# git-bisect

## 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-(bad|new)=<term-new> --term-(good|old)=<term-old>]
                                [--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<pathspec>...]
               git bisect (bad|new|<term-new>) [<rev>]
               git bisect (good|old|<term-old>) [<rev>...]
               git bisect terms [--term-(good|old) | --term-(bad|new)]
               git bisect skip [(<rev>|<range>)...]
               git bisect reset [<commit>]
               git bisect (visualize|view)
               git bisect replay <logfile>
               git bisect log
               git bisect run <cmd> [<arg>...]
               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 term with **git bisect terms --term-old** or
           **git bisect terms --term-good** ; **git bisect terms --term-new** and **git**
           **bisect terms --term-bad** can be used to learn how to call the
           commits more recent than the sought change.
    
           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
    
           Git detects a graphical environment through various environment
           variables: **DISPLAY** , which is set in X Window System environments
           on Unix systems. **SESSIONNAME** , which is set under Cygwin in
           interactive desktop sessions. **MSYSTEM** , which is set under Msys2
           and Git for Windows. **SECURITYSESSIONID** , which may be set on macOS
           in interactive desktop sessions.
    
           If none of these environment variables is 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 pathspec 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 the 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.
    
           --first-parent
               Follow only the first parent commit upon seeing a merge
               commit.
    
               In detecting regressions introduced through the merging of a
               branch, the merge commit will be identified as introduction of
               the bug and its ancestors will be ignored.
    
               This option is particularly useful in avoiding false positives
               when a merged branch contained broken or non-buildable
               commits, but the merge itself was OK.
    


## 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 --no-ff 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.
    


## GIT



           Part of the git(1) suite
    


## NOTES



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


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