This is tar.info, produced by Makeinfo version 3.12f from tar.texi. START-INFO-DIR-ENTRY * tar: (tar). Making tape (or disk) archives. END-INFO-DIR-ENTRY This file documents GNU `tar', a utility used to store, backup, and transport files. Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. This file documents GNU `tar', which is a utility used to store, backup, and transport files. `tar' is a tape (or disk) archiver. This manual documents the release 1.13. File: tar.info, Node: Blocking, Next: Many, Prev: Common Problems and Solutions, Up: Media Blocking ======== _(This message will disappear, once this node revised.)_ "Block" and "record" terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way. John Gilmore, the writer of the public domain `tar' from which GNU `tar' was originally derived, wrote (June 1995): The nomenclature of tape drives comes from IBM, where I believe they were invented for the IBM 650 or so. On IBM mainframes, what is recorded on tape are tape blocks. The logical organization of data is into records. There are various ways of putting records into blocks, including `F' (fixed sized records), `V' (variable sized records), `FB' (fixed blocked: fixed size records, N to a block), `VB' (variable size records, N to a block), `VSB' (variable spanned blocked: variable sized records that can occupy more than one block), etc. The `JCL' `DD RECFORM=' parameter specified this to the operating system. The Unix man page on `tar' was totally confused about this. When I wrote `PD TAR', I used the historically correct terminology (`tar' writes data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now Franc,ois has migrated that terminology back into the source code too. The term "physical block" means the basic transfer chunk from or to a device, after which reading or writing may stop without anything being lost. In this manual, the term "block" usually refers to a disk physical block, _assuming_ that each disk block is 512 bytes in length. It is true that some disk devices have different physical blocks, but `tar' ignore these differences in its own format, which is meant to be portable, so a `tar' block is always 512 bytes in length, and "block" always mean a `tar' block. The term "logical block" often represents the basic chunk of allocation of many disk blocks as a single entity, which the operating system treats somewhat atomically; this concept is only barely used in GNU `tar'. The term "physical record" is another way to speak of a physical block, those two terms are somewhat interchangeable. In this manual, the term "record" usually refers to a tape physical block, _assuming_ that the `tar' archive is kept on magnetic tape. It is true that archives may be put on disk or used with pipes, but nevertheless, `tar' tries to read and write the archive one "record" at a time, whatever the medium in use. One record is made up of an integral number of blocks, and this operation of putting many disk blocks into a single tape block is called "reblocking", or more simply, "blocking". The term "logical record" refers to the logical organization of many characters into something meaningful to the application. The term "unit record" describes a small set of characters which are transmitted whole to or by the application, and often refers to a line of text. Those two last terms are unrelated to what we call a "record" in GNU `tar'. When writing to tapes, `tar' writes the contents of the archive in chunks known as "records". To change the default blocking factor, use the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option. Each record will then be composed of 512-SIZE blocks. (Each `tar' block is 512 bytes. *Note Standard::.) Each file written to the archive uses at least one full record. As a result, using a larger record size can result in more wasted space for small files. On the other hand, a larger record size can often be read and written much more efficiently. Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking. When reading an archive, `tar' can usually figure out the record size on itself. When this is the case, and a non-standard record size was used when the archive was created, `tar' will print a message about a non-standard blocking factor, and then operate normally. On some tape devices, however, `tar' cannot figure out the record size itself. On most of those, you can specify a blocking factor (with `--blocking-factor=512-SIZE' (`-b 512-SIZE')) larger than the actual blocking factor, and then use the `--read-full-records' (`-B') option. (If you specify a blocking factor with `--blocking-factor=512-SIZE' (`-b 512-SIZE') and don't use the `--read-full-records' (`-B') option, then `tar' will not attempt to figure out the recording size itself.) On some devices, you must always specify the record size exactly with `--blocking-factor=512-SIZE' (`-b 512-SIZE') when reading, because `tar' cannot figure it out. In any case, use `--list' (`-t') before doing any extractions to see whether `tar' is reading the archive correctly. `tar' blocks are all fixed size (512 bytes), and its scheme for putting them into records is to put a whole number of them (one or more) into each record. `tar' records are all the same size; at the end of the file there's a block containing all zeros, which is how you tell that the remainder of the last record(s) are garbage. In a standard `tar' file (no options), the block size is 512 and the record size is 10240, for a blocking factor of 20. What the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option does is sets the blocking factor, changing the record size while leaving the block size at 512 bytes. 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives these days prefer much bigger records in order to stream and not waste tape. When writing tapes for myself, some tend to use a factor of the order of 2048, say, giving a record size of around one megabyte. If you use a blocking factor larger than 20, older `tar' programs might not be able to read the archive, so we recommend this as a limit to use in practice. GNU `tar', however, will support arbitrarily large record sizes, limited only by the amount of virtual memory or the physical characteristics of the tape device. * Menu: * Format Variations:: Format Variations * Blocking Factor:: The Blocking Factor of an Archive File: tar.info, Node: Format Variations, Next: Blocking Factor, Prev: Blocking, Up: Blocking Format Variations ----------------- _(This message will disappear, once this node revised.)_ Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive. To specify format parameters when accessing or creating an archive, you can use the options described in the following sections. If you do not specify any format parameters, `tar' uses default parameters. You cannot modify a compressed archive. If you create an archive with the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option specified (*note Blocking Factor::.), you must specify that blocking-factor when operating on the archive. *Note Formats::, for other examples of format parameter considerations. File: tar.info, Node: Blocking Factor, Prev: Format Variations, Up: Blocking The Blocking Factor of an Archive --------------------------------- _(This message will disappear, once this node revised.)_ The data in an archive is grouped into blocks, which are 512 bytes. Blocks are read and written in whole number multiples called "records". The number of blocks in a record (ie. the size of a record in units of 512 bytes) is called the "blocking factor". The `--blocking-factor=512-SIZE' (`-b 512-SIZE') option specifies the blocking factor of an archive. The default blocking factor is typically 20 (ie. 10240 bytes), but can be specified at installation. To find out the blocking factor of an existing archive, use `tar --list --file=ARCHIVE-NAME'. This may not work on some devices. Records are separated by gaps, which waste space on the archive media. If you are archiving on magnetic tape, using a larger blocking factor (and therefore larger records) provides faster throughput and allows you to fit more data on a tape (because there are fewer gaps). If you are archiving on cartridge, a very large blocking factor (say 126 or more) greatly increases performance. A smaller blocking factor, on the other hand, may be usefull when archiving small files, to avoid archiving lots of nulls as `tar' fills out the archive to the end of the record. In general, the ideal record size depends on the size of the inter-record gaps on the tape you are using, and the average size of the files you are archiving. *Note create::, for information on writing archives. Archives with blocking factors larger than 20 cannot be read by very old versions of `tar', or by some newer versions of `tar' running on old machines with small address spaces. With GNU `tar', the blocking factor of an archive is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported: Cannot write to /dev/dlt: Invalid argument In such cases, it sometimes happen that the `tar' bundled by the system is aware of block size idiosyncrasies, while GNU `tar' requires an explicit specification for the block size, which it cannot guess. This yields some people to consider GNU `tar' is misbehaving, because by comparison, `the bundle `tar' works OK'. Adding `-b 256', for example, might resolve the problem. If you use a non-default blocking factor when you create an archive, you must specify the same blocking factor when you modify that archive. Some archive devices will also require you to specify the blocking factor when reading that archive, however this is not typically the case. Usually, you can use `--list' (`-t') without specifying a blocking factor--`tar' reports a non-default record size and then lists the archive members as it would normally. To extract files from an archive with a non-standard blocking factor (particularly if you're not sure what the blocking factor is), you can usually use the `--read-full-records' (`-B') option while specifying a blocking factor larger then the blocking factor of the archive (ie. `tar --extract --read-full-records --blocking-factor=300'. *Note list::, for more information on the `--list' (`-t') operation. *Note Reading::, for a more detailed explanation of that option. `--blocking-factor=NUMBER' `-b NUMBER' Specifies the blocking factor of an archive. Can be used with any operation, but is usually not necessary with `--list' (`-t'). Device blocking `-b BLOCKS' `--blocking-factor=BLOCKS' Set record size to BLOCKS * 512 bytes. This option is used to specify a "blocking factor" for the archive. When reading or writing the archive, `tar', will do reads and writes of the archive in records of BLOCK*512 bytes. This is true even when the archive is compressed. Some devices requires that all write operations be a multiple of a certain size, and so, `tar' pads the archive out to the next record boundary. The default blocking factor is set when `tar' is compiled, and is typically 20. Blocking factors larger than 20 cannot be read by very old versions of `tar', or by some newer versions of `tar' running on old machines with small address spaces. With a magnetic tape, larger records give faster throughput and fit more data on a tape (because there are fewer inter-record gaps). If the archive is in a disk file or a pipe, you may want to specify a smaller blocking factor, since a large one will result in a large number of null bytes at the end of the archive. When writing cartridge or other streaming tapes, a much larger blocking factor (say 126 or more) will greatly increase performance. However, you must specify the same blocking factor when reading or updating the archive. Apparently, Exabyte drives have a physical block size of 8K bytes. If we choose our blocksize as a multiple of 8k bytes, then the problem seems to dissapper. Id est, we are using block size of 112 right now, and we haven't had the problem since we switched... With GNU `tar' the blocking factor is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. However, deblocking or reblocking is virtually avoided in a special case which often occurs in practice, but which requires all the following conditions to be simultaneously true: * the archive is subject to a compression option, * the archive is not handled through standard input or output, nor redirected nor piped, * the archive is directly handled to a local disk, instead of any special device, * `--blocking-factor=512-SIZE' (`-b 512-SIZE') is not explicitely specified on the `tar' invocation. In previous versions of GNU `tar', the `--compress-block' option (or even older: `--block-compress') was necessary to reblock compressed archives. It is now a dummy option just asking not to be used, and otherwise ignored. If the output goes directly to a local disk, and not through stdout, then the last write is not extended to a full record size. Otherwise, reblocking occurs. Here are a few other remarks on this topic: * `gzip' will complain about trailing garbage if asked to uncompress a compressed archive on tape, there is an option to turn the message off, but it breaks the regularity of simply having to use `PROG -d' for decompression. It would be nice if gzip was silently ignoring any number of trailing zeros. I'll ask Jean-loup Gailly, by sending a copy of this message to him. * `compress' does not show this problem, but as Jean-loup pointed out to Michael, `compress -d' silently adds garbage after the result of decompression, which tar ignores because it already recognized its end-of-file indicator. So this bug may be safely ignored. * `gzip -d -q' will be silent about the trailing zeros indeed, but will still return an exit status of 2 which tar reports in turn. `tar' might ignore the exit status returned, but I hate doing that, as it weakens the protection `tar' offers users against other possible problems at decompression time. If `gzip' was silently skipping trailing zeros _and_ also avoiding setting the exit status in this innocuous case, that would solve this situation. * `tar' should become more solid at not stopping to read a pipe at the first null block encountered. This inelegantly breaks the pipe. `tar' should rather drain the pipe out before exiting itself. `-i' `--ignore-zeros' Ignore blocks of zeros in archive (means EOF). The `--ignore-zeros' (`-i') option causes `tar' to ignore blocks of zeros in the archive. Normally a block of zeros indicates the end of the archive, but when reading a damaged archive, or one which was created by `cat'-ing several archives together, this option allows `tar' to read the entire archive. This option is not on by default because many versions of `tar' write garbage after the zeroed blocks. Note that this option causes `tar' to read to the end of the archive file, which may sometimes avoid problems when multiple files are stored on a single physical tape. `-B' `--read-full-records' Reblock as we read (for reading 4.2BSD pipes). If `--read-full-records' (`-B') is used, `tar' will not panic if an attempt to read a record from the archive does not return a full record. Instead, `tar' will keep reading until it has obtained a full record. This option is turned on by default when `tar' is reading an archive from standard input, or from a remote machine. This is because on BSD Unix systems, a read of a pipe will return however much happens to be in the pipe, even if it is less than `tar' requested. If this option was not used, `tar' would fail as soon as it read an incomplete record from the pipe. This option is also useful with the commands for updating an archive. Tape blocking When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A "tape gap" is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without loosing information. Using higher blocking (putting more disk blocks per tape block) will use the tape more efficiently as there will be less tape gaps. But reading such tapes may be more difficult for the system, as more memory will be required to receive at once the whole record. Further, if there is a reading error on a huge record, this is less likely that the system will succeed in recovering the information. So, blocking should not be too low, nor it should be too high. `tar' uses by default a blocking of 20 for historical reasons, and it does not really matter when reading or writing to disk. Current tape technology would easily accomodate higher blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs. We were told that for some DLT drives, the blocking should be a multiple of 4Kb, preferably 64Kb (`-b 128') or 256 for decent performance. Other manufacturers may use different recommendations for the same tapes. This might also depends of the buffering techniques used inside modern tape controllers. Some imposes a minimum blocking, or a maximum blocking. Others request blocking to be some exponent of two. So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable. I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time. I might also use `--number-blocks' instead of `--block-number', so `--block' will then expand to `--blocking-factor' unambiguously. File: tar.info, Node: Many, Next: Using Multiple Tapes, Prev: Blocking, Up: Media Many Archives on One Tape ========================= Most tape devices have two entries in the `/dev' directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example `/dev/tape', which often points to the only or usual tape device of a given system. There might be a corresponding `/dev/nrtape' or `/dev/ntape'. The simpler name is the _rewinding_ version of the device, while the name having `nr' in it is the _no rewinding_ version of the same device. A rewinding tape device will bring back the tape to its beginning point automatically when this device is opened or closed. Since `tar' opens the archive file before using it and closes it afterwards, this means that a simple: $ tar cf /dev/tape DIRECTORY will reposition the tape to its beginning both prior and after saving DIRECTORY contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved. So, a rewinding device is normally meant to hold one and only one file. If you want to put more than one `tar' archive on a given tape, you will need to avoid using the rewinding version of the tape device. You will also have to pay special attention to tape positioning. Errors in positionning may overwrite the valuable data already on your tape. Many people, burnt by past experiences, will only use rewinding devices and limit themselves to one file per tape, precisely to avoid the risk of such errors. Be fully aware that writing at the wrong position on a tape loses all information past this point and most probably until the end of the tape, and this destroyed information _cannot_ be recovered. To save DIRECTORY-1 as a first archive at the beginning of a tape, and leave that tape ready for a second archive, you should use: $ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape DIRECTORY-1 "Tape marks" are special magnetic patterns written on the tape media, which are later recognizable by the reading hardware. These marks are used after each file, when there are many on a single tape. An empty file (that is to say, two tape marks in a row) signal the logical end of the tape, after which no file exist. Usually, non-rewinding tape device drivers will react to the close request issued by `tar' by first writing two tape marks after your archive, and by backspacing over one of these. So, if you remove the tape at that time from the tape drive, it is properly terminated. But if you write another file at the current position, the second tape mark will be erased by the new information, leaving only one tape mark between files. So, you may now save DIRECTORY-2 as a second archive after the first on the same tape by issuing the command: $ tar cf /dev/nrtape DIRECTORY-2 and so on for all the archives you want to put on the same tape. Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving DIRECTORY-17, say, by using these commands: $ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape DIRECTORY-17 In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. *Note Blocking::. * Menu: * Tape Positioning:: Tape Positions and Tape Marks * mt:: The `mt' Utility File: tar.info, Node: Tape Positioning, Next: mt, Prev: Many, Up: Many Tape Positions and Tape Marks ----------------------------- _(This message will disappear, once this node revised.)_ Just as archives can store more than one file from the file system, tapes can store more than one archive file. To keep track of where archive files (or any other type of file stored on tape) begin and end, tape archive devices write magnetic "tape marks" on the archive media. Tape drives write one tape mark between files, two at the end of all the file entries. If you think of data as a series of records "rrrr"'s, and tape marks as "*"'s, a tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- Tape devices read and write tapes using a read/write "tape head"--a physical part of the device which can only access one point on the tape at a time. When you use `tar' to read or write archive data from a tape device, the device will begin reading or writing from wherever on the tape the tape head happens to be, regardless of which archive or what part of the archive the tape head is on. Before writing an archive, you should make sure that no data on the tape will be overwritten (unless it is no longer needed). Before reading an archive, you should make sure the tape head is at the beginning of the archive you want to read. (The `restore' script will find the archive automatically. . ). *Note mt::, for an explanation of the tape moving utility. If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**---------------- File: tar.info, Node: mt, Prev: Tape Positioning, Up: Many The `mt' Utility ---------------- _(This message will disappear, once this node revised.)_ *Note Blocking Factor::. You can use the `mt' utility to advance or rewind a tape past a specified number of archive files on the tape. This will allow you to move to the beginning of an archive before extracting or reading it, or to the end of all the archives before writing a new one. The syntax of the `mt' command is: mt [-f TAPENAME] OPERATION [NUMBER] where TAPENAME is the name of the tape device, NUMBER is the number of times an operation is performed (with a default of one), and OPERATION is one of the following: `eof' `weof' Writes NUMBER tape marks at the current position on the tape. `fsf' Moves tape position forward NUMBER files. `bsf' Moves tape position back NUMBER files. `rewind' Rewinds the tape. (Ignores NUMBER). `offline' `rewoff1' Rewinds the tape and takes the tape device off-line. (Ignores NUMBER). `status' Prints status information about the tape unit. If you don't specify a TAPENAME, `mt' uses the environment variable TAPE; if TAPE does not exist, `mt' uses the device `/dev/rmt12'. `mt' returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation failed. If you use `--extract' (`--get', `-x') with the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option specified, `tar' will read an archive label (the tape head has to be positioned on it) and print an error if the archive label doesn't match the ARCHIVE-NAME specified. ARCHIVE-NAME can be any regular expression. If the labels match, `tar' extracts the archive. *Note label::. . `tar --list --label' will cause `tar' to print the label. File: tar.info, Node: Using Multiple Tapes, Next: label, Prev: Many, Up: Media Using Multiple Tapes ==================== _(This message will disappear, once this node revised.)_ Often you might want to write a large archive, one larger than will fit on the actual tape you are using. In such a case, you can run multiple `tar' commands, but this can be inconvenient, particularly if you are using options like `--exclude=PATTERN' or dumping entire filesystems. Therefore, `tar' supports multiple tapes automatically. Use `--multi-volume' (`-M') on the command line, and then `tar' will, when it reaches the end of the tape, prompt for another tape, and continue the archive. Each tape will have an independent archive, and can be read without needing the other. (As an exception to this, the file that `tar' was archiving when it ran out of tape will usually be split between the two archives; in this case you need to extract from the first archive, using `--multi-volume' (`-M'), and then put in the second tape when prompted, so `tar' can restore both halves of the file.) GNU `tar' multi-volume archives do not use a truly portable format. You need GNU `tar' at both end to process them properly. When prompting for a new tape, `tar' accepts any of the following responses: `?' Request `tar' to explain possible responses `q' Request `tar' to exit immediately. `n FILE NAME' Request `tar' to write the next volume on the file FILE NAME. `!' Request `tar' to run a subshell. `y' Request `tar' to begin writing the next volume. (You should only type `y' after you have changed the tape; otherwise `tar' will write over the volume it just finished.) If you want more elaborate behavior than this, give `tar' the `--info-script=SCRIPT-NAME' (`--new-volume-script=SCRIPT-NAME', `-F SCRIPT-NAME') option. The file SCRIPT-NAME is expected to be a program (or shell script) to be run instead of the normal prompting procedure. When the program finishes, `tar' will immediately begin writing the next volume. The behavior of the `n' response to the normal tape-change prompt is not available if you use `--info-script=SCRIPT-NAME' (`--new-volume-script=SCRIPT-NAME', `-F SCRIPT-NAME'). The method `tar' uses to detect end of tape is not perfect, and fails on some operating systems or on some devices. You can use the `--tape-length=1024-SIZE' (`-L 1024-SIZE') option if `tar' can't detect the end of the tape itself. This option selects `--multi-volume' (`-M') automatically. The SIZE argument should then be the usable size of the tape. But for many devices, and floppy disks in particular, this option is never required for real, as far as we know. The volume number used by `tar' in its tape-change prompt can be changed; if you give the `--volno-file=FILE-OF-NUMBER' option, then FILE-OF-NUMBER should be an unexisting file to be created, or else, a file already containing a decimal number. That number will be used as the volume number of the first volume written. When `tar' is finished, it will rewrite the file with the now-current volume number. (This does not change the volume number written on a tape label, as per *Note label::, it _only_ affects the number used in the prompt.) If you want `tar' to cycle through a series of tape drives, then you can use the `n' response to the tape-change prompt. This is error prone, however, and doesn't work at all with `--info-script=SCRIPT-NAME' (`--new-volume-script=SCRIPT-NAME', `-F SCRIPT-NAME'). Therefore, if you give `tar' multiple `--file=ARCHIVE-NAME' (`-f ARCHIVE-NAME') options, then the specified files will be used, in sequence, as the successive volumes of the archive. Only when the first one in the sequence needs to be used again will `tar' prompt for a tape change (or run the info script). Multi-volume archives With `--multi-volume' (`-M'), `tar' will not abort when it cannot read or write any more data. Instead, it will ask you to prepare a new volume. If the archive is on a magnetic tape, you should change tapes now; if the archive is on a floppy disk, you should change disks, etc. Each volume of a multi-volume archive is an independent `tar' archive, complete in itself. For example, you can list or extract any volume alone; just don't specify `--multi-volume' (`-M'). However, if one file in the archive is split across volumes, the only way to extract it successfully is with a multi-volume extract command `--extract --multi-volume' (`-xM') starting on or before the volume where the file begins. For example, let's presume someone has two tape drives on a system named `/dev/tape0' and `/dev/tape1'. For having GNU `tar' to switch to the second drive when it needs to write the second tape, and then back to the first tape, etc., just do either of: $ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 FILES $ tar cMff /dev/tape0 /dev/tape1 FILES * Menu: * Multi-Volume Archives:: Archives Longer than One Tape or Disk * Tape Files:: Tape Files File: tar.info, Node: Multi-Volume Archives, Next: Tape Files, Prev: Using Multiple Tapes, Up: Using Multiple Tapes Archives Longer than One Tape or Disk ------------------------------------- _(This message will disappear, once this node revised.)_ To create an archive that is larger than will fit on a single unit of the media, use the `--multi-volume' (`-M') option in conjunction with the `--create' (`-c') option (*note create::.). A "multi-volume" archive can be manipulated like any other archive (provided the `--multi-volume' (`-M') option is specified), but is stored on more than one tape or disk. When you specify `--multi-volume' (`-M'), `tar' does not report an error when it comes to the end of an archive volume (when reading), or the end of the media (when writing). Instead, it prompts you to load a new storage volume. If the archive is on a magnetic tape, you should change tapes when you see the prompt; if the archive is on a floppy disk, you should change disks; etc. You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use `--list' (`-t'), without `--multi-volume' (`-M') specified. To extract an archive member from one volume (assuming it is described that volume), use `--extract' (`--get', `-x'), again without `--multi-volume' (`-M'). If an archive member is split across volumes (ie. its entry begins on one volume of the media and ends on another), you need to specify `--multi-volume' (`-M') to extract it successfully. In this case, you should load the volume where the archive member starts, and use `tar --extract --multi-volume'--`tar' will prompt for later volumes as it needs them. *Note extracting archives::, for more information about extracting archives. `--info-script=SCRIPT-NAME' (`--new-volume-script=SCRIPT-NAME', `-F SCRIPT-NAME') is like `--multi-volume' (`-M'), except that `tar' does not prompt you directly to change media volumes when a volume is full--instead, `tar' runs commands you have stored in SCRIPT-NAME. For example, this option can be used to eject cassettes, or to broadcast messages such as `Someone please come change my tape' when performing unattended backups. When SCRIPT-NAME is done, `tar' will assume that the media has been changed. Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive. If a multi-volume archive was labeled using `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') (*note label::.) when it was created, `tar' will not automatically label volumes which are added later. To label subsequent volumes, specify `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') again in conjunction with the `--append' (`-r'), `--update' (`-u') or `--concatenate' (`--catenate', `-A') operation. `--multi-volume' `-M' Creates a multi-volume archive, when used in conjunction with `--create' (`-c'). To perform any other operation on a multi-volume archive, specify `--multi-volume' (`-M') in conjunction with that operation. `--info-script=PROGRAM-FILE' `-F PROGRAM-FILE' Creates a multi-volume archive via a script. Used in conjunction with `--create' (`-c'). Beware that there is _no_ real standard about the proper way, for a `tar' archive, to span volume boundaries. If you have a multi-volume created by some vendor's `tar', there is almost no chance you could read all the volumes with GNU `tar'. The converse is also true: you may not expect multi-volume archives created by GNU `tar' to be fully recovered by vendor's `tar'. Since there is little chance that, in mixed system configurations, some vendor's `tar' will work on another vendor's machine, and there is a great chance that GNU `tar' will work on most of them, your best bet is to install GNU `tar' on all machines between which you know exchange of files is possible. File: tar.info, Node: Tape Files, Prev: Multi-Volume Archives, Up: Using Multiple Tapes Tape Files ---------- _(This message will disappear, once this node revised.)_ To give the archive a name which will be recorded in it, use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option. This will write a special block identifying VOLUME-LABEL as the name of the archive to the front of the archive which will be displayed when the archive is listed with `--list' (`-t'). If you are creating a multi-volume archive with `--multi-volume' (`-M') (), then the volume label will have `Volume NNN' appended to the name you give, where NNN is the number of the volume of the archive. (If you use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option when reading an archive, it checks to make sure the label on the tape matches the one you give. *Note label::. When `tar' writes an archive to tape, it creates a single tape file. If multiple archives are written to the same tape, one after the other, they each get written as separate tape files. When extracting, it is necessary to position the tape at the right place before running `tar'. To do this, use the `mt' command. For more information on the `mt' command and on the organization of tapes into a sequence of tape files, see *Note mt::. People seem to often do: --label="SOME-PREFIX `date +SOME-FORMAT`" or such, for pushing a common date in all volumes or an archive set. File: tar.info, Node: label, Next: verify, Prev: Using Multiple Tapes, Up: Media Including a Label in the Archive ================================ _(This message will disappear, once this node revised.)_ `-V NAME' `--label=NAME' Create archive with volume name NAME. This option causes `tar' to write out a "volume header" at the beginning of the archive. If `--multi-volume' (`-M') is used, each volume of the archive will have a volume header of `NAME Volume N', where N is 1 for the first volume, 2 for the next, and so on. To avoid problems caused by misplaced paper labels on the archive media, you can include a "label" entry--an archive member which contains the name of the archive--in the archive itself. Use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option in conjunction with the `--create' (`-c') operation to include a label entry in the archive as it is being created. If you create an archive using both `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') and `--multi-volume' (`-M'), each volume of the archive will have an archive label of the form `ARCHIVE-LABEL Volume N', where N is 1 for the first volume, 2 for the next, and so on. , for information on creating multiple volume archives. If you list or extract an archive using `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL'), `tar' will print an error if the archive label doesn't match the ARCHIVE-LABEL specified, and will then not list nor extract the archive. In those cases, ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which must match the actual magnetic volume label. *Note exclude::, for a precise description of how match is attempted(1). If the switch `--multi-volume' (`-M') is being used, the volume label matcher will also suffix ARCHIVE-LABEL by ` Volume [1-9]*' if the initial match fails, before giving up. Since the volume numbering is automatically added in labels at creation time, it sounded logical to equally help the user taking care of it when the archive is being read. The `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') was once called `--volume', but is not available under that name anymore. To find out an archive's label entry (or to find out if an archive has a label at all), use `tar --list --verbose'. `tar' will print the label first, and then print archive member information, as in the example below: $ tar --verbose --list --file=iamanarchive V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-rw-rw- ringo user 40 1990-05-21 13:30 iamafilename `--label=ARCHIVE-LABEL' `-V ARCHIVE-LABEL' Includes an "archive-label" at the beginning of the archive when the archive is being created, when used in conjunction with the `--create' (`-c') option. Checks to make sure the archive label matches the one specified (when used in conjunction with the `--extract' (`--get', `-x') option. To get a common information on all tapes of a series, use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option. For having this information different in each series created through a single script used on a regular basis, just manage to get some date string as part of the label. For example: $ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`" $ tar --create --file=/dev/tape --multi-volume \ --volume="Daily backup for `date +%Y-%m-%d`" Also note that each label has its own date and time, which corresponds to when GNU `tar' initially attempted to write it, often soon after the operator launches `tar' or types the carriage return telling that the next tape is ready. Comparing date labels does give an idea of tape throughput only if the delays for rewinding tapes and the operator switching them were negligible, which is ususally not the case. ---------- Footnotes ---------- (1) Previous versions of `tar' used full regular expression matching, or before that, only exact string matching, instead of wildcard matchers. We decided for the sake of simplicity to use a uniform matching device through `tar'. File: tar.info, Node: verify, Next: Write Protection, Prev: label, Up: Media Verifying Data as It is Stored ============================== `-W' `--verify' Attempt to verify the archive after writing. This option causes `tar' to verify the archive after writing it. Each volume is checked after it is written, and any discrepancies are recorded on the standard error output. Verification requires that the archive be on a back-space-able medium. This means pipes, some cartridge tape drives, and some other devices cannot be verified. You can insure the accuracy of an archive by comparing files in the system with archive members. `tar' can compare an archive to the file system as the archive is being written, to verify a write operation, or can compare a previously written archive, to insure that it is up to date. To check for discrepancies in an archive immediately after it is written, use the `--verify' (`-W') option in conjunction with the `--create' (`-c') operation. When this option is specified, `tar' checks archive members against their counterparts in the file system, and reports discrepancies on the standard error. In multi-volume archives, each volume is verified after it is written, before the next volume is written. To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified. One can explicitely compare an already made archive with the file system by using the `--compare' (`--diff', `-d') option, instead of using the more automatic `--verify' (`-W') option. *Note compare::. Note that these two options have a slightly different intent. The `--compare' (`--diff', `-d') option how identical are the logical contents of some archive with what is on your disks, while the `--verify' (`-W') option is really for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the `--verify' (`-W') operation, `tar' tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization undisturbed for the `--compare' (`--diff', `-d') option. If you nevertheless use `--compare' (`--diff', `-d') for media verification, you may have to defeat the in-memory cache yourself, maybe by opening and reclosing the door latch of your recording unit, forcing some doubt in your operating system about the fact this is really the same volume as the one just written or read. The `--verify' (`-W') option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned. File: tar.info, Node: Write Protection, Prev: verify, Up: Media Write Protection ================ Almost all tapes and diskettes, and in a few rare cases, even disks can be "write protected", to protect data on them from being changed. Once an archive is written, you should write protect the media to prevent the archive from being accidently overwritten or deleted. (This will protect the archive from being changed with a tape or floppy drive--it will not protect it from magnet fields or other physical hazards). The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.