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

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

 NAME
 ----
 git-bisect - Use binary search to find the commit 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 start [--term-{old,good}=<term> --term-{new,bad}=<term>]
 		  [--no-checkout] [<bad> [<good>...]] [--] [<paths>...]
  git bisect (bad|new|<term-new>) [<rev>]
  git bisect (good|old|<term-old>) [<rev>...]
  git bisect terms [--term-good | --term-bad]
  git bisect skip [(<rev>|<range>)...]
  git bisect reset [<commit>]
  git bisect (visualize|view)
  git bisect replay <logfile>
  git bisect log
  git bisect run <cmd>...
  git bisect help

 This command uses a binary search algorithm to find which commit in
 your project's history introduced a bug. You use it by first telling
 it a "bad" commit that is known to contain the bug, and a "good"
 commit that is known to be before the bug was introduced. Then `git
 bisect` picks a commit between those two endpoints and asks you
 whether the selected commit is "good" or "bad". It continues narrowing
 down the range until it finds the exact commit that introduced the
 change.

 In fact, `git bisect` can be used to find the commit that changed
 *any* property of your project; e.g., the commit that fixed a bug, or
 the commit that caused a benchmark's performance to improve. To
 support this more general usage, the terms "old" and "new" can be used
 in place of "good" and "bad", or you can choose your own terms. See
 section "Alternate terms" below for more information.

 Basic bisect commands: start, bad, good
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 As an example, suppose you are trying to find the commit that broke a
 feature that was known to work in version `v2.6.13-rc2` of your
 project. You start a bisect session as follows:

 ------------------------------------------------
 $ git bisect start
 $ git bisect bad                 # Current version is bad
 $ git bisect good v2.6.13-rc2    # v2.6.13-rc2 is known to be good
 ------------------------------------------------

 Once you have specified at least one bad and one good commit, `git
 bisect` selects a commit in the middle of that range of history,
 checks it out, and outputs something similar to the following:

 ------------------------------------------------
 Bisecting: 675 revisions left to test after this (roughly 10 steps)
 ------------------------------------------------

 You should now compile the checked-out version and test it. If that
 version works correctly, type

 ------------------------------------------------
 $ git bisect good
 ------------------------------------------------

 If that version is broken, type

 ------------------------------------------------
 $ git bisect bad
 ------------------------------------------------

 Then `git bisect` will respond with something like

 ------------------------------------------------
 Bisecting: 337 revisions left to test after this (roughly 9 steps)
 ------------------------------------------------

 Keep repeating the process: compile the tree, test it, and depending
 on whether it is good or bad run `git bisect good` or `git bisect bad`
 to ask for the next commit that needs testing.

 Eventually there will be no more revisions left to inspect, and the
 command will print out a description of the first bad commit. The
 reference `refs/bisect/bad` will be left pointing at that commit.


 Bisect reset
 ~~~~~~~~~~~~

 After a bisect session, to clean up the bisection state and return to
 the original HEAD, 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 bisect/bad` will check out the first
 bad revision, while `git bisect reset HEAD` will leave you on the
 current bisection commit and avoid switching commits at all.


 Alternate terms
 ~~~~~~~~~~~~~~~

 Sometimes you are not looking for the commit that introduced a
 breakage, but rather for a commit that caused a change between some
 other "old" state and "new" state. For example, you might be looking
 for the commit that introduced a particular fix. Or you might be
 looking for the first commit in which the source-code filenames were
 finally all converted to your company's naming standard. Or whatever.

 In such cases it can be very confusing to use the terms "good" and
 "bad" to refer to "the state before the change" and "the state after
 the change". So instead, you can use the terms "old" and "new",
 respectively, in place of "good" and "bad". (But note that you cannot
 mix "good" and "bad" with "old" and "new" in a single session.)

 In this more general usage, you provide `git bisect` with a "new"
 commit that has some property and an "old" commit that doesn't have that
 property. Each time `git bisect` checks out a commit, you test if that
 commit has the property. If it does, mark the commit as "new";
 otherwise, mark it as "old". When the bisection is done, `git bisect`
 will report which commit introduced the property.

 To use "old" and "new" instead of "good" and bad, you must run `git
 bisect start` without commits as argument and then run the following
 commands to add the commits:

 ------------------------------------------------
 git bisect old [<rev>]
 ------------------------------------------------

 to indicate that a commit was before the sought change, or

 ------------------------------------------------
 git bisect new [<rev>...]
 ------------------------------------------------

 to indicate that it was after.

 To get a reminder of the currently used terms, use

 ------------------------------------------------
 git bisect terms
 ------------------------------------------------

 You can get just the old (respectively new) term with `git bisect terms
 --term-old` or `git bisect terms --term-good`.

 If you would like to use your own terms instead of "bad"/"good" or
 "new"/"old", you can choose any names you like (except existing bisect
 subcommands like `reset`, `start`, ...) by starting the
 bisection using

 ------------------------------------------------
 git bisect start --term-old <term-old> --term-new <term-new>
 ------------------------------------------------

 For example, if you are looking for a commit that introduced a
 performance regression, you might use

 ------------------------------------------------
 git bisect start --term-old fast --term-new slow
 ------------------------------------------------

 Or if you are looking for the commit that fixed a bug, you might use

 ------------------------------------------------
 git bisect start --term-new fixed --term-old broken
 ------------------------------------------------

 Then, use `git bisect <term-old>` and `git bisect <term-new>` instead
 of `git bisect good` and `git bisect bad` to mark commits.

 Bisect visualize/view
 ~~~~~~~~~~~~~~~~~~~~~

 To see the currently remaining suspects in 'gitk', issue the following
 command during the bisection process (the subcommand `view` can be used
 as an alternative to `visualize`):

 ------------
 $ git bisect 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 visualize --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 suggested
 revision is not a good one to test (e.g. it fails to build and you
 know that the failure does not have anything to do with the bug you
 are chasing), you can manually select a nearby commit and test that
 one instead.

 For example:

 ------------
 $ git bisect good/bad			# previous round was good or bad.
 Bisecting: 337 revisions left to test after this (roughly 9 steps)
 $ 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 a nearby commit by yourself, you can ask Git to do
 it for you by issuing the command:

 ------------
 $ git bisect skip                 # Current version cannot be tested
 ------------

 However, if you skip a commit adjacent to the one you are looking for,
 Git will be unable to tell exactly which of those commits was the
 first bad one.

 You can also skip a range of commits, instead of just one commit,
 using range 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` and
 `v2.6` (inclusive) 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/old, and exit with a
 code between 1 and 127 (inclusive), except 125, if the current source
 code is bad/new.

 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'.

 * Look for a fix instead of a regression in the code
 +
 ------------
 $ git bisect start
 $ git bisect new HEAD    # current commit is marked as new
 $ git bisect old HEAD~10 # the tenth commit from now is marked as old
 ------------
 +
 or:
 ------------
 $ git bisect start --term-old broken --term-new fixed
 $ git bisect fixed
 $ git bisect broken HEAD~10
 ------------

 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.

 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 - Use binary search to find the commit 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 start [--term-{old,good}=<term> --term-{new,bad}=<term>]
	[--no-checkout] [<bad> [<good>...]] [--] [<paths>...]
	git bisect (bad\|new\|<term-new>) [<rev>]
	git bisect (good\|old\|<term-old>) [<rev>...]
	git bisect terms [--term-good \| --term-bad]
	git bisect skip [(<rev>\|<range>)...]
	git bisect reset [<commit>]
	git bisect (visualize\|view)
	git bisect replay <logfile>
	git bisect log
	git bisect run <cmd>...
	git bisect help

	This command uses a binary search algorithm to find which commit in
	your project's history introduced a bug. You use it by first telling
	it a "bad" commit that is known to contain the bug, and a "good"
	commit that is known to be before the bug was introduced. Then `git
	bisect` picks a commit between those two endpoints and asks you
	whether the selected commit is "good" or "bad". It continues narrowing
	down the range until it finds the exact commit that introduced the
	change.

	In fact, `git bisect` can be used to find the commit that changed
	any property of your project; e.g., the commit that fixed a bug, or
	the commit that caused a benchmark's performance to improve. To
	support this more general usage, the terms "old" and "new" can be used
	in place of "good" and "bad", or you can choose your own terms. See
	section "Alternate terms" below for more information.

	Basic bisect commands: start, bad, good
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

	As an example, suppose you are trying to find the commit that broke a
	feature that was known to work in version `v2.6.13-rc2` of your
	project. You start a bisect session as follows:

	------------------------------------------------
	$ git bisect start
	$ git bisect bad # Current version is bad
	$ git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good
	------------------------------------------------

	Once you have specified at least one bad and one good commit, `git
	bisect` selects a commit in the middle of that range of history,
	checks it out, and outputs something similar to the following:

	------------------------------------------------
	Bisecting: 675 revisions left to test after this (roughly 10 steps)
	------------------------------------------------

	You should now compile the checked-out version and test it. If that
	version works correctly, type

	------------------------------------------------
	$ git bisect good
	------------------------------------------------

	If that version is broken, type

	------------------------------------------------
	$ git bisect bad
	------------------------------------------------

	Then `git bisect` will respond with something like

	------------------------------------------------
	Bisecting: 337 revisions left to test after this (roughly 9 steps)
	------------------------------------------------

	Keep repeating the process: compile the tree, test it, and depending
	on whether it is good or bad run `git bisect good` or `git bisect bad`
	to ask for the next commit that needs testing.

	Eventually there will be no more revisions left to inspect, and the
	command will print out a description of the first bad commit. The
	reference `refs/bisect/bad` will be left pointing at that commit.


	Bisect reset
	~~~~~~~~~~~~

	After a bisect session, to clean up the bisection state and return to
	the original HEAD, 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 bisect/bad` will check out the first
	bad revision, while `git bisect reset HEAD` will leave you on the
	current bisection commit and avoid switching commits at all.


	Alternate terms
	~~~~~~~~~~~~~~~

	Sometimes you are not looking for the commit that introduced a
	breakage, but rather for a commit that caused a change between some
	other "old" state and "new" state. For example, you might be looking
	for the commit that introduced a particular fix. Or you might be
	looking for the first commit in which the source-code filenames were
	finally all converted to your company's naming standard. Or whatever.

	In such cases it can be very confusing to use the terms "good" and
	"bad" to refer to "the state before the change" and "the state after
	the change". So instead, you can use the terms "old" and "new",
	respectively, in place of "good" and "bad". (But note that you cannot
	mix "good" and "bad" with "old" and "new" in a single session.)

	In this more general usage, you provide `git bisect` with a "new"
	commit that has some property and an "old" commit that doesn't have that
	property. Each time `git bisect` checks out a commit, you test if that
	commit has the property. If it does, mark the commit as "new";
	otherwise, mark it as "old". When the bisection is done, `git bisect`
	will report which commit introduced the property.

	To use "old" and "new" instead of "good" and bad, you must run `git
	bisect start` without commits as argument and then run the following
	commands to add the commits:

	------------------------------------------------
	git bisect old [<rev>]
	------------------------------------------------

	to indicate that a commit was before the sought change, or

	------------------------------------------------
	git bisect new [<rev>...]
	------------------------------------------------

	to indicate that it was after.

	To get a reminder of the currently used terms, use

	------------------------------------------------
	git bisect terms
	------------------------------------------------

	You can get just the old (respectively new) term with `git bisect terms
	--term-old` or `git bisect terms --term-good`.

	If you would like to use your own terms instead of "bad"/"good" or
	"new"/"old", you can choose any names you like (except existing bisect
	subcommands like `reset`, `start`, ...) by starting the
	bisection using

	------------------------------------------------
	git bisect start --term-old <term-old> --term-new <term-new>
	------------------------------------------------

	For example, if you are looking for a commit that introduced a
	performance regression, you might use

	------------------------------------------------
	git bisect start --term-old fast --term-new slow
	------------------------------------------------

	Or if you are looking for the commit that fixed a bug, you might use

	------------------------------------------------
	git bisect start --term-new fixed --term-old broken
	------------------------------------------------

	Then, use `git bisect <term-old>` and `git bisect <term-new>` instead
	of `git bisect good` and `git bisect bad` to mark commits.

	Bisect visualize/view
	~~~~~~~~~~~~~~~~~~~~~

	To see the currently remaining suspects in 'gitk', issue the following
	command during the bisection process (the subcommand `view` can be used
	as an alternative to `visualize`):

	------------
	$ git bisect 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 visualize --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 suggested
	revision is not a good one to test (e.g. it fails to build and you
	know that the failure does not have anything to do with the bug you
	are chasing), you can manually select a nearby commit and test that
	one instead.

	For example:

	------------
	$ git bisect good/bad # previous round was good or bad.
	Bisecting: 337 revisions left to test after this (roughly 9 steps)
	$ 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 a nearby commit by yourself, you can ask Git to do
	it for you by issuing the command:

	------------
	$ git bisect skip # Current version cannot be tested
	------------

	However, if you skip a commit adjacent to the one you are looking for,
	Git will be unable to tell exactly which of those commits was the
	first bad one.

	You can also skip a range of commits, instead of just one commit,
	using range 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` and
	`v2.6` (inclusive) 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/old, and exit with a
	code between 1 and 127 (inclusive), except 125, if the current source
	code is bad/new.

	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'.

	* Look for a fix instead of a regression in the code
	+
	------------
	$ git bisect start
	$ git bisect new HEAD # current commit is marked as new
	$ git bisect old HEAD~10 # the tenth commit from now is marked as old
	------------
	+
	or:
	------------
	$ git bisect start --term-old broken --term-new fixed
	$ git bisect fixed
	$ git bisect broken HEAD~10
	------------

	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.

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

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