git-bisect.txt   [plain text]


git-bisect - Find by binary search the change that introduced a bug

'git bisect' <subcommand> <options>

The command takes various subcommands, and different options depending
on the subcommand:

 git bisect help
 git bisect start [--no-checkout] [<bad> [<good>...]] [--] [<paths>...]
 git bisect bad [<rev>]
 git bisect good [<rev>...]
 git bisect skip [(<rev>|<range>)...]
 git bisect reset [<commit>]
 git bisect visualize
 git bisect replay <logfile>
 git bisect log
 git bisect run <cmd>...

This command uses 'git rev-list --bisect' to help drive the
binary search process to find which change introduced a bug, given an
old "good" commit object name and a later "bad" commit object name.

Getting help

Use "git bisect" to get a short usage description, and "git bisect
help" or "git bisect -h" to get a long usage description.

Basic bisect commands: start, bad, good

Using the Linux kernel tree as an example, basic use of the bisect
command is as follows:

$ git bisect start
$ git bisect bad                 # Current version is bad
$ git bisect good v2.6.13-rc2    # v2.6.13-rc2 was the last version
                                 # tested that was good

When you have specified at least one bad and one good version, the
command bisects the revision tree and outputs something similar to
the following:

Bisecting: 675 revisions left to test after this

The state in the middle of the set of revisions is then checked out.
You would now compile that kernel and boot it. If the booted kernel
works correctly, you would then issue the following command:

$ git bisect good			# this one is good

The output of this command would be something similar to the following:

Bisecting: 337 revisions left to test after this

You keep repeating this process, compiling the tree, testing it, and
depending on whether it is good or bad issuing the command "git bisect good"
or "git bisect bad" to ask for the next bisection.

Eventually there will be no more revisions left to bisect, and you
will have been left with the first bad kernel revision in "refs/bisect/bad".

Bisect reset

After a bisect session, to clean up the bisection state and return to
the original HEAD (i.e., to quit bisecting), issue the following command:

$ git bisect reset

By default, this will return your tree to the commit that was checked
out before `git bisect start`.  (A new `git bisect start` will also do
that, as it cleans up the old bisection state.)

With an optional argument, you can return to a different commit

$ git bisect reset <commit>

For example, `git bisect reset HEAD` will leave you on the current
bisection commit and avoid switching commits at all, while `git bisect
reset bisect/bad` will check out the first bad revision.

Bisect visualize

To see the currently remaining suspects in 'gitk', issue the following
command during the bisection process:

$ git bisect visualize

`view` may also be used as a synonym for `visualize`.

If the 'DISPLAY' environment variable is not set, 'git log' is used
instead.  You can also give command line options such as `-p` and

$ git bisect view --stat

Bisect log and bisect replay

After having marked revisions as good or bad, issue the following
command to show what has been done so far:

$ git bisect log

If you discover that you made a mistake in specifying the status of a
revision, you can save the output of this command to a file, edit it to
remove the incorrect entries, and then issue the following commands to
return to a corrected state:

$ git bisect reset
$ git bisect replay that-file

Avoiding testing a commit

If, in the middle of a bisect session, you know that the next suggested
revision is not a good one to test (e.g. the change the commit
introduces is known not to work in your environment and you know it
does not have anything to do with the bug you are chasing), you may
want to find a nearby commit and try that instead.

For example:

$ git bisect good/bad			# previous round was good or bad.
Bisecting: 337 revisions left to test after this
$ git bisect visualize			# oops, that is uninteresting.
$ git reset --hard HEAD~3		# try 3 revisions before what
					# was suggested

Then compile and test the chosen revision, and afterwards mark
the revision as good or bad in the usual manner.

Bisect skip

Instead of choosing by yourself a nearby commit, you can ask Git
to do it for you by issuing the command:

$ git bisect skip                 # Current version cannot be tested

But Git may eventually be unable to tell the first bad commit among
a bad commit and one or more skipped commits.

You can even skip a range of commits, instead of just one commit,
using the "'<commit1>'..'<commit2>'" notation. For example:

$ git bisect skip v2.5..v2.6

This tells the bisect process that no commit after `v2.5`, up to and
including `v2.6`, should be tested.

Note that if you also want to skip the first commit of the range you
would issue the command:

$ git bisect skip v2.5 v2.5..v2.6

This tells the bisect process that the commits between `v2.5` included
and `v2.6` included should be skipped.

Cutting down bisection by giving more parameters to bisect start

You can further cut down the number of trials, if you know what part of
the tree is involved in the problem you are tracking down, by specifying
path parameters when issuing the `bisect start` command:

$ git bisect start -- arch/i386 include/asm-i386

If you know beforehand more than one good commit, you can narrow the
bisect space down by specifying all of the good commits immediately after
the bad commit when issuing the `bisect start` command:

$ git bisect start v2.6.20-rc6 v2.6.20-rc4 v2.6.20-rc1 --
                   # v2.6.20-rc6 is bad
                   # v2.6.20-rc4 and v2.6.20-rc1 are good

Bisect run

If you have a script that can tell if the current source code is good
or bad, you can bisect by issuing the command:

$ git bisect run my_script arguments

Note that the script (`my_script` in the above example) should
exit with code 0 if the current source code is good, and exit with a
code between 1 and 127 (inclusive), except 125, if the current
source code is bad.

Any other exit code will abort the bisect process. It should be noted
that a program that terminates via "exit(-1)" leaves $? = 255, (see the
exit(3) manual page), as the value is chopped with "& 0377".

The special exit code 125 should be used when the current source code
cannot be tested. If the script exits with this code, the current
revision will be skipped (see `git bisect skip` above). 125 was chosen
as the highest sensible value to use for this purpose, because 126 and 127
are used by POSIX shells to signal specific error status (127 is for
command not found, 126 is for command found but not executable---these
details do not matter, as they are normal errors in the script, as far as
"bisect run" is concerned).

You may often find that during a bisect session you want to have
temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a
header file, or "revision that does not have this commit needs this
patch applied to work around another problem this bisection is not
interested in") applied to the revision being tested.

To cope with such a situation, after the inner 'git bisect' finds the
next revision to test, the script can apply the patch
before compiling, run the real test, and afterwards decide if the
revision (possibly with the needed patch) passed the test and then
rewind the tree to the pristine state.  Finally the script should exit
with the status of the real test to let the "git bisect run" command loop
determine the eventual outcome of the bisect session.

Do not checkout the new working tree at each iteration of the bisection
process. Instead just update a special reference named 'BISECT_HEAD' to make
it point to the commit that should be tested.
This option may be useful when the test you would perform in each step
does not require a checked out tree.
If the repository is bare, `--no-checkout` is assumed.


* Automatically bisect a broken build between v1.2 and HEAD:
$ git bisect start HEAD v1.2 --      # HEAD is bad, v1.2 is good
$ git bisect run make                # "make" builds the app
$ git bisect reset                   # quit the bisect session

* Automatically bisect a test failure between origin and HEAD:
$ git bisect start HEAD origin --    # HEAD is bad, origin is good
$ git bisect run make test           # "make test" builds and tests
$ git bisect reset                   # quit the bisect session

* Automatically bisect a broken test case:
$ cat ~/
make || exit 125                     # this skips broken builds
~/                 # does the test case pass?
$ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
$ git bisect run ~/
$ git bisect reset                   # quit the bisect session
Here we use a "" custom script. In this script, if "make"
fails, we skip the current commit.
"" should "exit 0" if the test case passes,
and "exit 1" otherwise.
It is safer if both "" and "" are
outside the repository to prevent interactions between the bisect,
make and test processes and the scripts.

* Automatically bisect with temporary modifications (hot-fix):
$ cat ~/

# tweak the working tree by merging the hot-fix branch
# and then attempt a build
if	git merge --no-commit hot-fix &&
	# run project specific test and report its status
	# tell the caller this is untestable

# undo the tweak to allow clean flipping to the next commit
git reset --hard

# return control
exit $status
This applies modifications from a hot-fix branch before each test run,
e.g. in case your build or test environment changed so that older
revisions may need a fix which newer ones have already. (Make sure the
hot-fix branch is based off a commit which is contained in all revisions
which you are bisecting, so that the merge does not pull in too much, or
use `git cherry-pick` instead of `git merge`.)

* Automatically bisect a broken test case:
$ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
$ git bisect run sh -c "make || exit 125; ~/"
$ git bisect reset                   # quit the bisect session
This shows that you can do without a run script if you write the test
on a single line.

* Locate a good region of the object graph in a damaged repository
$ git bisect start HEAD <known-good-commit> [ <boundary-commit> ... ] --no-checkout
$ git bisect run sh -c '
	GOOD=$(git for-each-ref "--format=%(objectname)" refs/bisect/good-*) &&
	git rev-list --objects BISECT_HEAD --not $GOOD >tmp.$$ &&
	git pack-objects --stdout >/dev/null <tmp.$$
	rm -f tmp.$$
	test $rc = 0'

$ git bisect reset                   # quit the bisect session
In this case, when 'git bisect run' finishes, bisect/bad will refer to a commit that
has at least one parent whose reachable graph is fully traversable in the sense
required by 'git pack objects'.

link:git-bisect-lk2009.html[Fighting regressions with git bisect],

Part of the linkgit:git[1] suite