GIT-BISECT(1)                                                    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 [<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, 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.


EXAMPLES
       o  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


       o  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


       o  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


          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.

       o  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.)

       o  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"


          This shows that you can do without a run script if you write the
          test on a single line.

SEE ALSO
       Fighting regressions with git bisect: git-bisect-lk2009.html,
       git-blame(1).


GIT
       Part of the git(1) suite




                                                                 GIT-BISECT(1)
