bisect: explain the rationale behind 125
[git/git.git] / Documentation / git-bisect.txt
1 git-bisect(1)
2 =============
3
4 NAME
5 ----
6 git-bisect - Find by binary search the change that introduced a bug
7
8
9 SYNOPSIS
10 --------
11 'git bisect' <subcommand> <options>
12
13 DESCRIPTION
14 -----------
15 The command takes various subcommands, and different options depending
16 on the subcommand:
17
18 git bisect help
19 git bisect start [<bad> [<good>...]] [--] [<paths>...]
20 git bisect bad [<rev>]
21 git bisect good [<rev>...]
22 git bisect skip [(<rev>|<range>)...]
23 git bisect reset [<commit>]
24 git bisect visualize
25 git bisect replay <logfile>
26 git bisect log
27 git bisect run <cmd>...
28
29 This command uses 'git rev-list --bisect' to help drive the
30 binary search process to find which change introduced a bug, given an
31 old "good" commit object name and a later "bad" commit object name.
32
33 Getting help
34 ~~~~~~~~~~~~
35
36 Use "git bisect" to get a short usage description, and "git bisect
37 help" or "git bisect -h" to get a long usage description.
38
39 Basic bisect commands: start, bad, good
40 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
41
42 Using the Linux kernel tree as an example, basic use of the bisect
43 command is as follows:
44
45 ------------------------------------------------
46 $ git bisect start
47 $ git bisect bad # Current version is bad
48 $ git bisect good v2.6.13-rc2 # v2.6.13-rc2 was the last version
49 # tested that was good
50 ------------------------------------------------
51
52 When you have specified at least one bad and one good version, the
53 command bisects the revision tree and outputs something similar to
54 the following:
55
56 ------------------------------------------------
57 Bisecting: 675 revisions left to test after this
58 ------------------------------------------------
59
60 The state in the middle of the set of revisions is then checked out.
61 You would now compile that kernel and boot it. If the booted kernel
62 works correctly, you would then issue the following command:
63
64 ------------------------------------------------
65 $ git bisect good # this one is good
66 ------------------------------------------------
67
68 The output of this command would be something similar to the following:
69
70 ------------------------------------------------
71 Bisecting: 337 revisions left to test after this
72 ------------------------------------------------
73
74 You keep repeating this process, compiling the tree, testing it, and
75 depending on whether it is good or bad issuing the command "git bisect good"
76 or "git bisect bad" to ask for the next bisection.
77
78 Eventually there will be no more revisions left to bisect, and you
79 will have been left with the first bad kernel revision in "refs/bisect/bad".
80
81 Bisect reset
82 ~~~~~~~~~~~~
83
84 After a bisect session, to clean up the bisection state and return to
85 the original HEAD, issue the following command:
86
87 ------------------------------------------------
88 $ git bisect reset
89 ------------------------------------------------
90
91 By default, this will return your tree to the commit that was checked
92 out before `git bisect start`. (A new `git bisect start` will also do
93 that, as it cleans up the old bisection state.)
94
95 With an optional argument, you can return to a different commit
96 instead:
97
98 ------------------------------------------------
99 $ git bisect reset <commit>
100 ------------------------------------------------
101
102 For example, `git bisect reset HEAD` will leave you on the current
103 bisection commit and avoid switching commits at all, while `git bisect
104 reset bisect/bad` will check out the first bad revision.
105
106 Bisect visualize
107 ~~~~~~~~~~~~~~~~
108
109 To see the currently remaining suspects in 'gitk', issue the following
110 command during the bisection process:
111
112 ------------
113 $ git bisect visualize
114 ------------
115
116 `view` may also be used as a synonym for `visualize`.
117
118 If the 'DISPLAY' environment variable is not set, 'git log' is used
119 instead. You can also give command line options such as `-p` and
120 `--stat`.
121
122 ------------
123 $ git bisect view --stat
124 ------------
125
126 Bisect log and bisect replay
127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
128
129 After having marked revisions as good or bad, issue the following
130 command to show what has been done so far:
131
132 ------------
133 $ git bisect log
134 ------------
135
136 If you discover that you made a mistake in specifying the status of a
137 revision, you can save the output of this command to a file, edit it to
138 remove the incorrect entries, and then issue the following commands to
139 return to a corrected state:
140
141 ------------
142 $ git bisect reset
143 $ git bisect replay that-file
144 ------------
145
146 Avoiding testing a commit
147 ~~~~~~~~~~~~~~~~~~~~~~~~~
148
149 If, in the middle of a bisect session, you know that the next suggested
150 revision is not a good one to test (e.g. the change the commit
151 introduces is known not to work in your environment and you know it
152 does not have anything to do with the bug you are chasing), you may
153 want to find a nearby commit and try that instead.
154
155 For example:
156
157 ------------
158 $ git bisect good/bad # previous round was good or bad.
159 Bisecting: 337 revisions left to test after this
160 $ git bisect visualize # oops, that is uninteresting.
161 $ git reset --hard HEAD~3 # try 3 revisions before what
162 # was suggested
163 ------------
164
165 Then compile and test the chosen revision, and afterwards mark
166 the revision as good or bad in the usual manner.
167
168 Bisect skip
169 ~~~~~~~~~~~~
170
171 Instead of choosing by yourself a nearby commit, you can ask git
172 to do it for you by issuing the command:
173
174 ------------
175 $ git bisect skip # Current version cannot be tested
176 ------------
177
178 But git may eventually be unable to tell the first bad commit among
179 a bad commit and one or more skipped commits.
180
181 You can even skip a range of commits, instead of just one commit,
182 using the "'<commit1>'..'<commit2>'" notation. For example:
183
184 ------------
185 $ git bisect skip v2.5..v2.6
186 ------------
187
188 This tells the bisect process that no commit after `v2.5`, up to and
189 including `v2.6`, should be tested.
190
191 Note that if you also want to skip the first commit of the range you
192 would issue the command:
193
194 ------------
195 $ git bisect skip v2.5 v2.5..v2.6
196 ------------
197
198 This tells the bisect process that the commits between `v2.5` included
199 and `v2.6` included should be skipped.
200
201
202 Cutting down bisection by giving more parameters to bisect start
203 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
204
205 You can further cut down the number of trials, if you know what part of
206 the tree is involved in the problem you are tracking down, by specifying
207 path parameters when issuing the `bisect start` command:
208
209 ------------
210 $ git bisect start -- arch/i386 include/asm-i386
211 ------------
212
213 If you know beforehand more than one good commit, you can narrow the
214 bisect space down by specifying all of the good commits immediately after
215 the bad commit when issuing the `bisect start` command:
216
217 ------------
218 $ git bisect start v2.6.20-rc6 v2.6.20-rc4 v2.6.20-rc1 --
219 # v2.6.20-rc6 is bad
220 # v2.6.20-rc4 and v2.6.20-rc1 are good
221 ------------
222
223 Bisect run
224 ~~~~~~~~~~
225
226 If you have a script that can tell if the current source code is good
227 or bad, you can bisect by issuing the command:
228
229 ------------
230 $ git bisect run my_script arguments
231 ------------
232
233 Note that the script (`my_script` in the above example) should
234 exit with code 0 if the current source code is good, and exit with a
235 code between 1 and 127 (inclusive), except 125, if the current
236 source code is bad.
237
238 Any other exit code will abort the bisect process. It should be noted
239 that a program that terminates via "exit(-1)" leaves $? = 255, (see the
240 exit(3) manual page), as the value is chopped with "& 0377".
241
242 The special exit code 125 should be used when the current source code
243 cannot be tested. If the script exits with this code, the current
244 revision will be skipped (see `git bisect skip` above). 125 was chosen
245 as the highest sensible value to use for this purpose, because 126 and 127
246 are used by POSIX shells to signal specific error status (127 is for
247 command not found, 126 is for command found but not executable---these
248 details do not matter, as they are normal errors in the script, as far as
249 "bisect run" is concerned).
250
251 You may often find that during a bisect session you want to have
252 temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a
253 header file, or "revision that does not have this commit needs this
254 patch applied to work around another problem this bisection is not
255 interested in") applied to the revision being tested.
256
257 To cope with such a situation, after the inner 'git bisect' finds the
258 next revision to test, the script can apply the patch
259 before compiling, run the real test, and afterwards decide if the
260 revision (possibly with the needed patch) passed the test and then
261 rewind the tree to the pristine state. Finally the script should exit
262 with the status of the real test to let the "git bisect run" command loop
263 determine the eventual outcome of the bisect session.
264
265 EXAMPLES
266 --------
267
268 * Automatically bisect a broken build between v1.2 and HEAD:
269 +
270 ------------
271 $ git bisect start HEAD v1.2 -- # HEAD is bad, v1.2 is good
272 $ git bisect run make # "make" builds the app
273 ------------
274
275 * Automatically bisect a test failure between origin and HEAD:
276 +
277 ------------
278 $ git bisect start HEAD origin -- # HEAD is bad, origin is good
279 $ git bisect run make test # "make test" builds and tests
280 ------------
281
282 * Automatically bisect a broken test suite:
283 +
284 ------------
285 $ cat ~/test.sh
286 #!/bin/sh
287 make || exit 125 # this skips broken builds
288 make test # "make test" runs the test suite
289 $ git bisect start v1.3 v1.1 -- # v1.3 is bad, v1.1 is good
290 $ git bisect run ~/test.sh
291 ------------
292 +
293 Here we use a "test.sh" custom script. In this script, if "make"
294 fails, we skip the current commit.
295 +
296 It is safer to use a custom script outside the repository to prevent
297 interactions between the bisect, make and test processes and the
298 script.
299 +
300 "make test" should "exit 0", if the test suite passes, and
301 "exit 1" otherwise.
302
303 * Automatically bisect a broken test case:
304 +
305 ------------
306 $ cat ~/test.sh
307 #!/bin/sh
308 make || exit 125 # this skips broken builds
309 ~/check_test_case.sh # does the test case passes ?
310 $ git bisect start HEAD HEAD~10 -- # culprit is among the last 10
311 $ git bisect run ~/test.sh
312 ------------
313 +
314 Here "check_test_case.sh" should "exit 0" if the test case passes,
315 and "exit 1" otherwise.
316 +
317 It is safer if both "test.sh" and "check_test_case.sh" scripts are
318 outside the repository to prevent interactions between the bisect,
319 make and test processes and the scripts.
320
321 * Automatically bisect a broken test suite:
322 +
323 ------------
324 $ git bisect start HEAD HEAD~10 -- # culprit is among the last 10
325 $ git bisect run sh -c "make || exit 125; ~/check_test_case.sh"
326 ------------
327 +
328 Does the same as the previous example, but on a single line.
329
330 Author
331 ------
332 Written by Linus Torvalds <torvalds@osdl.org>
333
334 Documentation
335 -------------
336 Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.
337
338 SEE ALSO
339 --------
340 link:git-bisect-lk2009.html[Fighting regressions with git bisect],
341 linkgit:git-blame[1].
342
343 GIT
344 ---
345 Part of the linkgit:git[1] suite