Documentation/git-bisect.txt - jrn/git - Git at Google

 git-bisect(1)
 =============

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


 SYNOPSIS
 --------
 [verse]
 'git bisect' <subcommand> <options>

 DESCRIPTION
 -----------
 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
 instead:

 ------------------------------------------------
 $ 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
 `--stat`.

 ------------
 $ 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.

 OPTIONS
 -------
 --no-checkout::
 +
 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.

 EXAMPLES
 --------

 * 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 ~/test.sh
 #!/bin/sh
 make || exit 125                     # this skips broken builds
 ~/check_test_case.sh                 # does the test case pass?
 $ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
 $ git bisect run ~/test.sh
 $ git bisect reset                   # quit the bisect session
 ------------
 +
 Here we use a "test.sh" custom script. In this script, if "make"
 fails, we skip the current commit.
 "check_test_case.sh" should "exit 0" if the test case passes,
 and "exit 1" otherwise.
 +
 It is safer if both "test.sh" and "check_test_case.sh" 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 ~/test.sh
 #!/bin/sh

 # tweak the working tree by merging the hot-fix branch
 # and then attempt a build
 if	git merge --no-commit hot-fix &&
 	make
 then
 	# run project specific test and report its status
 	~/check_test_case.sh
 	status=$?
 else
 	# tell the caller this is untestable
 	status=125
 fi

 # 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; ~/check_test_case.sh"
 $ 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.$$ &&
 	git pack-objects --stdout >/dev/null <tmp.$$
 	rc=$?
 	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'.


 SEE ALSO
 --------
 link:git-bisect-lk2009.html[Fighting regressions with git bisect],
 linkgit:git-blame[1].

 GIT
 ---
 Part of the linkgit:git[1] suite
	git-bisect(1)
	=============

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


	SYNOPSIS
	--------
	[verse]
	'git bisect' <subcommand> <options>

	DESCRIPTION
	-----------
	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
	instead:

	------------------------------------------------
	$ 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
	`--stat`.

	------------
	$ 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.

	OPTIONS
	-------
	--no-checkout::
	+
	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.

	EXAMPLES
	--------

	* 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 ~/test.sh
	#!/bin/sh
	make \|\| exit 125 # this skips broken builds
	~/check_test_case.sh # does the test case pass?
	$ git bisect start HEAD HEAD~10 -- # culprit is among the last 10
	$ git bisect run ~/test.sh
	$ git bisect reset # quit the bisect session
	------------
	+
	Here we use a "test.sh" custom script. In this script, if "make"
	fails, we skip the current commit.
	"check_test_case.sh" should "exit 0" if the test case passes,
	and "exit 1" otherwise.
	+
	It is safer if both "test.sh" and "check_test_case.sh" 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 ~/test.sh
	#!/bin/sh

	# tweak the working tree by merging the hot-fix branch
	# and then attempt a build
	if git merge --no-commit hot-fix &&
	make
	then
	# run project specific test and report its status
	~/check_test_case.sh
	status=$?
	else
	# tell the caller this is untestable
	status=125
	fi

	# 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; ~/check_test_case.sh"
	$ 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.$$ &&
	git pack-objects --stdout >/dev/null <tmp.$$
	rc=$?
	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'.


	SEE ALSO
	--------
	link:git-bisect-lk2009.html[Fighting regressions with git bisect],
	linkgit:git-blame[1].

	GIT
	---
	Part of the linkgit:git[1] suite