The command takes various subcommands, and different options
depending on the subcommand:
git bisect start [--term-{new,bad}=<term> --term-{old,good}=<term>]
[--no-checkout] [--first-parent] [<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.
