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git-bisect - Find by binary search the change that introduced a bug
'git bisect' <subcommand> <options>
The command takes various subcommands, and different options depending on the subcommand:
git bisect help 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>...
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.
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".
After a bisect session, to clean up the bisection state and return to the original HEAD (i.e., to quit bisecting), 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 HEAD will leave you on the current
bisection commit and avoid switching commits at all, while
reset bisect/bad will check out the first bad revision.
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 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.
$ 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.
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
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 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
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.
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.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-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.
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