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git-bisect - Use binary search to find the commit that introduced a bug
git bisect <subcommand> <options>
The command takes various subcommands, and different options depending on the subcommand:
git bisect start [--no-checkout] [<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>... 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
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
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,
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
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
refs/bisect/bad will be left pointing at that commit.
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
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>
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.
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
If the DISPLAY environment variable is not set, git log is used
instead. You can also give command-line options such as
$ 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 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.
$ 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.
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
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
v2.6 (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
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
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.
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,
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.shcustom script. In this script, if
makefails, we skip the current commit.
exit 0if the test case passes, and
It is safer if both
check_test_case.share 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-pickinstead of
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.
git bisect to get a short usage description, and
git bisect -h to get a long usage description.
Part of the git suite