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510 lines
17 KiB
git-bisect(1) |
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============= |
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NAME |
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---- |
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git-bisect - Use binary search to find the commit that introduced a bug |
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SYNOPSIS |
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-------- |
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[verse] |
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'git bisect' <subcommand> <options> |
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DESCRIPTION |
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----------- |
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The command takes various subcommands, and different options depending |
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on the subcommand: |
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git bisect start [--term-{new,bad}=<term> --term-{old,good}=<term>] |
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[--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<paths>...] |
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git bisect (bad|new|<term-new>) [<rev>] |
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git bisect (good|old|<term-old>) [<rev>...] |
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git bisect terms [--term-good | --term-bad] |
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git bisect skip [(<rev>|<range>)...] |
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git bisect reset [<commit>] |
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git bisect (visualize|view) |
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git bisect replay <logfile> |
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git bisect log |
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git bisect run <cmd>... |
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git bisect help |
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This command uses a binary search algorithm to find which commit in |
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your project's history introduced a bug. You use it by first telling |
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it a "bad" commit that is known to contain the bug, and a "good" |
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commit that is known to be before the bug was introduced. Then `git |
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bisect` picks a commit between those two endpoints and asks you |
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whether the selected commit is "good" or "bad". It continues narrowing |
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down the range until it finds the exact commit that introduced the |
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change. |
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In fact, `git bisect` can be used to find the commit that changed |
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*any* property of your project; e.g., the commit that fixed a bug, or |
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the commit that caused a benchmark's performance to improve. To |
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support this more general usage, the terms "old" and "new" can be used |
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in place of "good" and "bad", or you can choose your own terms. See |
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section "Alternate terms" below for more information. |
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Basic bisect commands: start, bad, good |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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As an example, suppose you are trying to find the commit that broke a |
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feature that was known to work in version `v2.6.13-rc2` of your |
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project. You start a bisect session as follows: |
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------------------------------------------------ |
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$ git bisect start |
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$ git bisect bad # Current version is bad |
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$ git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good |
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------------------------------------------------ |
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Once you have specified at least one bad and one good commit, `git |
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bisect` selects a commit in the middle of that range of history, |
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checks it out, and outputs something similar to the following: |
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------------------------------------------------ |
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Bisecting: 675 revisions left to test after this (roughly 10 steps) |
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------------------------------------------------ |
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You should now compile the checked-out version and test it. If that |
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version works correctly, type |
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------------------------------------------------ |
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$ git bisect good |
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------------------------------------------------ |
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If that version is broken, type |
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------------------------------------------------ |
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$ git bisect bad |
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------------------------------------------------ |
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Then `git bisect` will respond with something like |
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------------------------------------------------ |
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Bisecting: 337 revisions left to test after this (roughly 9 steps) |
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------------------------------------------------ |
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Keep repeating the process: compile the tree, test it, and depending |
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on whether it is good or bad run `git bisect good` or `git bisect bad` |
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to ask for the next commit that needs testing. |
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Eventually there will be no more revisions left to inspect, and the |
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command will print out a description of the first bad commit. The |
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reference `refs/bisect/bad` will be left pointing at that commit. |
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Bisect reset |
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~~~~~~~~~~~~ |
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After a bisect session, to clean up the bisection state and return to |
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the original HEAD, issue the following command: |
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------------------------------------------------ |
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$ git bisect reset |
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------------------------------------------------ |
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By default, this will return your tree to the commit that was checked |
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out before `git bisect start`. (A new `git bisect start` will also do |
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that, as it cleans up the old bisection state.) |
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With an optional argument, you can return to a different commit |
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instead: |
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------------------------------------------------ |
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$ git bisect reset <commit> |
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------------------------------------------------ |
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For example, `git bisect reset bisect/bad` will check out the first |
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bad revision, while `git bisect reset HEAD` will leave you on the |
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current bisection commit and avoid switching commits at all. |
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Alternate terms |
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~~~~~~~~~~~~~~~ |
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Sometimes you are not looking for the commit that introduced a |
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breakage, but rather for a commit that caused a change between some |
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other "old" state and "new" state. For example, you might be looking |
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for the commit that introduced a particular fix. Or you might be |
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looking for the first commit in which the source-code filenames were |
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finally all converted to your company's naming standard. Or whatever. |
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In such cases it can be very confusing to use the terms "good" and |
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"bad" to refer to "the state before the change" and "the state after |
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the change". So instead, you can use the terms "old" and "new", |
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respectively, in place of "good" and "bad". (But note that you cannot |
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mix "good" and "bad" with "old" and "new" in a single session.) |
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In this more general usage, you provide `git bisect` with a "new" |
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commit that has some property and an "old" commit that doesn't have that |
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property. Each time `git bisect` checks out a commit, you test if that |
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commit has the property. If it does, mark the commit as "new"; |
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otherwise, mark it as "old". When the bisection is done, `git bisect` |
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will report which commit introduced the property. |
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To use "old" and "new" instead of "good" and bad, you must run `git |
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bisect start` without commits as argument and then run the following |
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commands to add the commits: |
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------------------------------------------------ |
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git bisect old [<rev>] |
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------------------------------------------------ |
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to indicate that a commit was before the sought change, or |
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------------------------------------------------ |
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git bisect new [<rev>...] |
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------------------------------------------------ |
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to indicate that it was after. |
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To get a reminder of the currently used terms, use |
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------------------------------------------------ |
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git bisect terms |
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------------------------------------------------ |
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You can get just the old (respectively new) term with `git bisect terms |
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--term-old` or `git bisect terms --term-good`. |
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If you would like to use your own terms instead of "bad"/"good" or |
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"new"/"old", you can choose any names you like (except existing bisect |
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subcommands like `reset`, `start`, ...) by starting the |
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bisection using |
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------------------------------------------------ |
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git bisect start --term-old <term-old> --term-new <term-new> |
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------------------------------------------------ |
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For example, if you are looking for a commit that introduced a |
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performance regression, you might use |
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------------------------------------------------ |
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git bisect start --term-old fast --term-new slow |
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------------------------------------------------ |
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Or if you are looking for the commit that fixed a bug, you might use |
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------------------------------------------------ |
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git bisect start --term-new fixed --term-old broken |
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------------------------------------------------ |
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Then, use `git bisect <term-old>` and `git bisect <term-new>` instead |
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of `git bisect good` and `git bisect bad` to mark commits. |
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Bisect visualize/view |
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~~~~~~~~~~~~~~~~~~~~~ |
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To see the currently remaining suspects in 'gitk', issue the following |
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command during the bisection process (the subcommand `view` can be used |
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as an alternative to `visualize`): |
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------------ |
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$ git bisect visualize |
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------------ |
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If the `DISPLAY` environment variable is not set, 'git log' is used |
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instead. You can also give command-line options such as `-p` and |
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`--stat`. |
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------------ |
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$ git bisect visualize --stat |
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------------ |
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Bisect log and bisect replay |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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After having marked revisions as good or bad, issue the following |
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command to show what has been done so far: |
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------------ |
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$ git bisect log |
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------------ |
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If you discover that you made a mistake in specifying the status of a |
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revision, you can save the output of this command to a file, edit it to |
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remove the incorrect entries, and then issue the following commands to |
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return to a corrected state: |
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------------ |
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$ git bisect reset |
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$ git bisect replay that-file |
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------------ |
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Avoiding testing a commit |
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~~~~~~~~~~~~~~~~~~~~~~~~~ |
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If, in the middle of a bisect session, you know that the suggested |
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revision is not a good one to test (e.g. it fails to build and you |
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know that the failure does not have anything to do with the bug you |
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are chasing), you can manually select a nearby commit and test that |
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one instead. |
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For example: |
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------------ |
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$ git bisect good/bad # previous round was good or bad. |
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Bisecting: 337 revisions left to test after this (roughly 9 steps) |
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$ git bisect visualize # oops, that is uninteresting. |
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$ git reset --hard HEAD~3 # try 3 revisions before what |
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# was suggested |
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------------ |
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Then compile and test the chosen revision, and afterwards mark |
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the revision as good or bad in the usual manner. |
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Bisect skip |
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~~~~~~~~~~~ |
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Instead of choosing a nearby commit by yourself, you can ask Git to do |
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it for you by issuing the command: |
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------------ |
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$ git bisect skip # Current version cannot be tested |
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------------ |
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However, if you skip a commit adjacent to the one you are looking for, |
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Git will be unable to tell exactly which of those commits was the |
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first bad one. |
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You can also skip a range of commits, instead of just one commit, |
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using range notation. For example: |
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------------ |
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$ git bisect skip v2.5..v2.6 |
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------------ |
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This tells the bisect process that no commit after `v2.5`, up to and |
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including `v2.6`, should be tested. |
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Note that if you also want to skip the first commit of the range you |
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would issue the command: |
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------------ |
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$ git bisect skip v2.5 v2.5..v2.6 |
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------------ |
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This tells the bisect process that the commits between `v2.5` and |
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`v2.6` (inclusive) should be skipped. |
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Cutting down bisection by giving more parameters to bisect start |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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You can further cut down the number of trials, if you know what part of |
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the tree is involved in the problem you are tracking down, by specifying |
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path parameters when issuing the `bisect start` command: |
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------------ |
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$ git bisect start -- arch/i386 include/asm-i386 |
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------------ |
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If you know beforehand more than one good commit, you can narrow the |
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bisect space down by specifying all of the good commits immediately after |
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the bad commit when issuing the `bisect start` command: |
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------------ |
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$ git bisect start v2.6.20-rc6 v2.6.20-rc4 v2.6.20-rc1 -- |
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# v2.6.20-rc6 is bad |
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# v2.6.20-rc4 and v2.6.20-rc1 are good |
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------------ |
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Bisect run |
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~~~~~~~~~~ |
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If you have a script that can tell if the current source code is good |
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or bad, you can bisect by issuing the command: |
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------------ |
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$ git bisect run my_script arguments |
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------------ |
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Note that the script (`my_script` in the above example) should exit |
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with code 0 if the current source code is good/old, and exit with a |
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code between 1 and 127 (inclusive), except 125, if the current source |
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code is bad/new. |
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Any other exit code will abort the bisect process. It should be noted |
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that a program that terminates via `exit(-1)` leaves $? = 255, (see the |
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exit(3) manual page), as the value is chopped with `& 0377`. |
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The special exit code 125 should be used when the current source code |
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cannot be tested. If the script exits with this code, the current |
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revision will be skipped (see `git bisect skip` above). 125 was chosen |
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as the highest sensible value to use for this purpose, because 126 and 127 |
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are used by POSIX shells to signal specific error status (127 is for |
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command not found, 126 is for command found but not executable--these |
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details do not matter, as they are normal errors in the script, as far as |
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`bisect run` is concerned). |
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You may often find that during a bisect session you want to have |
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temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a |
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header file, or "revision that does not have this commit needs this |
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patch applied to work around another problem this bisection is not |
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interested in") applied to the revision being tested. |
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To cope with such a situation, after the inner 'git bisect' finds the |
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next revision to test, the script can apply the patch |
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before compiling, run the real test, and afterwards decide if the |
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revision (possibly with the needed patch) passed the test and then |
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rewind the tree to the pristine state. Finally the script should exit |
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with the status of the real test to let the `git bisect run` command loop |
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determine the eventual outcome of the bisect session. |
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OPTIONS |
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------- |
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--no-checkout:: |
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+ |
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Do not checkout the new working tree at each iteration of the bisection |
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process. Instead just update a special reference named `BISECT_HEAD` to make |
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it point to the commit that should be tested. |
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+ |
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This option may be useful when the test you would perform in each step |
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does not require a checked out tree. |
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+ |
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If the repository is bare, `--no-checkout` is assumed. |
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--first-parent:: |
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+ |
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Follow only the first parent commit upon seeing a merge commit. |
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+ |
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In detecting regressions introduced through the merging of a branch, the merge |
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commit will be identified as introduction of the bug and its ancestors will be |
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ignored. |
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+ |
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This option is particularly useful in avoiding false positives when a merged |
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branch contained broken or non-buildable commits, but the merge itself was OK. |
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EXAMPLES |
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-------- |
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* Automatically bisect a broken build between v1.2 and HEAD: |
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+ |
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------------ |
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$ git bisect start HEAD v1.2 -- # HEAD is bad, v1.2 is good |
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$ git bisect run make # "make" builds the app |
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$ git bisect reset # quit the bisect session |
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------------ |
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* Automatically bisect a test failure between origin and HEAD: |
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+ |
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------------ |
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$ git bisect start HEAD origin -- # HEAD is bad, origin is good |
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$ git bisect run make test # "make test" builds and tests |
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$ git bisect reset # quit the bisect session |
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------------ |
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* Automatically bisect a broken test case: |
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+ |
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------------ |
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$ cat ~/test.sh |
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#!/bin/sh |
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make || exit 125 # this skips broken builds |
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~/check_test_case.sh # does the test case pass? |
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$ git bisect start HEAD HEAD~10 -- # culprit is among the last 10 |
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$ git bisect run ~/test.sh |
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$ git bisect reset # quit the bisect session |
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------------ |
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+ |
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Here we use a `test.sh` custom script. In this script, if `make` |
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fails, we skip the current commit. |
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`check_test_case.sh` should `exit 0` if the test case passes, |
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and `exit 1` otherwise. |
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+ |
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It is safer if both `test.sh` and `check_test_case.sh` are |
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outside the repository to prevent interactions between the bisect, |
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make and test processes and the scripts. |
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* Automatically bisect with temporary modifications (hot-fix): |
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+ |
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------------ |
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$ cat ~/test.sh |
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#!/bin/sh |
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# tweak the working tree by merging the hot-fix branch |
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# and then attempt a build |
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if git merge --no-commit --no-ff hot-fix && |
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make |
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then |
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# run project specific test and report its status |
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~/check_test_case.sh |
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status=$? |
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else |
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# tell the caller this is untestable |
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status=125 |
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fi |
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# undo the tweak to allow clean flipping to the next commit |
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git reset --hard |
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# return control |
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exit $status |
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------------ |
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+ |
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This applies modifications from a hot-fix branch before each test run, |
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e.g. in case your build or test environment changed so that older |
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revisions may need a fix which newer ones have already. (Make sure the |
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hot-fix branch is based off a commit which is contained in all revisions |
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which you are bisecting, so that the merge does not pull in too much, or |
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use `git cherry-pick` instead of `git merge`.) |
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* Automatically bisect a broken test case: |
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+ |
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------------ |
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$ git bisect start HEAD HEAD~10 -- # culprit is among the last 10 |
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$ git bisect run sh -c "make || exit 125; ~/check_test_case.sh" |
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$ git bisect reset # quit the bisect session |
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------------ |
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+ |
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This shows that you can do without a run script if you write the test |
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on a single line. |
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* Locate a good region of the object graph in a damaged repository |
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+ |
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------------ |
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$ git bisect start HEAD <known-good-commit> [ <boundary-commit> ... ] --no-checkout |
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$ git bisect run sh -c ' |
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GOOD=$(git for-each-ref "--format=%(objectname)" refs/bisect/good-*) && |
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git rev-list --objects BISECT_HEAD --not $GOOD >tmp.$$ && |
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git pack-objects --stdout >/dev/null <tmp.$$ |
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rc=$? |
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rm -f tmp.$$ |
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test $rc = 0' |
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$ git bisect reset # quit the bisect session |
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------------ |
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+ |
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In this case, when 'git bisect run' finishes, bisect/bad will refer to a commit that |
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has at least one parent whose reachable graph is fully traversable in the sense |
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required by 'git pack objects'. |
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* Look for a fix instead of a regression in the code |
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+ |
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------------ |
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$ git bisect start |
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$ git bisect new HEAD # current commit is marked as new |
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$ git bisect old HEAD~10 # the tenth commit from now is marked as old |
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------------ |
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+ |
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or: |
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------------ |
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$ git bisect start --term-old broken --term-new fixed |
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$ git bisect fixed |
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$ git bisect broken HEAD~10 |
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------------ |
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Getting help |
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~~~~~~~~~~~~ |
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Use `git bisect` to get a short usage description, and `git bisect |
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help` or `git bisect -h` to get a long usage description. |
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SEE ALSO |
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-------- |
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link:git-bisect-lk2009.html[Fighting regressions with git bisect], |
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linkgit:git-blame[1]. |
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GIT |
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--- |
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Part of the linkgit:git[1] suite
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