Git 2.8.4
[git/git.git] / Documentation / CodingGuidelines
CommitLineData
6d0618a8 1Like other projects, we also have some guidelines to keep to the
6c3b2afe 2code. For Git in general, a few rough rules are:
6d0618a8
JS
3
4 - Most importantly, we never say "It's in POSIX; we'll happily
5 ignore your needs should your system not conform to it."
6 We live in the real world.
7
8 - However, we often say "Let's stay away from that construct,
9 it's not even in POSIX".
10
11 - In spite of the above two rules, we sometimes say "Although
12 this is not in POSIX, it (is so convenient | makes the code
13 much more readable | has other good characteristics) and
14 practically all the platforms we care about support it, so
15 let's use it".
16
17 Again, we live in the real world, and it is sometimes a
18 judgement call, the decision based more on real world
19 constraints people face than what the paper standard says.
20
dd30800b
JH
21 - Fixing style violations while working on a real change as a
22 preparatory clean-up step is good, but otherwise avoid useless code
23 churn for the sake of conforming to the style.
24
25 "Once it _is_ in the tree, it's not really worth the patch noise to
26 go and fix it up."
27 Cf. http://article.gmane.org/gmane.linux.kernel/943020
28
c5e366b1 29Make your code readable and sensible, and don't try to be clever.
6d0618a8
JS
30
31As for more concrete guidelines, just imitate the existing code
32(this is a good guideline, no matter which project you are
dfb047b9 33contributing to). It is always preferable to match the _local_
2de9b711 34convention. New code added to Git suite is expected to match
dfb047b9
NS
35the overall style of existing code. Modifications to existing
36code is expected to match the style the surrounding code already
37uses (even if it doesn't match the overall style of existing code).
38
39But if you must have a list of rules, here they are.
6d0618a8
JS
40
41For shell scripts specifically (not exhaustive):
42
f36a4fa8
GB
43 - We use tabs for indentation.
44
79fc3ca1
JH
45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
f36a4fa8 56
48f359bf
TH
57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
6a49909b
JH
64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
6d0618a8
JS
72 - We prefer $( ... ) for command substitution; unlike ``, it
73 properly nests. It should have been the way Bourne spelled
74 it from day one, but unfortunately isn't.
75
860f70f9
TH
76 - If you want to find out if a command is available on the user's
77 $PATH, you should use 'type <command>', instead of 'which <command>'.
78 The output of 'which' is not machine parseable and its exit code
79 is not reliable across platforms.
80
bc979945
JH
81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
6d0618a8 83
bc979945
JH
84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
6d0618a8 86
bc979945
JH
87 - We use ${parameter#word} and its [#%] siblings, and their
88 doubled "longest matching" form.
6d0618a8 89
bc979945 90 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 91
bc979945 92 - No shell arrays.
6d0618a8 93
bc979945 94 - No strlen ${#parameter}.
6d0618a8 95
bc979945 96 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 97
bc979945
JH
98 - We use Arithmetic Expansion $(( ... )).
99
100 - Inside Arithmetic Expansion, spell shell variables with $ in front
101 of them, as some shells do not grok $((x)) while accepting $(($x))
102 just fine (e.g. dash older than 0.5.4).
6d0618a8
JS
103
104 - We do not use Process Substitution <(list) or >(list).
105
03b05c7d
HV
106 - Do not write control structures on a single line with semicolon.
107 "then" should be on the next line for if statements, and "do"
108 should be on the next line for "while" and "for".
109
9dbe7801
JH
110 (incorrect)
111 if test -f hello; then
112 do this
113 fi
114
115 (correct)
116 if test -f hello
117 then
118 do this
119 fi
120
6d0618a8
JS
121 - We prefer "test" over "[ ... ]".
122
123 - We do not write the noiseword "function" in front of shell
124 functions.
125
6117a3d4
JH
126 - We prefer a space between the function name and the parentheses,
127 and no space inside the parentheses. The opening "{" should also
128 be on the same line.
129
130 (incorrect)
131 my_function(){
132 ...
133
134 (correct)
135 my_function () {
136 ...
03b05c7d 137
009c98ee 138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 139 [::], [==], or [..]) for portability.
009c98ee
JH
140
141 - We do not use \{m,n\};
142
143 - We do not use -E;
144
a58088ab 145 - We do not use ? or + (which are \{0,1\} and \{1,\}
009c98ee
JH
146 respectively in BRE) but that goes without saying as these
147 are ERE elements not BRE (note that \? and \+ are not even part
148 of BRE -- making them accessible from BRE is a GNU extension).
149
5e9637c6
ÆAB
150 - Use Git's gettext wrappers in git-sh-i18n to make the user
151 interface translatable. See "Marking strings for translation" in
152 po/README.
153
897f964c
JH
154 - We do not write our "test" command with "-a" and "-o" and use "&&"
155 or "||" to concatenate multiple "test" commands instead, because
156 the use of "-a/-o" is often error-prone. E.g.
157
158 test -n "$x" -a "$a" = "$b"
159
160 is buggy and breaks when $x is "=", but
161
162 test -n "$x" && test "$a" = "$b"
163
164 does not have such a problem.
165
166
6d0618a8
JS
167For C programs:
168
169 - We use tabs to indent, and interpret tabs as taking up to
170 8 spaces.
171
172 - We try to keep to at most 80 characters per line.
173
658df95a
LS
174 - As a Git developer we assume you have a reasonably modern compiler
175 and we recommend you to enable the DEVELOPER makefile knob to
176 ensure your patch is clear of all compiler warnings we care about,
177 by e.g. "echo DEVELOPER=1 >>config.mak".
178
2de9b711 179 - We try to support a wide range of C compilers to compile Git with,
a26fd033
AS
180 including old ones. That means that you should not use C99
181 initializers, even if a lot of compilers grok it.
182
183 - Variables have to be declared at the beginning of the block.
184
185 - NULL pointers shall be written as NULL, not as 0.
186
6d0618a8
JS
187 - When declaring pointers, the star sides with the variable
188 name, i.e. "char *string", not "char* string" or
189 "char * string". This makes it easier to understand code
190 like "char *string, c;".
191
f57b6cfd
JK
192 - Use whitespace around operators and keywords, but not inside
193 parentheses and not around functions. So:
194
195 while (condition)
196 func(bar + 1);
197
198 and not:
199
200 while( condition )
201 func (bar+1);
202
6d0618a8
JS
203 - We avoid using braces unnecessarily. I.e.
204
205 if (bla) {
206 x = 1;
207 }
208
209 is frowned upon. A gray area is when the statement extends
210 over a few lines, and/or you have a lengthy comment atop of
211 it. Also, like in the Linux kernel, if there is a long list
212 of "else if" statements, it can make sense to add braces to
213 single line blocks.
214
691d0dd0 215 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 216
6d0618a8
JS
217 - Try to make your code understandable. You may put comments
218 in, but comments invariably tend to stale out when the code
219 they were describing changes. Often splitting a function
220 into two makes the intention of the code much clearer.
221
b75a6ca7 222 - Multi-line comments include their delimiters on separate lines from
223 the text. E.g.
224
225 /*
226 * A very long
227 * multi-line comment.
228 */
229
cbcfd4e3
JH
230 Note however that a comment that explains a translatable string to
231 translators uses a convention of starting with a magic token
232 "TRANSLATORS: " immediately after the opening delimiter, even when
233 it spans multiple lines. We do not add an asterisk at the beginning
234 of each line, either. E.g.
235
236 /* TRANSLATORS: here is a comment that explains the string
237 to be translated, that follows immediately after it */
238 _("Here is a translatable string explained by the above.");
239
6d0618a8
JS
240 - Double negation is often harder to understand than no negation
241 at all.
242
5db9ab82
JH
243 - There are two schools of thought when it comes to comparison,
244 especially inside a loop. Some people prefer to have the less stable
245 value on the left hand side and the more stable value on the right hand
246 side, e.g. if you have a loop that counts variable i down to the
247 lower bound,
248
249 while (i > lower_bound) {
250 do something;
251 i--;
252 }
253
254 Other people prefer to have the textual order of values match the
255 actual order of values in their comparison, so that they can
256 mentally draw a number line from left to right and place these
257 values in order, i.e.
258
259 while (lower_bound < i) {
260 do something;
261 i--;
262 }
263
264 Both are valid, and we use both. However, the more "stable" the
265 stable side becomes, the more we tend to prefer the former
266 (comparison with a constant, "i > 0", is an extreme example).
267 Just do not mix styles in the same part of the code and mimic
268 existing styles in the neighbourhood.
269
f26443da
JH
270 - There are two schools of thought when it comes to splitting a long
271 logical line into multiple lines. Some people push the second and
272 subsequent lines far enough to the right with tabs and align them:
273
274 if (the_beginning_of_a_very_long_expression_that_has_to ||
275 span_more_than_a_single_line_of ||
276 the_source_text) {
277 ...
278
279 while other people prefer to align the second and the subsequent
280 lines with the column immediately inside the opening parenthesis,
281 with tabs and spaces, following our "tabstop is always a multiple
282 of 8" convention:
283
284 if (the_beginning_of_a_very_long_expression_that_has_to ||
285 span_more_than_a_single_line_of ||
286 the_source_text) {
287 ...
288
289 Both are valid, and we use both. Again, just do not mix styles in
290 the same part of the code and mimic existing styles in the
291 neighbourhood.
292
293 - When splitting a long logical line, some people change line before
294 a binary operator, so that the result looks like a parse tree when
295 you turn your head 90-degrees counterclockwise:
296
297 if (the_beginning_of_a_very_long_expression_that_has_to
298 || span_more_than_a_single_line_of_the_source_text) {
299
300 while other people prefer to leave the operator at the end of the
301 line:
302
303 if (the_beginning_of_a_very_long_expression_that_has_to ||
304 span_more_than_a_single_line_of_the_source_text) {
305
306 Both are valid, but we tend to use the latter more, unless the
307 expression gets fairly complex, in which case the former tends to
308 be easier to read. Again, just do not mix styles in the same part
309 of the code and mimic existing styles in the neighbourhood.
310
311 - When splitting a long logical line, with everything else being
312 equal, it is preferable to split after the operator at higher
313 level in the parse tree. That is, this is more preferable:
314
315 if (a_very_long_variable * that_is_used_in +
316 a_very_long_expression) {
317 ...
318
319 than
320
321 if (a_very_long_variable *
322 that_is_used_in + a_very_long_expression) {
323 ...
324
6d0618a8
JS
325 - Some clever tricks, like using the !! operator with arithmetic
326 constructs, can be extremely confusing to others. Avoid them,
327 unless there is a compelling reason to use them.
328
329 - Use the API. No, really. We have a strbuf (variable length
330 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 331 string_list for sorted string lists, a hash map (mapping struct
6d0618a8
JS
332 objects) named "struct decorate", amongst other things.
333
334 - When you come up with an API, document it.
335
412cb2ec
JH
336 - The first #include in C files, except in platform specific compat/
337 implementations, must be either "git-compat-util.h", "cache.h" or
338 "builtin.h". You do not have to include more than one of these.
339
340 - A C file must directly include the header files that declare the
341 functions and the types it uses, except for the functions and types
342 that are made available to it by including one of the header files
343 it must include by the previous rule.
6d0618a8
JS
344
345 - If you are planning a new command, consider writing it in shell
346 or perl first, so that changes in semantics can be easily
2de9b711 347 changed and discussed. Many Git commands started out like
6d0618a8
JS
348 that, and a few are still scripts.
349
2de9b711 350 - Avoid introducing a new dependency into Git. This means you
6d0618a8 351 usually should stay away from scripting languages not already
2de9b711 352 used in the Git core command set (unless your command is clearly
6d0618a8 353 separate from it, such as an importer to convert random-scm-X
2de9b711 354 repositories to Git).
57199892
KB
355
356 - When we pass <string, length> pair to functions, we should try to
357 pass them in that order.
c455bd89 358
5e9637c6
ÆAB
359 - Use Git's gettext wrappers to make the user interface
360 translatable. See "Marking strings for translation" in po/README.
361
c5e366b1
TZ
362For Perl programs:
363
364 - Most of the C guidelines above apply.
365
366 - We try to support Perl 5.8 and later ("use Perl 5.008").
367
368 - use strict and use warnings are strongly preferred.
369
370 - Don't overuse statement modifiers unless using them makes the
371 result easier to follow.
372
373 ... do something ...
374 do_this() unless (condition);
375 ... do something else ...
376
377 is more readable than:
378
379 ... do something ...
380 unless (condition) {
381 do_this();
382 }
383 ... do something else ...
384
385 *only* when the condition is so rare that do_this() will be almost
386 always called.
387
388 - We try to avoid assignments inside "if ()" conditions.
389
390 - Learn and use Git.pm if you need that functionality.
391
392 - For Emacs, it's useful to put the following in
393 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
394
395 ;; note the first part is useful for C editing, too
396 ((nil . ((indent-tabs-mode . t)
397 (tab-width . 8)
398 (fill-column . 80)))
399 (cperl-mode . ((cperl-indent-level . 8)
400 (cperl-extra-newline-before-brace . nil)
401 (cperl-merge-trailing-else . t))))
402
9ef43dd7
JK
403For Python scripts:
404
405 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
406
407 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
408
409 - Where required libraries do not restrict us to Python 2, we try to
410 also be compatible with Python 3.1 and later.
411
412 - When you must differentiate between Unicode literals and byte string
413 literals, it is OK to use the 'b' prefix. Even though the Python
414 documentation for version 2.6 does not mention this prefix, it has
415 been supported since version 2.6.0.
416
0ae0e882
PO
417Error Messages
418
419 - Do not end error messages with a full stop.
420
421 - Do not capitalize ("unable to open %s", not "Unable to open %s")
422
423 - Say what the error is first ("cannot open %s", not "%s: cannot open")
424
425
35840a3e
JH
426Externally Visible Names
427
428 - For configuration variable names, follow the existing convention:
429
430 . The section name indicates the affected subsystem.
431
432 . The subsection name, if any, indicates which of an unbounded set
433 of things to set the value for.
434
435 . The variable name describes the effect of tweaking this knob.
436
437 The section and variable names that consist of multiple words are
438 formed by concatenating the words without punctuations (e.g. `-`),
439 and are broken using bumpyCaps in documentation as a hint to the
440 reader.
441
442 When choosing the variable namespace, do not use variable name for
443 specifying possibly unbounded set of things, most notably anything
444 an end user can freely come up with (e.g. branch names). Instead,
445 use subsection names or variable values, like the existing variable
446 branch.<name>.description does.
447
448
c455bd89
ŠN
449Writing Documentation:
450
48bc1755
DW
451 Most (if not all) of the documentation pages are written in the
452 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
453 processed into HTML and manpages (e.g. git.html and git.1 in the
454 same directory).
bb9f2aec 455
42e0fae9
MB
456 The documentation liberally mixes US and UK English (en_US/UK)
457 norms for spelling and grammar, which is somewhat unfortunate.
458 In an ideal world, it would have been better if it consistently
459 used only one and not the other, and we would have picked en_US
460 (if you wish to correct the English of some of the existing
461 documentation, please see the documentation-related advice in the
462 Documentation/SubmittingPatches file).
463
c455bd89
ŠN
464 Every user-visible change should be reflected in the documentation.
465 The same general rule as for code applies -- imitate the existing
ca03c368
JSJ
466 conventions.
467
468 A few commented examples follow to provide reference when writing or
469 modifying command usage strings and synopsis sections in the manual
470 pages:
c455bd89 471
b1afe49d 472 Placeholders are spelled in lowercase and enclosed in angle brackets:
c455bd89
ŠN
473 <file>
474 --sort=<key>
475 --abbrev[=<n>]
476
9c9b4f2f
AH
477 If a placeholder has multiple words, they are separated by dashes:
478 <new-branch-name>
479 --template=<template-directory>
480
469bfc96 481 Possibility of multiple occurrences is indicated by three dots:
c455bd89
ŠN
482 <file>...
483 (One or more of <file>.)
484
485 Optional parts are enclosed in square brackets:
486 [<extra>]
487 (Zero or one <extra>.)
488
489 --exec-path[=<path>]
490 (Option with an optional argument. Note that the "=" is inside the
491 brackets.)
492
493 [<patch>...]
494 (Zero or more of <patch>. Note that the dots are inside, not
495 outside the brackets.)
496
9c9b4f2f 497 Multiple alternatives are indicated with vertical bars:
c455bd89
ŠN
498 [-q | --quiet]
499 [--utf8 | --no-utf8]
500
501 Parentheses are used for grouping:
9c9b4f2f 502 [(<rev> | <range>)...]
c455bd89
ŠN
503 (Any number of either <rev> or <range>. Parens are needed to make
504 it clear that "..." pertains to both <rev> and <range>.)
505
506 [(-p <parent>)...]
507 (Any number of option -p, each with one <parent> argument.)
508
509 git remote set-head <name> (-a | -d | <branch>)
510 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
511 brackets) be provided.)
512
513 And a somewhat more contrived example:
514 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
515 Here "=" is outside the brackets, because "--diff-filter=" is a
516 valid usage. "*" has its own pair of brackets, because it can
517 (optionally) be specified only when one or more of the letters is
518 also provided.
48a8c26c
TA
519
520 A note on notation:
521 Use 'git' (all lowercase) when talking about commands i.e. something
522 the user would type into a shell and use 'Git' (uppercase first letter)
523 when talking about the version control system and its properties.
ca03c368
JSJ
524
525 A few commented examples follow to provide reference when writing or
526 modifying paragraphs or option/command explanations that contain options
527 or commands:
528
529 Literal examples (e.g. use of command-line options, command names, and
530 configuration variables) are typeset in monospace, and if you can use
531 `backticks around word phrases`, do so.
532 `--pretty=oneline`
533 `git rev-list`
da0005b8 534 `remote.pushDefault`
ca03c368
JSJ
535
536 Word phrases enclosed in `backtick characters` are rendered literally
537 and will not be further expanded. The use of `backticks` to achieve the
538 previous rule means that literal examples should not use AsciiDoc
539 escapes.
540 Correct:
541 `--pretty=oneline`
542 Incorrect:
543 `\--pretty=oneline`
544
545 If some place in the documentation needs to typeset a command usage
546 example with inline substitutions, it is fine to use +monospaced and
547 inline substituted text+ instead of `monospaced literal text`, and with
548 the former, the part that should not get substituted must be
549 quoted/escaped.