path: root/Documentation/git-bisect.txt

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

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


SYNOPSIS
--------
'git bisect' <subcommand> <options>

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

 git bisect help
 git bisect start [<bad> [<good>...]] [--] [<paths>...]
 git bisect bad [<rev>]
 git bisect good [<rev>...]
 git bisect skip [(<rev>|<range>)...]
 git bisect reset [<branch>]
 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:

------------------------------------------------
Bisecting: 675 revisions left to test after this
------------------------------------------------

and then checks out the state in the middle. 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
------------------------------------------------

which would then output something similar to:

------------------------------------------------
Bisecting: 337 revisions left to test after this
------------------------------------------------

and you continue along, compiling that one, 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
~~~~~~~~~~~~

To return to the original head after a bisect session, you issue the
command:

------------------------------------------------
$ git bisect reset
------------------------------------------------

This resets the tree to the original branch instead of being on the
bisection commit ("git bisect start" will also do that, as it resets
the bisection state).

Bisect visualize
~~~~~~~~~~~~~~~~

During the bisection process, you issue the command:

------------
$ git bisect visualize
------------

to see the currently remaining suspects in 'gitk'.  `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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The good/bad input is logged, and:

------------
$ git bisect log
------------

shows what you have done so far. You can truncate its output somewhere
and save it in a file, and run:

------------
$ git bisect replay that-file
------------

if you find later that you made a mistake specifying revisions as good/bad.

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/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. Afterwards the revision
is marked as good/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 computing the commit to test may be slower afterwards and git may
eventually not be able 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
------------

would mean that no commit between `v2.5` excluded and `v2.6` included
can 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
------------

and the commit pointed to by `v2.5` would also 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, like this:

------------
$ 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
------------

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).

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
to determine the eventual outcome of the bisect session.

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
------------

* Automatically bisect a broken test suite:
+
------------
$ cat ~/test.sh
#!/bin/sh
make || exit 125                   # this skips broken builds
make test                          # "make test" runs the test suite
$ git bisect start v1.3 v1.1 --    # v1.3 is bad, v1.1 is good
$ git bisect run ~/test.sh
------------
+
Here we use a "test.sh" custom script. In this script, if "make"
fails, we skip the current commit.
+
It is safer to use a custom script outside the repository to prevent
interactions between the bisect, make and test processes and the
script.
+
"make test" should "exit 0", if the test suite passes, and
"exit 1" otherwise.

* 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 passes ?
$ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
$ git bisect run ~/test.sh
------------
+
Here "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" scripts are
outside the repository to prevent interactions between the bisect,
make and test processes and the scripts.

Author
------
Written by Linus Torvalds <torvalds@osdl.org>

Documentation
-------------
Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.

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