Merge branch 'jc/em-dash-in-doc'
[git/git.git] / Documentation / git-bisect.txt
1 git-bisect(1)
2 =============
3
4 NAME
5 ----
6 git-bisect - Use binary search to find the commit that introduced a bug
7
8
9 SYNOPSIS
10 --------
11 [verse]
12 'git bisect' <subcommand> <options>
13
14 DESCRIPTION
15 -----------
16 The command takes various subcommands, and different options depending
17 on the subcommand:
18
19 git bisect start [--term-{old,good}=<term> --term-{new,bad}=<term>]
20 [--no-checkout] [<bad> [<good>...]] [--] [<paths>...]
21 git bisect (bad|new) [<rev>]
22 git bisect (good|old) [<rev>...]
23 git bisect terms [--term-good | --term-bad]
24 git bisect skip [(<rev>|<range>)...]
25 git bisect reset [<commit>]
26 git bisect visualize
27 git bisect replay <logfile>
28 git bisect log
29 git bisect run <cmd>...
30 git bisect help
31
32 This command uses a binary search algorithm to find which commit in
33 your project's history introduced a bug. You use it by first telling
34 it a "bad" commit that is known to contain the bug, and a "good"
35 commit that is known to be before the bug was introduced. Then `git
36 bisect` picks a commit between those two endpoints and asks you
37 whether the selected commit is "good" or "bad". It continues narrowing
38 down the range until it finds the exact commit that introduced the
39 change.
40
41 In fact, `git bisect` can be used to find the commit that changed
42 *any* property of your project; e.g., the commit that fixed a bug, or
43 the commit that caused a benchmark's performance to improve. To
44 support this more general usage, the terms "old" and "new" can be used
45 in place of "good" and "bad", or you can choose your own terms. See
46 section "Alternate terms" below for more information.
47
48 Basic bisect commands: start, bad, good
49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
50
51 As an example, suppose you are trying to find the commit that broke a
52 feature that was known to work in version `v2.6.13-rc2` of your
53 project. You start a bisect session as follows:
54
55 ------------------------------------------------
56 $ git bisect start
57 $ git bisect bad # Current version is bad
58 $ git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good
59 ------------------------------------------------
60
61 Once you have specified at least one bad and one good commit, `git
62 bisect` selects a commit in the middle of that range of history,
63 checks it out, and outputs something similar to the following:
64
65 ------------------------------------------------
66 Bisecting: 675 revisions left to test after this (roughly 10 steps)
67 ------------------------------------------------
68
69 You should now compile the checked-out version and test it. If that
70 version works correctly, type
71
72 ------------------------------------------------
73 $ git bisect good
74 ------------------------------------------------
75
76 If that version is broken, type
77
78 ------------------------------------------------
79 $ git bisect bad
80 ------------------------------------------------
81
82 Then `git bisect` will respond with something like
83
84 ------------------------------------------------
85 Bisecting: 337 revisions left to test after this (roughly 9 steps)
86 ------------------------------------------------
87
88 Keep repeating the process: compile the tree, test it, and depending
89 on whether it is good or bad run `git bisect good` or `git bisect bad`
90 to ask for the next commit that needs testing.
91
92 Eventually there will be no more revisions left to inspect, and the
93 command will print out a description of the first bad commit. The
94 reference `refs/bisect/bad` will be left pointing at that commit.
95
96
97 Bisect reset
98 ~~~~~~~~~~~~
99
100 After a bisect session, to clean up the bisection state and return to
101 the original HEAD, issue the following command:
102
103 ------------------------------------------------
104 $ git bisect reset
105 ------------------------------------------------
106
107 By default, this will return your tree to the commit that was checked
108 out before `git bisect start`. (A new `git bisect start` will also do
109 that, as it cleans up the old bisection state.)
110
111 With an optional argument, you can return to a different commit
112 instead:
113
114 ------------------------------------------------
115 $ git bisect reset <commit>
116 ------------------------------------------------
117
118 For example, `git bisect reset bisect/bad` will check out the first
119 bad revision, while `git bisect reset HEAD` will leave you on the
120 current bisection commit and avoid switching commits at all.
121
122
123 Alternate terms
124 ~~~~~~~~~~~~~~~
125
126 Sometimes you are not looking for the commit that introduced a
127 breakage, but rather for a commit that caused a change between some
128 other "old" state and "new" state. For example, you might be looking
129 for the commit that introduced a particular fix. Or you might be
130 looking for the first commit in which the source-code filenames were
131 finally all converted to your company's naming standard. Or whatever.
132
133 In such cases it can be very confusing to use the terms "good" and
134 "bad" to refer to "the state before the change" and "the state after
135 the change". So instead, you can use the terms "old" and "new",
136 respectively, in place of "good" and "bad". (But note that you cannot
137 mix "good" and "bad" with "old" and "new" in a single session.)
138
139 In this more general usage, you provide `git bisect` with a "new"
140 commit has some property and an "old" commit that doesn't have that
141 property. Each time `git bisect` checks out a commit, you test if that
142 commit has the property. If it does, mark the commit as "new";
143 otherwise, mark it as "old". When the bisection is done, `git bisect`
144 will report which commit introduced the property.
145
146 To use "old" and "new" instead of "good" and bad, you must run `git
147 bisect start` without commits as argument and then run the following
148 commands to add the commits:
149
150 ------------------------------------------------
151 git bisect old [<rev>]
152 ------------------------------------------------
153
154 to indicate that a commit was before the sought change, or
155
156 ------------------------------------------------
157 git bisect new [<rev>...]
158 ------------------------------------------------
159
160 to indicate that it was after.
161
162 To get a reminder of the currently used terms, use
163
164 ------------------------------------------------
165 git bisect terms
166 ------------------------------------------------
167
168 You can get just the old (respectively new) term with `git bisect term
169 --term-old` or `git bisect term --term-good`.
170
171 If you would like to use your own terms instead of "bad"/"good" or
172 "new"/"old", you can choose any names you like (except existing bisect
173 subcommands like `reset`, `start`, ...) by starting the
174 bisection using
175
176 ------------------------------------------------
177 git bisect start --term-old <term-old> --term-new <term-new>
178 ------------------------------------------------
179
180 For example, if you are looking for a commit that introduced a
181 performance regression, you might use
182
183 ------------------------------------------------
184 git bisect start --term-old fast --term-new slow
185 ------------------------------------------------
186
187 Or if you are looking for the commit that fixed a bug, you might use
188
189 ------------------------------------------------
190 git bisect start --term-new fixed --term-old broken
191 ------------------------------------------------
192
193 Then, use `git bisect <term-old>` and `git bisect <term-new>` instead
194 of `git bisect good` and `git bisect bad` to mark commits.
195
196 Bisect visualize
197 ~~~~~~~~~~~~~~~~
198
199 To see the currently remaining suspects in 'gitk', issue the following
200 command during the bisection process:
201
202 ------------
203 $ git bisect visualize
204 ------------
205
206 `view` may also be used as a synonym for `visualize`.
207
208 If the 'DISPLAY' environment variable is not set, 'git log' is used
209 instead. You can also give command-line options such as `-p` and
210 `--stat`.
211
212 ------------
213 $ git bisect view --stat
214 ------------
215
216 Bisect log and bisect replay
217 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
218
219 After having marked revisions as good or bad, issue the following
220 command to show what has been done so far:
221
222 ------------
223 $ git bisect log
224 ------------
225
226 If you discover that you made a mistake in specifying the status of a
227 revision, you can save the output of this command to a file, edit it to
228 remove the incorrect entries, and then issue the following commands to
229 return to a corrected state:
230
231 ------------
232 $ git bisect reset
233 $ git bisect replay that-file
234 ------------
235
236 Avoiding testing a commit
237 ~~~~~~~~~~~~~~~~~~~~~~~~~
238
239 If, in the middle of a bisect session, you know that the suggested
240 revision is not a good one to test (e.g. it fails to build and you
241 know that the failure does not have anything to do with the bug you
242 are chasing), you can manually select a nearby commit and test that
243 one instead.
244
245 For example:
246
247 ------------
248 $ git bisect good/bad # previous round was good or bad.
249 Bisecting: 337 revisions left to test after this (roughly 9 steps)
250 $ git bisect visualize # oops, that is uninteresting.
251 $ git reset --hard HEAD~3 # try 3 revisions before what
252 # was suggested
253 ------------
254
255 Then compile and test the chosen revision, and afterwards mark
256 the revision as good or bad in the usual manner.
257
258 Bisect skip
259 ~~~~~~~~~~~
260
261 Instead of choosing a nearby commit by yourself, you can ask Git to do
262 it for you by issuing the command:
263
264 ------------
265 $ git bisect skip # Current version cannot be tested
266 ------------
267
268 However, if you skip a commit adjacent to the one you are looking for,
269 Git will be unable to tell exactly which of those commits was the
270 first bad one.
271
272 You can also skip a range of commits, instead of just one commit,
273 using range notation. For example:
274
275 ------------
276 $ git bisect skip v2.5..v2.6
277 ------------
278
279 This tells the bisect process that no commit after `v2.5`, up to and
280 including `v2.6`, should be tested.
281
282 Note that if you also want to skip the first commit of the range you
283 would issue the command:
284
285 ------------
286 $ git bisect skip v2.5 v2.5..v2.6
287 ------------
288
289 This tells the bisect process that the commits between `v2.5` and
290 `v2.6` (inclusive) should be skipped.
291
292
293 Cutting down bisection by giving more parameters to bisect start
294 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
295
296 You can further cut down the number of trials, if you know what part of
297 the tree is involved in the problem you are tracking down, by specifying
298 path parameters when issuing the `bisect start` command:
299
300 ------------
301 $ git bisect start -- arch/i386 include/asm-i386
302 ------------
303
304 If you know beforehand more than one good commit, you can narrow the
305 bisect space down by specifying all of the good commits immediately after
306 the bad commit when issuing the `bisect start` command:
307
308 ------------
309 $ git bisect start v2.6.20-rc6 v2.6.20-rc4 v2.6.20-rc1 --
310 # v2.6.20-rc6 is bad
311 # v2.6.20-rc4 and v2.6.20-rc1 are good
312 ------------
313
314 Bisect run
315 ~~~~~~~~~~
316
317 If you have a script that can tell if the current source code is good
318 or bad, you can bisect by issuing the command:
319
320 ------------
321 $ git bisect run my_script arguments
322 ------------
323
324 Note that the script (`my_script` in the above example) should exit
325 with code 0 if the current source code is good/old, and exit with a
326 code between 1 and 127 (inclusive), except 125, if the current source
327 code is bad/new.
328
329 Any other exit code will abort the bisect process. It should be noted
330 that a program that terminates via `exit(-1)` leaves $? = 255, (see the
331 exit(3) manual page), as the value is chopped with `& 0377`.
332
333 The special exit code 125 should be used when the current source code
334 cannot be tested. If the script exits with this code, the current
335 revision will be skipped (see `git bisect skip` above). 125 was chosen
336 as the highest sensible value to use for this purpose, because 126 and 127
337 are used by POSIX shells to signal specific error status (127 is for
338 command not found, 126 is for command found but not executable--these
339 details do not matter, as they are normal errors in the script, as far as
340 `bisect run` is concerned).
341
342 You may often find that during a bisect session you want to have
343 temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a
344 header file, or "revision that does not have this commit needs this
345 patch applied to work around another problem this bisection is not
346 interested in") applied to the revision being tested.
347
348 To cope with such a situation, after the inner 'git bisect' finds the
349 next revision to test, the script can apply the patch
350 before compiling, run the real test, and afterwards decide if the
351 revision (possibly with the needed patch) passed the test and then
352 rewind the tree to the pristine state. Finally the script should exit
353 with the status of the real test to let the `git bisect run` command loop
354 determine the eventual outcome of the bisect session.
355
356 OPTIONS
357 -------
358 --no-checkout::
359 +
360 Do not checkout the new working tree at each iteration of the bisection
361 process. Instead just update a special reference named 'BISECT_HEAD' to make
362 it point to the commit that should be tested.
363 +
364 This option may be useful when the test you would perform in each step
365 does not require a checked out tree.
366 +
367 If the repository is bare, `--no-checkout` is assumed.
368
369 EXAMPLES
370 --------
371
372 * Automatically bisect a broken build between v1.2 and HEAD:
373 +
374 ------------
375 $ git bisect start HEAD v1.2 -- # HEAD is bad, v1.2 is good
376 $ git bisect run make # "make" builds the app
377 $ git bisect reset # quit the bisect session
378 ------------
379
380 * Automatically bisect a test failure between origin and HEAD:
381 +
382 ------------
383 $ git bisect start HEAD origin -- # HEAD is bad, origin is good
384 $ git bisect run make test # "make test" builds and tests
385 $ git bisect reset # quit the bisect session
386 ------------
387
388 * Automatically bisect a broken test case:
389 +
390 ------------
391 $ cat ~/test.sh
392 #!/bin/sh
393 make || exit 125 # this skips broken builds
394 ~/check_test_case.sh # does the test case pass?
395 $ git bisect start HEAD HEAD~10 -- # culprit is among the last 10
396 $ git bisect run ~/test.sh
397 $ git bisect reset # quit the bisect session
398 ------------
399 +
400 Here we use a `test.sh` custom script. In this script, if `make`
401 fails, we skip the current commit.
402 `check_test_case.sh` should `exit 0` if the test case passes,
403 and `exit 1` otherwise.
404 +
405 It is safer if both `test.sh` and `check_test_case.sh` are
406 outside the repository to prevent interactions between the bisect,
407 make and test processes and the scripts.
408
409 * Automatically bisect with temporary modifications (hot-fix):
410 +
411 ------------
412 $ cat ~/test.sh
413 #!/bin/sh
414
415 # tweak the working tree by merging the hot-fix branch
416 # and then attempt a build
417 if git merge --no-commit hot-fix &&
418 make
419 then
420 # run project specific test and report its status
421 ~/check_test_case.sh
422 status=$?
423 else
424 # tell the caller this is untestable
425 status=125
426 fi
427
428 # undo the tweak to allow clean flipping to the next commit
429 git reset --hard
430
431 # return control
432 exit $status
433 ------------
434 +
435 This applies modifications from a hot-fix branch before each test run,
436 e.g. in case your build or test environment changed so that older
437 revisions may need a fix which newer ones have already. (Make sure the
438 hot-fix branch is based off a commit which is contained in all revisions
439 which you are bisecting, so that the merge does not pull in too much, or
440 use `git cherry-pick` instead of `git merge`.)
441
442 * Automatically bisect a broken test case:
443 +
444 ------------
445 $ git bisect start HEAD HEAD~10 -- # culprit is among the last 10
446 $ git bisect run sh -c "make || exit 125; ~/check_test_case.sh"
447 $ git bisect reset # quit the bisect session
448 ------------
449 +
450 This shows that you can do without a run script if you write the test
451 on a single line.
452
453 * Locate a good region of the object graph in a damaged repository
454 +
455 ------------
456 $ git bisect start HEAD <known-good-commit> [ <boundary-commit> ... ] --no-checkout
457 $ git bisect run sh -c '
458 GOOD=$(git for-each-ref "--format=%(objectname)" refs/bisect/good-*) &&
459 git rev-list --objects BISECT_HEAD --not $GOOD >tmp.$$ &&
460 git pack-objects --stdout >/dev/null <tmp.$$
461 rc=$?
462 rm -f tmp.$$
463 test $rc = 0'
464
465 $ git bisect reset # quit the bisect session
466 ------------
467 +
468 In this case, when 'git bisect run' finishes, bisect/bad will refer to a commit that
469 has at least one parent whose reachable graph is fully traversable in the sense
470 required by 'git pack objects'.
471
472 * Look for a fix instead of a regression in the code
473 +
474 ------------
475 $ git bisect start
476 $ git bisect new HEAD # current commit is marked as new
477 $ git bisect old HEAD~10 # the tenth commit from now is marked as old
478 ------------
479 +
480 or:
481 ------------
482 $ git bisect start --term-old broken --term-new fixed
483 $ git bisect fixed
484 $ git bisect broken HEAD~10
485 ------------
486
487 Getting help
488 ~~~~~~~~~~~~
489
490 Use `git bisect` to get a short usage description, and `git bisect
491 help` or `git bisect -h` to get a long usage description.
492
493 SEE ALSO
494 --------
495 link:git-bisect-lk2009.html[Fighting regressions with git bisect],
496 linkgit:git-blame[1].
497
498 GIT
499 ---
500 Part of the linkgit:git[1] suite