified signal's disposition is reset to the value it had when the shell was started. If <VAR>arg</VAR> is the null string, then the signal specified by each <VAR>sigspec</VAR> is ignored by the shell and commands it invokes. If <VAR>arg</VAR> is not present and <SAMP>`-p'</SAMP> has been supplied, the shell displays the trap commands associated with each <VAR>sigspec</VAR>. If no arguments are supplied, or only <SAMP>`-p'</SAMP> is given, <CODE>trap</CODE> prints the list of commands associated with each signal number in a form that may be reused as shell input. The <SAMP>`-l'</SAMP> option causes the shell to print a list of signal names and their corresponding numbers. Each <VAR>sigspec</VAR> is either a signal name or a signal number. Signal names are case insensitive and the <CODE>SIG</CODE> prefix is optional. If a <VAR>sigspec</VAR> is <CODE>0</CODE> or <CODE>EXIT</CODE>, <VAR>arg</VAR> is executed when the shell exits. If a <VAR>sigspec</VAR> is <CODE>DEBUG</CODE>, the command <VAR>arg</VAR> is executed before every simple command, <CODE>for</CODE> command, <CODE>case</CODE> command, <CODE>select</CODE> command, every arithmetic <CODE>for</CODE> command, and before the first command executes in a shell function. Refer to the description of the <CODE>extglob</CODE> option to the <CODE>shopt</CODE> builtin (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>) for details of its effect on the <CODE>DEBUG</CODE> trap. If a <VAR>sigspec</VAR> is <CODE>ERR</CODE>, the command <VAR>arg</VAR> is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The <CODE>ERR</CODE> trap is not executed if the failed command is part of the command list immediately following an <CODE>until</CODE> or <CODE>while</CODE> keyword, part of the test in an <CODE>if</CODE> statement, part of a <CODE>&&</CODE> or <CODE>||</CODE> list, or if the command's return status is being inverted using <CODE>!</CODE>. These are the same conditions obeyed by the <CODE>errexit</CODE> option. If a <VAR>sigspec</VAR> is <CODE>RETURN</CODE>, the command <VAR>arg</VAR> is executed each time a shell function or a script executed with the <CODE>.</CODE> or <CODE>source</CODE> builtins finishes executing. <P> Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created. </P><P> The return status is zero unless a <VAR>sigspec</VAR> does not specify a valid signal. </P><P> <DT><CODE>umask</CODE> <DD><A NAME="IDX87"></A> <TABLE><tr><td> </td><td class=example><pre>umask [-p] [-S] [<VAR>mode</VAR>] </pre></td></tr></table>Set the shell process's file creation mask to <VAR>mode</VAR>. If <VAR>mode</VAR> begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the <CODE>chmod</CODE> command. If <VAR>mode</VAR> is omitted, the current value of the mask is printed. If the <SAMP>`-S'</SAMP> option is supplied without a <VAR>mode</VAR> argument, the mask is printed in a symbolic format. If the <SAMP>`-p'</SAMP> option is supplied, and <VAR>mode</VAR> is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no <VAR>mode</VAR> argument is supplied, and non-zero otherwise. <P> Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from <CODE>7</CODE>. Thus, a umask of <CODE>022</CODE> results in permissions of <CODE>755</CODE>. </P><P> <DT><CODE>unset</CODE> <DD><A NAME="IDX88"></A> <TABLE><tr><td> </td><td class=example><pre>unset [-fv] [<VAR>name</VAR>] </pre></td></tr></table>Each variable or function <VAR>name</VAR> is removed. If no options are supplied, or the <SAMP>`-v'</SAMP> option is given, each <VAR>name</VAR> refers to a shell variable. If the <SAMP>`-f'</SAMP> option is given, the <VAR>name</VAR>s refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a <VAR>name</VAR> is readonly. </DL> <P> <A NAME="Bash Builtins"></A> <HR SIZE="6"> <A NAME="SEC58"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC57"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC59"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC59"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 4.2 Bash Builtin Commands </H2> <!--docid::SEC58::--> <P> This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX 1003.2 standard. </P><P> <DL COMPACT> <DT><CODE>alias</CODE> <DD><A NAME="IDX89"></A> <TABLE><tr><td> </td><td class=example><pre>alias [<CODE>-p</CODE>] [<VAR>name</VAR>[=<VAR>value</VAR>] <small>...</small>] </pre></td></tr></table><P> Without arguments or with the <SAMP>`-p'</SAMP> option, <CODE>alias</CODE> prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, an alias is defined for each <VAR>name</VAR> whose <VAR>value</VAR> is given. If no <VAR>value</VAR> is given, the name and value of the alias is printed. Aliases are described in <A HREF="bashref.html#SEC80">6.6 Aliases</A>. </P><P> <DT><CODE>bind</CODE> <DD><A NAME="IDX90"></A> <TABLE><tr><td> </td><td class=example><pre>bind [-m <VAR>keymap</VAR>] [-lpsvPSV] bind [-m <VAR>keymap</VAR>] [-q <VAR>function</VAR>] [-u <VAR>function</VAR>] [-r <VAR>keyseq</VAR>] bind [-m <VAR>keymap</VAR>] -f <VAR>filename</VAR> bind [-m <VAR>keymap</VAR>] -x <VAR>keyseq:shell-command</VAR> bind [-m <VAR>keymap</VAR>] <VAR>keyseq:function-name</VAR> bind <VAR>readline-command</VAR> </pre></td></tr></table><P> Display current Readline (see section <A HREF="bashref.html#SEC91">8. Command Line Editing</A>) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a a Readline initialization file (see section <A HREF="bashref.html#SEC99">8.3 Readline Init File</A>), but each binding or command must be passed as a separate argument; e.g., <SAMP>`"\C-x\C-r":re-read-init-file'</SAMP>. Options, if supplied, have the following meanings: </P><P> <DL COMPACT> <DT><CODE>-m <VAR>keymap</VAR></CODE> <DD>Use <VAR>keymap</VAR> as the keymap to be affected by the subsequent bindings. Acceptable <VAR>keymap</VAR> names are <CODE>emacs</CODE>, <CODE>emacs-standard</CODE>, <CODE>emacs-meta</CODE>, <CODE>emacs-ctlx</CODE>, <CODE>vi</CODE>, <CODE>vi-move</CODE>, <CODE>vi-command</CODE>, and <CODE>vi-insert</CODE>. <CODE>vi</CODE> is equivalent to <CODE>vi-command</CODE>; <CODE>emacs</CODE> is equivalent to <CODE>emacs-standard</CODE>. <P> <DT><CODE>-l</CODE> <DD>List the names of all Readline functions. <P> <DT><CODE>-p</CODE> <DD>Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file. <P> <DT><CODE>-P</CODE> <DD>List current Readline function names and bindings. <P> <DT><CODE>-v</CODE> <DD>Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file. <P> <DT><CODE>-V</CODE> <DD>List current Readline variable names and values. <P> <DT><CODE>-s</CODE> <DD>Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file. <P> <DT><CODE>-S</CODE> <DD>Display Readline key sequences bound to macros and the strings they output. <P> <DT><CODE>-f <VAR>filename</VAR></CODE> <DD>Read key bindings from <VAR>filename</VAR>. <P> <DT><CODE>-q <VAR>function</VAR></CODE> <DD>Query about which keys invoke the named <VAR>function</VAR>. <P> <DT><CODE>-u <VAR>function</VAR></CODE> <DD>Unbind all keys bound to the named <VAR>function</VAR>. <P> <DT><CODE>-r <VAR>keyseq</VAR></CODE> <DD>Remove any current binding for <VAR>keyseq</VAR>. <P> <DT><CODE>-x <VAR>keyseq:shell-command</VAR></CODE> <DD>Cause <VAR>shell-command</VAR> to be executed whenever <VAR>keyseq</VAR> is entered. <P> </DL> <P> The return status is zero unless an invalid option is supplied or an error occurs. </P><P> <DT><CODE>builtin</CODE> <DD><A NAME="IDX91"></A> <TABLE><tr><td> </td><td class=example><pre>builtin [<VAR>shell-builtin</VAR> [<VAR>args</VAR>]] </pre></td></tr></table>Run a shell builtin, passing it <VAR>args</VAR>, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if <VAR>shell-builtin</VAR> is not a shell builtin command. <P> <DT><CODE>caller</CODE> <DD><A NAME="IDX92"></A> <TABLE><tr><td> </td><td class=example><pre>caller [<VAR>expr</VAR>] </pre></td></tr></table>Returns the context of any active subroutine call (a shell function or a script executed with the <CODE>.</CODE> or <CODE>source</CODE> builtins). <P> Without <VAR>expr</VAR>, <CODE>caller</CODE> displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as <VAR>expr</VAR>, <CODE>caller</CODE> displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0. </P><P> The return value is 0 unless the shell is not executing a subroutine call or <VAR>expr</VAR> does not correspond to a valid position in the call stack. </P><P> <DT><CODE>command</CODE> <DD><A NAME="IDX93"></A> <TABLE><tr><td> </td><td class=example><pre>command [-pVv] <VAR>command</VAR> [<VAR>arguments</VAR> <small>...</small>] </pre></td></tr></table>Runs <VAR>command</VAR> with <VAR>arguments</VAR> ignoring any shell function named <VAR>command</VAR>. Only shell builtin commands or commands found by searching the <CODE>PATH</CODE> are executed. If there is a shell function named <CODE>ls</CODE>, running <SAMP>`command ls'</SAMP> within the function will execute the external command <CODE>ls</CODE> instead of calling the function recursively. The <SAMP>`-p'</SAMP> option means to use a default value for <CODE>PATH</CODE> that is guaranteed to find all of the standard utilities. The return status in this case is 127 if <VAR>command</VAR> cannot be found or an error occurred, and the exit status of <VAR>command</VAR> otherwise. <P> If either the <SAMP>`-V'</SAMP> or <SAMP>`-v'</SAMP> option is supplied, a description of <VAR>command</VAR> is printed. The <SAMP>`-v'</SAMP> option causes a single word indicating the command or file name used to invoke <VAR>command</VAR> to be displayed; the <SAMP>`-V'</SAMP> option produces a more verbose description. In this case, the return status is zero if <VAR>command</VAR> is found, and non-zero if not. </P><P> <DT><CODE>declare</CODE> <DD><A NAME="IDX94"></A> <TABLE><tr><td> </td><td class=example><pre>declare [-afFirtx] [-p] [<VAR>name</VAR>[=<VAR>value</VAR>] <small>...</small>] </pre></td></tr></table><P> Declare variables and give them attributes. If no <VAR>name</VAR>s are given, then display the values of variables instead. </P><P> The <SAMP>`-p'</SAMP> option will display the attributes and values of each <VAR>name</VAR>. When <SAMP>`-p'</SAMP> is used, additional options are ignored. The <SAMP>`-F'</SAMP> option inhibits the display of function definitions; only the function name and attributes are printed. If the <CODE>extdebug</CODE> shell option is enabled using <CODE>shopt</CODE> (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>), the source file name and line number where the function is defined are displayed as well. <SAMP>`-F'</SAMP> implies <SAMP>`-f'</SAMP>. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes: </P><P> <DL COMPACT> <DT><CODE>-a</CODE> <DD>Each <VAR>name</VAR> is an array variable (see section <A HREF="bashref.html#SEC81">6.7 Arrays</A>). <P> <DT><CODE>-f</CODE> <DD>Use function names only. <P> <DT><CODE>-i</CODE> <DD>The variable is to be treated as an integer; arithmetic evaluation (see section <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>) is performed when the variable is assigned a value. <P> <DT><CODE>-r</CODE> <DD>Make <VAR>name</VAR>s readonly. These names cannot then be assigned values by subsequent assignment statements or unset. <P> <DT><CODE>-t</CODE> <DD>Give each <VAR>name</VAR> the <CODE>trace</CODE> attribute. Traced functions inherit the <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> traps from the calling shell. The trace attribute has no special meaning for variables. <P> <DT><CODE>-x</CODE> <DD>Mark each <VAR>name</VAR> for export to subsequent commands via the environment. </DL> <P> Using <SAMP>`+'</SAMP> instead of <SAMP>`-'</SAMP> turns off the attribute instead. When used in a function, <CODE>declare</CODE> makes each <VAR>name</VAR> local, as with the <CODE>local</CODE> command. If a variable name is followed by =<VAR>value</VAR>, the value of the variable is set to <VAR>value</VAR>. </P><P> The return status is zero unless an invalid option is encountered, an attempt is made to define a function using <SAMP>`-f foo=bar'</SAMP>, an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see section <A HREF="bashref.html#SEC81">6.7 Arrays</A>), one of the <VAR>names</VAR> is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with <SAMP>`-f'</SAMP>. </P><P> <DT><CODE>echo</CODE> <DD><A NAME="IDX95"></A> <TABLE><tr><td> </td><td class=example><pre>echo [-neE] [<VAR>arg</VAR> <small>...</small>] </pre></td></tr></table>Output the <VAR>arg</VAR>s, separated by spaces, terminated with a newline. The return status is always 0. If <SAMP>`-n'</SAMP> is specified, the trailing newline is suppressed. If the <SAMP>`-e'</SAMP> option is given, interpretation of the following backslash-escaped characters is enabled. The <SAMP>`-E'</SAMP> option disables the interpretation of these escape characters, even on systems where they are interpreted by default. The <CODE>xpg_echo</CODE> shell option may be used to dynamically determine whether or not <CODE>echo</CODE> expands these escape characters by default. <CODE>echo</CODE> does not interpret <SAMP>`--'</SAMP> to mean the end of options. <P> <CODE>echo</CODE> interprets the following escape sequences: <DL COMPACT> <DT><CODE>\a</CODE> <DD>alert (bell) <DT><CODE>\b</CODE> <DD>backspace <DT><CODE>\c</CODE> <DD>suppress trailing newline <DT><CODE>\e</CODE> <DD>escape <DT><CODE>\f</CODE> <DD>form feed <DT><CODE>\n</CODE> <DD>new line <DT><CODE>\r</CODE> <DD>carriage return <DT><CODE>\t</CODE> <DD>horizontal tab <DT><CODE>\v</CODE> <DD>vertical tab <DT><CODE>\\</CODE> <DD>backslash <DT><CODE>\0<VAR>nnn</VAR></CODE> <DD>the eight-bit character whose value is the octal value <VAR>nnn</VAR> (zero to three octal digits) <DT><CODE>\<VAR>nnn</VAR></CODE> <DD>the eight-bit character whose value is the octal value <VAR>nnn</VAR> (one to three octal digits) <DT><CODE>\x<VAR>HH</VAR></CODE> <DD>the eight-bit character whose value is the hexadecimal value <VAR>HH</VAR> (one or two hex digits) </DL> <P> <DT><CODE>enable</CODE> <DD><A NAME="IDX96"></A> <TABLE><tr><td> </td><td class=example><pre>enable [-n] [-p] [-f <VAR>filename</VAR>] [-ads] [<VAR>name</VAR> <small>...</small>] </pre></td></tr></table>Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If <SAMP>`-n'</SAMP> is used, the <VAR>name</VAR>s become disabled. Otherwise <VAR>name</VAR>s are enabled. For example, to use the <CODE>test</CODE> binary found via <CODE>$PATH</CODE> instead of the shell builtin version, type <SAMP>`enable -n test'</SAMP>. <P> If the <SAMP>`-p'</SAMP> option is supplied, or no <VAR>name</VAR> arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The <SAMP>`-a'</SAMP> option means to list each builtin with an indication of whether or not it is enabled. </P><P> The <SAMP>`-f'</SAMP> option means to load the new builtin command <VAR>name</VAR> from shared object <VAR>filename</VAR>, on systems that support dynamic loading. The <SAMP>`-d'</SAMP> option will delete a builtin loaded with <SAMP>`-f'</SAMP>. </P><P> If there are no options, a list of the shell builtins is displayed. The <SAMP>`-s'</SAMP> option restricts <CODE>enable</CODE> to the POSIX special builtins. If <SAMP>`-s'</SAMP> is used with <SAMP>`-f'</SAMP>, the new builtin becomes a special builtin (see section <A HREF="bashref.html#SEC60">4.4 Special Builtins</A>). </P><P> The return status is zero unless a <VAR>name</VAR> is not a shell builtin or there is an error loading a new builtin from a shared object. </P><P> <DT><CODE>help</CODE> <DD><A NAME="IDX97"></A> <TABLE><tr><td> </td><td class=example><pre>help [-s] [<VAR>pattern</VAR>] </pre></td></tr></table>Display helpful information about builtin commands. If <VAR>pattern</VAR> is specified, <CODE>help</CODE> gives detailed help on all commands matching <VAR>pattern</VAR>, otherwise a list of the builtins is printed. The <SAMP>`-s'</SAMP> option restricts the information displayed to a short usage synopsis. The return status is zero unless no command matches <VAR>pattern</VAR>. <P> <DT><CODE>let</CODE> <DD><A NAME="IDX98"></A> <TABLE><tr><td> </td><td class=example><pre>let <VAR>expression</VAR> [<VAR>expression</VAR>] </pre></td></tr></table>The <CODE>let</CODE> builtin allows arithmetic to be performed on shell variables. Each <VAR>expression</VAR> is evaluated according to the rules given below in <A HREF="bashref.html#SEC79">6.5 Shell Arithmetic</A>. If the last <VAR>expression</VAR> evaluates to 0, <CODE>let</CODE> returns 1; otherwise 0 is returned. <P> <DT><CODE>local</CODE> <DD><A NAME="IDX99"></A> <TABLE><tr><td> </td><td class=example><pre>local [<VAR>option</VAR>] <VAR>name</VAR>[=<VAR>value</VAR>] <small>...</small> </pre></td></tr></table>For each argument, a local variable named <VAR>name</VAR> is created, and assigned <VAR>value</VAR>. The <VAR>option</VAR> can be any of the options accepted by <CODE>declare</CODE>. <CODE>local</CODE> can only be used within a function; it makes the variable <VAR>name</VAR> have a visible scope restricted to that function and its children. The return status is zero unless <CODE>local</CODE> is used outside a function, an invalid <VAR>name</VAR> is supplied, or <VAR>name</VAR> is a readonly variable. <P> <DT><CODE>logout</CODE> <DD><A NAME="IDX100"></A> <TABLE><tr><td> </td><td class=example><pre>logout [<VAR>n</VAR>] </pre></td></tr></table>Exit a login shell, returning a status of <VAR>n</VAR> to the shell's parent. <P> <DT><CODE>printf</CODE> <DD><A NAME="IDX101"></A> <TABLE><tr><td> </td><td class=example><pre><CODE>printf</CODE> [-v <VAR>var</VAR>] <VAR>format</VAR> [<VAR>arguments</VAR>] </pre></td></tr></table>Write the formatted <VAR>arguments</VAR> to the standard output under the control of the <VAR>format</VAR>. The <VAR>format</VAR> is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive <VAR>argument</VAR>. In addition to the standard <CODE>printf(1)</CODE> formats, <SAMP>`%b'</SAMP> causes <CODE>printf</CODE> to expand backslash escape sequences in the corresponding <VAR>argument</VAR>, (except that <SAMP>`\c'</SAMP> terminates output, backslashes in <SAMP>`\''</SAMP>, <SAMP>`\"'</SAMP>, and <SAMP>`\?'</SAMP> are not removed, and octal escapes beginning with <SAMP>`\0'</SAMP> may contain up to four digits), and <SAMP>`%q'</SAMP> causes <CODE>printf</CODE> to output the corresponding <VAR>argument</VAR> in a format that can be reused as shell input. <P> The <SAMP>`-v'</SAMP> option causes the output to be assigned to the variable <VAR>var</VAR> rather than being printed to the standard output. </P><P> The <VAR>format</VAR> is reused as necessary to consume all of the <VAR>arguments</VAR>. If the <VAR>format</VAR> requires more <VAR>arguments</VAR> than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure. </P><P> <DT><CODE>read</CODE> <DD><A NAME="IDX102"></A> <TABLE><tr><td> </td><td class=example><pre>read [-ers] [-a <VAR>aname</VAR>] [-d <VAR>delim</VAR>] [-n <VAR>nchars</VAR>] [-p <VAR>prompt</VAR>] [-t <VAR>timeout</VAR>] [-u <VAR>fd</VAR>] [<VAR>name</VAR> <small>...</small>] </pre></td></tr></table>One line is read from the standard input, or from the file descriptor <VAR>fd</VAR> supplied as an argument to the <SAMP>`-u'</SAMP> option, and the first word is assigned to the first <VAR>name</VAR>, the second word to the second <VAR>name</VAR>, and so on, with leftover words and their intervening separators assigned to the last <VAR>name</VAR>. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the <CODE>IFS</CODE> variable are used to split the line into words. The backslash character <SAMP>`\'</SAMP> may be used to remove any special meaning for the next character read and for line continuation. If no names are supplied, the line read is assigned to the variable <CODE>REPLY</CODE>. The return code is zero, unless end-of-file is encountered, <CODE>read</CODE> times out, or an invalid file descriptor is supplied as the argument to <SAMP>`-u'</SAMP>. Options, if supplied, have the following meanings: <P> <DL COMPACT> <DT><CODE>-a <VAR>aname</VAR></CODE> <DD>The words are assigned to sequential indices of the array variable <VAR>aname</VAR>, starting at 0. All elements are removed from <VAR>aname</VAR> before the assignment. Other <VAR>name</VAR> arguments are ignored. <P> <DT><CODE>-d <VAR>delim</VAR></CODE> <DD>The first character of <VAR>delim</VAR> is used to terminate the input line, rather than newline. <P> <DT><CODE>-e</CODE> <DD>Readline (see section <A HREF="bashref.html#SEC91">8. Command Line Editing</A>) is used to obtain the line. <P> <DT><CODE>-n <VAR>nchars</VAR></CODE> <DD><CODE>read</CODE> returns after reading <VAR>nchars</VAR> characters rather than waiting for a complete line of input. <P> <DT><CODE>-p <VAR>prompt</VAR></CODE> <DD>Display <VAR>prompt</VAR>, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal. <P> <DT><CODE>-r</CODE> <DD>If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not be used as a line continuation. <P> <DT><CODE>-s</CODE> <DD>Silent mode. If input is coming from a terminal, characters are not echoed. <P> <DT><CODE>-t <VAR>timeout</VAR></CODE> <DD>Cause <CODE>read</CODE> to time out and return failure if a complete line of input is not read within <VAR>timeout</VAR> seconds. This option has no effect if <CODE>read</CODE> is not reading input from the terminal or a pipe. <P> <DT><CODE>-u <VAR>fd</VAR></CODE> <DD>Read input from file descriptor <VAR>fd</VAR>. <P> </DL> <P> <DT><CODE>shopt</CODE> <DD><A NAME="IDX103"></A> <TABLE><tr><td> </td><td class=example><pre>shopt [-pqsu] [-o] [<VAR>optname</VAR> <small>...</small>] </pre></td></tr></table>Toggle the values of variables controlling optional shell behavior. With no options, or with the <SAMP>`-p'</SAMP> option, a list of all settable options is displayed, with an indication of whether or not each is set. The <SAMP>`-p'</SAMP> option causes output to be displayed in a form that may be reused as input. Other options have the following meanings: <P> <DL COMPACT> <DT><CODE>-s</CODE> <DD>Enable (set) each <VAR>optname</VAR>. <P> <DT><CODE>-u</CODE> <DD>Disable (unset) each <VAR>optname</VAR>. <P> <DT><CODE>-q</CODE> <DD>Suppresses normal output; the return status indicates whether the <VAR>optname</VAR> is set or unset. If multiple <VAR>optname</VAR> arguments are given with <SAMP>`-q'</SAMP>, the return status is zero if all <VAR>optnames</VAR> are enabled; non-zero otherwise. <P> <DT><CODE>-o</CODE> <DD>Restricts the values of <VAR>optname</VAR> to be those defined for the <SAMP>`-o'</SAMP> option to the <CODE>set</CODE> builtin (see section <A HREF="bashref.html#SEC59">4.3 The Set Builtin</A>). </DL> <P> If either <SAMP>`-s'</SAMP> or <SAMP>`-u'</SAMP> is used with no <VAR>optname</VAR> arguments, the display is limited to those options which are set or unset, respectively. </P><P> Unless otherwise noted, the <CODE>shopt</CODE> options are disabled (off) by default. </P><P> The return status when listing options is zero if all <VAR>optnames</VAR> are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an <VAR>optname</VAR> is not a valid shell option. </P><P> The list of <CODE>shopt</CODE> options is: <DL COMPACT> <DT><CODE>cdable_vars</CODE> <DD>If this is set, an argument to the <CODE>cd</CODE> builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to. <P> <DT><CODE>cdspell</CODE> <DD>If set, minor errors in the spelling of a directory component in a <CODE>cd</CODE> command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells. <P> <DT><CODE>checkhash</CODE> <DD>If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed. <P> <DT><CODE>checkwinsize</CODE> <DD>If set, Bash checks the window size after each command and, if necessary, updates the values of <CODE>LINES</CODE> and <CODE>COLUMNS</CODE>. <P> <DT><CODE>cmdhist</CODE> <DD>If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. <P> <DT><CODE>dotglob</CODE> <DD>If set, Bash includes filenames beginning with a `.' in the results of filename expansion. <P> <DT><CODE>execfail</CODE> <DD>If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the <CODE>exec</CODE> builtin command. An interactive shell does not exit if <CODE>exec</CODE> fails. <P> <DT><CODE>expand_aliases</CODE> <DD>If set, aliases are expanded as described below under Aliases, <A HREF="bashref.html#SEC80">6.6 Aliases</A>. This option is enabled by default for interactive shells. <P> <DT><CODE>extdebug</CODE> <DD>If set, behavior intended for use by debuggers is enabled: <P> <OL> <LI> The <SAMP>`-F'</SAMP> option to the <CODE>declare</CODE> builtin (see section <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A>) displays the source file name and line number corresponding to each function name supplied as an argument. <P> <LI> If the command run by the <CODE>DEBUG</CODE> trap returns a non-zero value, the next command is skipped and not executed. <P> <LI> If the command run by the <CODE>DEBUG</CODE> trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the <CODE>.</CODE> or <CODE>source</CODE> builtins), a call to <CODE>return</CODE> is simulated. <P> <LI> <CODE>BASH_ARGC</CODE> and <CODE>BASH_ARGV</CODE> are updated as described in their descriptions (see section <A HREF="bashref.html#SEC63">5.2 Bash Variables</A>). <P> <LI> Function tracing is enabled: command substitution, shell functions, and subshells invoked with <CODE>( <VAR>command</VAR> )</CODE> inherit the <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> traps. <P> <LI> Error tracing is enabled: command substitution, shell functions, and subshells invoked with <CODE>( <VAR>command</VAR> )</CODE> inherit the <CODE>ERROR</CODE> trap. </OL> <P> <DT><CODE>extglob</CODE> <DD>If set, the extended pattern matching features described above (see section <A HREF="bashref.html#SEC36">3.5.8.1 Pattern Matching</A>) are enabled. <P> <DT><CODE>extquote</CODE> <DD>If set, <CODE>$'<VAR>string</VAR>'</CODE> and <CODE>$"<VAR>string</VAR>"</CODE> quoting is performed within <CODE>${<VAR>parameter</VAR>}</CODE> expansions enclosed in double quotes. This option is enabled by default. <P> <DT><CODE>failglob</CODE> <DD>If set, patterns which fail to match filenames during pathname expansion result in an expansion error. <P> <DT><CODE>force_fignore</CODE> <DD>If set, the suffixes specified by the <CODE>FIGNORE</CODE> shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See section <A HREF="bashref.html#SEC63">5.2 Bash Variables</A>, for a description of <CODE>FIGNORE</CODE>. This option is enabled by default. <P> <DT><CODE>gnu_errfmt</CODE> <DD>If set, shell error messages are written in the standard GNU error message format. <P> <DT><CODE>histappend</CODE> <DD>If set, the history list is appended to the file named by the value of the <CODE>HISTFILE</CODE> variable when the shell exits, rather than overwriting the file. <P> <DT><CODE>histreedit</CODE> <DD>If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution. <P> <DT><CODE>histverify</CODE> <DD>If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification. <P> <DT><CODE>hostcomplete</CODE> <DD>If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a <SAMP>`@'</SAMP> is being completed (see section <A HREF="bashref.html#SEC109">8.4.6 Letting Readline Type For You</A>). This option is enabled by default. <P> <DT><CODE>huponexit</CODE> <DD>If set, Bash will send <CODE>SIGHUP</CODE> to all jobs when an interactive login shell exits (see section <A HREF="bashref.html#SEC54">3.7.6 Signals</A>). <P> <DT><CODE>interactive_comments</CODE> <DD>Allow a word beginning with <SAMP>`#'</SAMP> to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default. <P> <DT><CODE>lithist</CODE> <DD>If enabled, and the <CODE>cmdhist</CODE> option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible. <P> <DT><CODE>login_shell</CODE> <DD>The shell sets this option if it is started as a login shell (see section <A HREF="bashref.html#SEC65">6.1 Invoking Bash</A>). The value may not be changed. <P> <DT><CODE>mailwarn</CODE> <DD>If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message <CODE>"The mail in <VAR>mailfile</VAR> has been read"</CODE> is displayed. <P> <DT><CODE>no_empty_cmd_completion</CODE> <DD>If set, and Readline is being used, Bash will not attempt to search the <CODE>PATH</CODE> for possible completions when completion is attempted on an empty line. <P> <DT><CODE>nocaseglob</CODE> <DD>If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion. <P> <DT><CODE>nocasematch</CODE> <DD>If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing <CODE>case</CODE> or <CODE>[[</CODE> conditional commands. <P> <DT><CODE>nullglob</CODE> <DD>If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves. <P> <DT><CODE>progcomp</CODE> <DD>If set, the programmable completion facilities (see section <A HREF="bashref.html#SEC113">8.6 Programmable Completion</A>) are enabled. This option is enabled by default. <P> <DT><CODE>promptvars</CODE> <DD>If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (see section <A HREF="bashref.html#SEC84">6.9 Controlling the Prompt</A>). This option is enabled by default. <P> <DT><CODE>restricted_shell</CODE> <DD>The shell sets this option if it is started in restricted mode (see section <A HREF="bashref.html#SEC85">6.10 The Restricted Shell</A>). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted. <P> <DT><CODE>shift_verbose</CODE> <DD>If this is set, the <CODE>shift</CODE> builtin prints an error message when the shift count exceeds the number of positional parameters. <P> <DT><CODE>sourcepath</CODE> <DD>If set, the <CODE>source</CODE> builtin uses the value of <CODE>PATH</CODE> to find the directory containing the file supplied as an argument. This option is enabled by default. <P> <DT><CODE>xpg_echo</CODE> <DD>If set, the <CODE>echo</CODE> builtin expands backslash-escape sequences by default. <P> </DL> <P> The return status when listing options is zero if all <VAR>optnames</VAR> are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an <VAR>optname</VAR> is not a valid shell option. </P><P> <DT><CODE>source</CODE> <DD><A NAME="IDX104"></A> <TABLE><tr><td> </td><td class=example><pre>source <VAR>filename</VAR> </pre></td></tr></table>A synonym for <CODE>.</CODE> (see section <A HREF="bashref.html#SEC57">4.1 Bourne Shell Builtins</A>). <P> <DT><CODE>type</CODE> <DD><A NAME="IDX105"></A> <TABLE><tr><td> </td><td class=example><pre>type [-afptP] [<VAR>name</VAR> <small>...</small>] </pre></td></tr></table>For each <VAR>name</VAR>, indicate how it would be interpreted if used as a command name. <P> If the <SAMP>`-t'</SAMP> option is used, <CODE>type</CODE> prints a single word which is one of <SAMP>`alias'</SAMP>, <SAMP>`function'</SAMP>, <SAMP>`builtin'</SAMP>, <SAMP>`file'</SAMP> or <SAMP>`keyword'</SAMP>, if <VAR>name</VAR> is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the <VAR>name</VAR> is not found, then nothing is printed, and <CODE>type</CODE> returns a failure status. </P><P> If the <SAMP>`-p'</SAMP> option is used, <CODE>type</CODE> either returns the name of the disk file that would be executed, or nothing if <SAMP>`-t'</SAMP> would not return <SAMP>`file'</SAMP>. </P><P> The <SAMP>`-P'</SAMP> option forces a path search for each <VAR>name</VAR>, even if <SAMP>`-t'</SAMP> would not return <SAMP>`file'</SAMP>. </P><P> If a command is hashed, <SAMP>`-p'</SAMP> and <SAMP>`-P'</SAMP> print the hashed value, not necessarily the file that appears first in <CODE>$PATH</CODE>. </P><P> If the <SAMP>`-a'</SAMP> option is used, <CODE>type</CODE> returns all of the places that contain an executable named <VAR>file</VAR>. This includes aliases and functions, if and only if the <SAMP>`-p'</SAMP> option is not also used. </P><P> If the <SAMP>`-f'</SAMP> option is used, <CODE>type</CODE> does not attempt to find shell functions, as with the <CODE>command</CODE> builtin. </P><P> The return status is zero if any of the <VAR>names</VAR> are found, non-zero if none are found. </P><P> <DT><CODE>typeset</CODE> <DD><A NAME="IDX106"></A> <TABLE><tr><td> </td><td class=example><pre>typeset [-afFrxi] [-p] [<VAR>name</VAR>[=<VAR>value</VAR>] <small>...</small>] </pre></td></tr></table>The <CODE>typeset</CODE> command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the <CODE>declare</CODE> builtin command. <P> <DT><CODE>ulimit</CODE> <DD><A NAME="IDX107"></A> <TABLE><tr><td> </td><td class=example><pre>ulimit [-acdflmnpstuvSH] [<VAR>limit</VAR>] </pre></td></tr></table><CODE>ulimit</CODE> provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows: <DL COMPACT> <DT><CODE>-S</CODE> <DD>Change and report the soft limit associated with a resource. <P> <DT><CODE>-H</CODE> <DD>Change and report the hard limit associated with a resource. <P> <DT><CODE>-a</CODE> <DD>All current limits are reported. <P> <DT><CODE>-c</CODE> <DD>The maximum size of core files created. <P> <DT><CODE>-d</CODE> <DD>The maximum size of a process's data segment. <P> <DT><CODE>-f</CODE> <DD>The maximum size of files created by the shell. <P> <DT><CODE>-l</CODE> <DD>The maximum size that may be locked into memory. <P> <DT><CODE>-m</CODE> <DD>The maximum resident set size. <P> <DT><CODE>-n</CODE> <DD>The maximum number of open file descriptors. <P> <DT><CODE>-p</CODE> <DD>The pipe buffer size. <P> <DT><CODE>-s</CODE> <DD>The maximum stack size. <P> <DT><CODE>-t</CODE> <DD>The maximum amount of cpu time in seconds. <P> <DT><CODE>-u</CODE> <DD>The maximum number of processes available to a single user. <P> <DT><CODE>-v</CODE> <DD>The maximum amount of virtual memory available to the process. <P> </DL> <P> If <VAR>limit</VAR> is given, it is the new value of the specified resource; the special <VAR>limit</VAR> values <CODE>hard</CODE>, <CODE>soft</CODE>, and <CODE>unlimited</CODE> stand for the current hard limit, the current soft limit, and no limit, respectively. Otherwise, the current value of the soft limit for the specified resource is printed, unless the <SAMP>`-H'</SAMP> option is supplied. When setting new limits, if neither <SAMP>`-H'</SAMP> nor <SAMP>`-S'</SAMP> is supplied, both the hard and soft limits are set. If no option is given, then <SAMP>`-f'</SAMP> is assumed. Values are in 1024-byte increments, except for <SAMP>`-t'</SAMP>, which is in seconds, <SAMP>`-p'</SAMP>, which is in units of 512-byte blocks, and <SAMP>`-n'</SAMP> and <SAMP>`-u'</SAMP>, which are unscaled values. </P><P> The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit. </P><P> <DT><CODE>unalias</CODE> <DD><A NAME="IDX108"></A> <TABLE><tr><td> </td><td class=example><pre>unalias [-a] [<VAR>name</VAR> <small>...</small> ] </pre></td></tr></table><P> Remove each <VAR>name</VAR> from the list of aliases. If <SAMP>`-a'</SAMP> is supplied, all aliases are removed. Aliases are described in <A HREF="bashref.html#SEC80">6.6 Aliases</A>. </P><P> </DL> <P> <A NAME="The Set Builtin"></A> <HR SIZE="6"> <A NAME="SEC59"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC58"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC60"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC60"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 4.3 The Set Builtin </H2> <!--docid::SEC59::--> <P> This builtin is so complicated that it deserves its own section. </P><P> <DL COMPACT> <DT><CODE>set</CODE> <DD><A NAME="IDX109"></A> <TABLE><tr><td> </td><td class=example><pre>set [--abefhkmnptuvxBCHP] [-o <VAR>option</VAR>] [<VAR>argument</VAR> <small>...</small>] </pre></td></tr></table><P> If no options or arguments are supplied, <CODE>set</CODE> displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In POSIX mode, only shell variables are listed. </P><P> When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings: </P><P> <DL COMPACT> <DT><CODE>-a</CODE> <DD>Mark variables and function which are modified or created for export to the environment of subsequent commands. <P> <DT><CODE>-b</CODE> <DD>Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt. <P> <DT><CODE>-e</CODE> <DD>Exit immediately if a simple command (see section <A HREF="bashref.html#SEC16">3.2.1 Simple Commands</A>) exits with a non-zero status, unless the command that fails is part of the command list immediately following a <CODE>while</CODE> or <CODE>until</CODE> keyword, part of the test in an <CODE>if</CODE> statement, part of a <CODE>&&</CODE> or <CODE>||</CODE> list, or if the command's return status is being inverted using <CODE>!</CODE>. A trap on <CODE>ERR</CODE>, if set, is executed before the shell exits. <P> <DT><CODE>-f</CODE> <DD>Disable file name generation (globbing). <P> <DT><CODE>-h</CODE> <DD>Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default. <P> <DT><CODE>-k</CODE> <DD>All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name. <P> <DT><CODE>-m</CODE> <DD>Job control is enabled (see section <A HREF="bashref.html#SEC87">7. Job Control</A>). <P> <DT><CODE>-n</CODE> <DD>Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells. <P> <DT><CODE>-o <VAR>option-name</VAR></CODE> <DD><P> Set the option corresponding to <VAR>option-name</VAR>: </P><P> <DL COMPACT> <DT><CODE>allexport</CODE> <DD>Same as <CODE>-a</CODE>. <P> <DT><CODE>braceexpand</CODE> <DD>Same as <CODE>-B</CODE>. <P> <DT><CODE>emacs</CODE> <DD>Use an <CODE>emacs</CODE>-style line editing interface (see section <A HREF="bashref.html#SEC91">8. Command Line Editing</A>). <P> <DT><CODE>errexit</CODE> <DD>Same as <CODE>-e</CODE>. <P> <DT><CODE>errtrace</CODE> <DD>Same as <CODE>-E</CODE>. <P> <DT><CODE>functrace</CODE> <DD>Same as <CODE>-T</CODE>. <P> <DT><CODE>hashall</CODE> <DD>Same as <CODE>-h</CODE>. <P> <DT><CODE>histexpand</CODE> <DD>Same as <CODE>-H</CODE>. <P> <DT><CODE>history</CODE> <DD>Enable command history, as described in <A HREF="bashref.html#SEC116">9.1 Bash History Facilities</A>. This option is on by default in interactive shells. <P> <DT><CODE>ignoreeof</CODE> <DD>An interactive shell will not exit upon reading EOF. <P> <DT><CODE>keyword</CODE> <DD>Same as <CODE>-k</CODE>. <P> <DT><CODE>monitor</CODE> <DD>Same as <CODE>-m</CODE>. <P> <DT><CODE>noclobber</CODE> <DD>Same as <CODE>-C</CODE>. <P> <DT><CODE>noexec</CODE> <DD>Same as <CODE>-n</CODE>. <P> <DT><CODE>noglob</CODE> <DD>Same as <CODE>-f</CODE>. <P> <DT><CODE>nolog</CODE> <DD>Currently ignored. <P> <DT><CODE>notify</CODE> <DD>Same as <CODE>-b</CODE>. <P> <DT><CODE>nounset</CODE> <DD>Same as <CODE>-u</CODE>. <P> <DT><CODE>onecmd</CODE> <DD>Same as <CODE>-t</CODE>. <P> <DT><CODE>physical</CODE> <DD>Same as <CODE>-P</CODE>. <P> <DT><CODE>pipefail</CODE> <DD>If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default. <P> <DT><CODE>posix</CODE> <DD>Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard (see section <A HREF="bashref.html#SEC86">6.11 Bash POSIX Mode</A>). This is intended to make Bash behave as a strict superset of that standard. <P> <DT><CODE>privileged</CODE> <DD>Same as <CODE>-p</CODE>. <P> <DT><CODE>verbose</CODE> <DD>Same as <CODE>-v</CODE>. <P> <DT><CODE>vi</CODE> <DD>Use a <CODE>vi</CODE>-style line editing interface. <P> <DT><CODE>xtrace</CODE> <DD>Same as <CODE>-x</CODE>. </DL> <P> <DT><CODE>-p</CODE> <DD>Turn on privileged mode. In this mode, the <CODE>$BASH_ENV</CODE> and <CODE>$ENV</CODE> files are not processed, shell functions are not inherited from the environment, and the <CODE>SHELLOPTS</CODE> variable, if it appears in the environment, is ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the <CODE>-p</CODE> option is not supplied, these actions are taken and the effective user id is set to the real user id. If the <CODE>-p</CODE> option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids. <P> <DT><CODE>-t</CODE> <DD>Exit after reading and executing one command. <P> <DT><CODE>-u</CODE> <DD>Treat unset variables as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit. <P> <DT><CODE>-v</CODE> <DD>Print shell input lines as they are read. <P> <DT><CODE>-x</CODE> <DD>Print a trace of simple commands, \fBfor\fP commands, \fBcase\fP commands, \fBselect\fP commands, and arithmetic \fBfor\fP commands and their arguments or associated word lists after they are expanded and before they are executed. The value of the <CODE>PS4</CODE> variable is expanded and the resultant value is printed before the command and its expanded arguments. <P> <DT><CODE>-B</CODE> <DD>The shell will perform brace expansion (see section <A HREF="bashref.html#SEC28">3.5.1 Brace Expansion</A>). This option is on by default. <P> <DT><CODE>-C</CODE> <DD>Prevent output redirection using <SAMP>`>'</SAMP>, <SAMP>`>&'</SAMP>, and <SAMP>`<>'</SAMP> from overwriting existing files. <P> <DT><CODE>-E</CODE> <DD>If set, any trap on <CODE>ERR</CODE> is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The <CODE>ERR</CODE> trap is normally not inherited in such cases. <P> <DT><CODE>-H</CODE> <DD>Enable <SAMP>`!'</SAMP> style history substitution (see section <A HREF="bashref.html#SEC118">9.3 History Expansion</A>). This option is on by default for interactive shells. <P> <DT><CODE>-P</CODE> <DD>If set, do not follow symbolic links when performing commands such as <CODE>cd</CODE> which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory. <P> For example, if <TT>`/usr/sys'</TT> is a symbolic link to <TT>`/usr/local/sys'</TT> then: <TABLE><tr><td> </td><td class=example><pre>$ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usr </pre></td></tr></table></P><P> If <CODE>set -P</CODE> is on, then: <TABLE><tr><td> </td><td class=example><pre>$ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local </pre></td></tr></table></P><P> <DT><CODE>-T</CODE> <DD>If set, any trap on <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The <CODE>DEBUG</CODE> and <CODE>RETURN</CODE> traps are normally not inherited in such cases. <P> <DT><CODE>--</CODE> <DD>If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the <VAR>arguments</VAR>, even if some of them begin with a <SAMP>`-'</SAMP>. <P> <DT><CODE>-</CODE> <DD>Signal the end of options, cause all remaining <VAR>arguments</VAR> to be assigned to the positional parameters. The <SAMP>`-x'</SAMP> and <SAMP>`-v'</SAMP> options are turned off. If there are no arguments, the positional parameters remain unchanged. </DL> <P> Using <SAMP>`+'</SAMP> rather than <SAMP>`-'</SAMP> causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in <CODE>$-</CODE>. </P><P> The remaining N <VAR>arguments</VAR> are positional parameters and are assigned, in order, to <CODE>$1</CODE>, <CODE>$2</CODE>, <small>...</small> <CODE>$N</CODE>. The special parameter <CODE>#</CODE> is set to N. </P><P> The return status is always zero unless an invalid option is supplied. </DL> <P> <A NAME="Special Builtins"></A> <HR SIZE="6"> <A NAME="SEC60"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC59"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC56"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 4.4 Special Builtins </H2> <!--docid::SEC60::--> <P> For historical reasons, the POSIX 1003.2 standard has classified several builtin commands as <EM>special</EM>. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects: </P><P> <OL> <LI> Special builtins are found before shell functions during command lookup. <P> <LI> If a special builtin returns an error status, a non-interactive shell exits. <P> <LI> Assignment statements preceding the command stay in effect in the shell environment after the command completes. </OL> <P> When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in <A HREF="bashref.html#SEC86">6.11 Bash POSIX Mode</A>. </P><P> These are the POSIX special builtins: <TABLE><tr><td> </td><td class=example><pre>break : . continue eval exec exit export readonly return set shift trap unset </pre></td></tr></table></P><P> <A NAME="Shell Variables"></A> <HR SIZE="6"> <A NAME="SEC61"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC60"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC62"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC64"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC64"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H1> 5. Shell Variables </H1> <!--docid::SEC61::--> <P> <BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC62">5.1 Bourne Shell Variables</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">Variables which Bash uses in the same way as the Bourne Shell.</TD></TR> <TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC63">5.2 Bash Variables</A></TD><TD> </TD><TD ALIGN="left" VALIGN="TOP">List of variables that exist in Bash.</TD></TR> </TABLE></BLOCKQUOTE> <P> This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables. </P><P> <A NAME="Bourne Shell Variables"></A> <HR SIZE="6"> <A NAME="SEC62"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC63"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC64"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 5.1 Bourne Shell Variables </H2> <!--docid::SEC62::--> <P> Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable. </P><P> <DL COMPACT> <A NAME="IDX110"></A> <DT><CODE>CDPATH</CODE> <DD><A NAME="IDX111"></A> A colon-separated list of directories used as a search path for the <CODE>cd</CODE> builtin command. <P> <A NAME="IDX112"></A> <DT><CODE>HOME</CODE> <DD><A NAME="IDX113"></A> The current user's home directory; the default for the <CODE>cd</CODE> builtin command. The value of this variable is also used by tilde expansion (see section <A HREF="bashref.html#SEC29">3.5.2 Tilde Expansion</A>). <P> <A NAME="IDX114"></A> <DT><CODE>IFS</CODE> <DD><A NAME="IDX115"></A> A list of characters that separate fields; used when the shell splits words as part of expansion. <P> <A NAME="IDX116"></A> <DT><CODE>MAIL</CODE> <DD><A NAME="IDX117"></A> If this parameter is set to a filename and the <CODE>MAILPATH</CODE> variable is not set, Bash informs the user of the arrival of mail in the specified file. <P> <A NAME="IDX118"></A> <DT><CODE>MAILPATH</CODE> <DD><A NAME="IDX119"></A> A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the file name from the message with a <SAMP>`?'</SAMP>. When used in the text of the message, <CODE>$_</CODE> expands to the name of the current mail file. <P> <A NAME="IDX120"></A> <DT><CODE>OPTARG</CODE> <DD><A NAME="IDX121"></A> The value of the last option argument processed by the <CODE>getopts</CODE> builtin. <P> <A NAME="IDX122"></A> <DT><CODE>OPTIND</CODE> <DD><A NAME="IDX123"></A> The index of the last option argument processed by the <CODE>getopts</CODE> builtin. <P> <A NAME="IDX124"></A> <DT><CODE>PATH</CODE> <DD><A NAME="IDX125"></A> A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of <CODE>PATH</CODE> indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. <P> <A NAME="IDX126"></A> <DT><CODE>PS1</CODE> <DD><A NAME="IDX127"></A> The primary prompt string. The default value is <SAMP>`\s-\v\$ '</SAMP>. See section <A HREF="bashref.html#SEC84">6.9 Controlling the Prompt</A>, for the complete list of escape sequences that are expanded before <CODE>PS1</CODE> is displayed. <P> <A NAME="IDX128"></A> <DT><CODE>PS2</CODE> <DD><A NAME="IDX129"></A> The secondary prompt string. The default value is <SAMP>`> '</SAMP>. <P> </DL> <P> <A NAME="Bash Variables"></A> <HR SIZE="6"> <A NAME="SEC63"></A> <TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0> <TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC62"> < </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC64"> > </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> << </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC61"> Up </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC64"> >> </A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Top">Top</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_Contents">Contents</A>]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[Index]</TD> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD> </TR></TABLE> <H2> 5.2 Bash Variables </H2> <!--docid::SEC63::--> <P> These variables are set or used by Bash, but other shells do not normally treat them specially. </P><P> A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see section <A HREF="bashref.html#SEC90">7.3 Job Control Variables</A>). </P><P> <DL COMPACT> <A NAME="IDX130"></A> <DT><CODE>BASH</CODE> <DD><A NAME="IDX131"></A> The full pathname used to execute the current instance of Bash. <P> <A NAME="IDX132"></A> <DT><CODE>BASH_ARGC</CODE> <DD><A NAME="IDX133"></A> An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with <CODE>.</CODE> or <CODE>source</CODE>) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto <CODE>BASH_ARGC</CODE>. The shell sets <CODE>BASH_ARGC</CODE> only when in extended debugging mode (see <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A> for a description of the <CODE>extdebug</CODE> option to the <CODE>shopt</CODE> builtin). <P> <A NAME="IDX134"></A> <DT><CODE>BASH_ARGV</CODE> <DD><A NAME="IDX135"></A> An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto <CODE>BASH_ARGV</CODE>. The shell sets <CODE>BASH_ARGV</CODE> only when in extended debugging mode (see <A HREF="bashref.html#SEC58">4.2 Bash Builtin Commands</A> for a description of the <CODE>extdebug</CODE> option to the <CODE>shopt</CODE> builtin). <P> <A NAME="IDX136"></A> <DT><CODE>BASH_COMMAND</CODE> <DD><A NAME="IDX137"></A> The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. <P> <A NAME="IDX138"></A> <DT><CODE>BASH_ENV</CODE> <DD><A NAME="IDX139"></A> If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. See section <A HREF="bashref.html#SEC66">6.2 Bash Startup Files</A>. <P> <A NAME="IDX140"></A> <DT><CODE>BASH_EXECUTION_STRING</CODE> <DD><A NAME="IDX141"></A> The command argument to the <SAMP>`-c'</SAMP> invocation option. <P> <A NAME="IDX142">