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NAME

       git-bisect - Use binary search to find the commit that introduced a bug

SYNOPSIS

       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

       Fighting regressions with git bisect[1], git-blame(1).

GIT

       Part of the git(1) suite

NOTES

        1. Fighting regressions with git bisect
           file:///usr/share/doc/git/html/git-bisect-lk2009.html