t/README: reformat Do, Don't, Keep in mind lists
[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."
2e477d8d 27 Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
dd30800b 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
1797dc51
JK
209 is frowned upon. But there are a few exceptions:
210
211 - When the statement extends over a few lines (e.g., a while loop
212 with an embedded conditional, or a comment). E.g.:
213
214 while (foo) {
215 if (x)
216 one();
217 else
218 two();
219 }
220
221 if (foo) {
222 /*
223 * This one requires some explanation,
224 * so we're better off with braces to make
225 * it obvious that the indentation is correct.
226 */
227 doit();
228 }
229
230 - When there are multiple arms to a conditional and some of them
231 require braces, enclose even a single line block in braces for
232 consistency. E.g.:
233
234 if (foo) {
235 doit();
236 } else {
237 one();
238 two();
239 three();
240 }
6d0618a8 241
691d0dd0 242 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 243
6d0618a8
JS
244 - Try to make your code understandable. You may put comments
245 in, but comments invariably tend to stale out when the code
246 they were describing changes. Often splitting a function
247 into two makes the intention of the code much clearer.
248
b75a6ca7 249 - Multi-line comments include their delimiters on separate lines from
250 the text. E.g.
251
252 /*
253 * A very long
254 * multi-line comment.
255 */
256
cbcfd4e3
JH
257 Note however that a comment that explains a translatable string to
258 translators uses a convention of starting with a magic token
66f5f6dc 259 "TRANSLATORS: ", e.g.
cbcfd4e3 260
66f5f6dc
ÆAB
261 /*
262 * TRANSLATORS: here is a comment that explains the string to
263 * be translated, that follows immediately after it.
264 */
cbcfd4e3
JH
265 _("Here is a translatable string explained by the above.");
266
6d0618a8
JS
267 - Double negation is often harder to understand than no negation
268 at all.
269
5db9ab82
JH
270 - There are two schools of thought when it comes to comparison,
271 especially inside a loop. Some people prefer to have the less stable
272 value on the left hand side and the more stable value on the right hand
273 side, e.g. if you have a loop that counts variable i down to the
274 lower bound,
275
276 while (i > lower_bound) {
277 do something;
278 i--;
279 }
280
281 Other people prefer to have the textual order of values match the
282 actual order of values in their comparison, so that they can
283 mentally draw a number line from left to right and place these
284 values in order, i.e.
285
286 while (lower_bound < i) {
287 do something;
288 i--;
289 }
290
291 Both are valid, and we use both. However, the more "stable" the
292 stable side becomes, the more we tend to prefer the former
293 (comparison with a constant, "i > 0", is an extreme example).
294 Just do not mix styles in the same part of the code and mimic
295 existing styles in the neighbourhood.
296
f26443da
JH
297 - There are two schools of thought when it comes to splitting a long
298 logical line into multiple lines. Some people push the second and
299 subsequent lines far enough to the right with tabs and align them:
300
301 if (the_beginning_of_a_very_long_expression_that_has_to ||
302 span_more_than_a_single_line_of ||
303 the_source_text) {
304 ...
305
306 while other people prefer to align the second and the subsequent
307 lines with the column immediately inside the opening parenthesis,
308 with tabs and spaces, following our "tabstop is always a multiple
309 of 8" convention:
310
311 if (the_beginning_of_a_very_long_expression_that_has_to ||
312 span_more_than_a_single_line_of ||
313 the_source_text) {
314 ...
315
316 Both are valid, and we use both. Again, just do not mix styles in
317 the same part of the code and mimic existing styles in the
318 neighbourhood.
319
320 - When splitting a long logical line, some people change line before
321 a binary operator, so that the result looks like a parse tree when
322 you turn your head 90-degrees counterclockwise:
323
324 if (the_beginning_of_a_very_long_expression_that_has_to
325 || span_more_than_a_single_line_of_the_source_text) {
326
327 while other people prefer to leave the operator at the end of the
328 line:
329
330 if (the_beginning_of_a_very_long_expression_that_has_to ||
331 span_more_than_a_single_line_of_the_source_text) {
332
333 Both are valid, but we tend to use the latter more, unless the
334 expression gets fairly complex, in which case the former tends to
335 be easier to read. Again, just do not mix styles in the same part
336 of the code and mimic existing styles in the neighbourhood.
337
338 - When splitting a long logical line, with everything else being
339 equal, it is preferable to split after the operator at higher
340 level in the parse tree. That is, this is more preferable:
341
342 if (a_very_long_variable * that_is_used_in +
343 a_very_long_expression) {
344 ...
345
346 than
347
348 if (a_very_long_variable *
349 that_is_used_in + a_very_long_expression) {
350 ...
351
6d0618a8
JS
352 - Some clever tricks, like using the !! operator with arithmetic
353 constructs, can be extremely confusing to others. Avoid them,
354 unless there is a compelling reason to use them.
355
356 - Use the API. No, really. We have a strbuf (variable length
357 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 358 string_list for sorted string lists, a hash map (mapping struct
6d0618a8
JS
359 objects) named "struct decorate", amongst other things.
360
361 - When you come up with an API, document it.
362
412cb2ec
JH
363 - The first #include in C files, except in platform specific compat/
364 implementations, must be either "git-compat-util.h", "cache.h" or
365 "builtin.h". You do not have to include more than one of these.
366
367 - A C file must directly include the header files that declare the
368 functions and the types it uses, except for the functions and types
369 that are made available to it by including one of the header files
370 it must include by the previous rule.
6d0618a8
JS
371
372 - If you are planning a new command, consider writing it in shell
373 or perl first, so that changes in semantics can be easily
2de9b711 374 changed and discussed. Many Git commands started out like
6d0618a8
JS
375 that, and a few are still scripts.
376
2de9b711 377 - Avoid introducing a new dependency into Git. This means you
6d0618a8 378 usually should stay away from scripting languages not already
2de9b711 379 used in the Git core command set (unless your command is clearly
6d0618a8 380 separate from it, such as an importer to convert random-scm-X
2de9b711 381 repositories to Git).
57199892
KB
382
383 - When we pass <string, length> pair to functions, we should try to
384 pass them in that order.
c455bd89 385
5e9637c6
ÆAB
386 - Use Git's gettext wrappers to make the user interface
387 translatable. See "Marking strings for translation" in po/README.
388
89a9f2c8
JK
389 - Variables and functions local to a given source file should be marked
390 with "static". Variables that are visible to other source files
391 must be declared with "extern" in header files. However, function
392 declarations should not use "extern", as that is already the default.
393
c5e366b1
TZ
394For Perl programs:
395
396 - Most of the C guidelines above apply.
397
398 - We try to support Perl 5.8 and later ("use Perl 5.008").
399
400 - use strict and use warnings are strongly preferred.
401
402 - Don't overuse statement modifiers unless using them makes the
403 result easier to follow.
404
405 ... do something ...
406 do_this() unless (condition);
407 ... do something else ...
408
409 is more readable than:
410
411 ... do something ...
412 unless (condition) {
413 do_this();
414 }
415 ... do something else ...
416
417 *only* when the condition is so rare that do_this() will be almost
418 always called.
419
420 - We try to avoid assignments inside "if ()" conditions.
421
422 - Learn and use Git.pm if you need that functionality.
423
424 - For Emacs, it's useful to put the following in
425 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
426
427 ;; note the first part is useful for C editing, too
428 ((nil . ((indent-tabs-mode . t)
429 (tab-width . 8)
430 (fill-column . 80)))
431 (cperl-mode . ((cperl-indent-level . 8)
432 (cperl-extra-newline-before-brace . nil)
433 (cperl-merge-trailing-else . t))))
434
9ef43dd7
JK
435For Python scripts:
436
437 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
438
439 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
440
441 - Where required libraries do not restrict us to Python 2, we try to
442 also be compatible with Python 3.1 and later.
443
444 - When you must differentiate between Unicode literals and byte string
445 literals, it is OK to use the 'b' prefix. Even though the Python
446 documentation for version 2.6 does not mention this prefix, it has
447 been supported since version 2.6.0.
448
0ae0e882
PO
449Error Messages
450
451 - Do not end error messages with a full stop.
452
453 - Do not capitalize ("unable to open %s", not "Unable to open %s")
454
455 - Say what the error is first ("cannot open %s", not "%s: cannot open")
456
457
35840a3e
JH
458Externally Visible Names
459
460 - For configuration variable names, follow the existing convention:
461
462 . The section name indicates the affected subsystem.
463
464 . The subsection name, if any, indicates which of an unbounded set
465 of things to set the value for.
466
467 . The variable name describes the effect of tweaking this knob.
468
469 The section and variable names that consist of multiple words are
470 formed by concatenating the words without punctuations (e.g. `-`),
471 and are broken using bumpyCaps in documentation as a hint to the
472 reader.
473
474 When choosing the variable namespace, do not use variable name for
475 specifying possibly unbounded set of things, most notably anything
476 an end user can freely come up with (e.g. branch names). Instead,
477 use subsection names or variable values, like the existing variable
478 branch.<name>.description does.
479
480
c455bd89
ŠN
481Writing Documentation:
482
48bc1755
DW
483 Most (if not all) of the documentation pages are written in the
484 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
485 processed into HTML and manpages (e.g. git.html and git.1 in the
486 same directory).
bb9f2aec 487
42e0fae9
MB
488 The documentation liberally mixes US and UK English (en_US/UK)
489 norms for spelling and grammar, which is somewhat unfortunate.
490 In an ideal world, it would have been better if it consistently
491 used only one and not the other, and we would have picked en_US
492 (if you wish to correct the English of some of the existing
493 documentation, please see the documentation-related advice in the
494 Documentation/SubmittingPatches file).
495
c455bd89
ŠN
496 Every user-visible change should be reflected in the documentation.
497 The same general rule as for code applies -- imitate the existing
ca03c368
JSJ
498 conventions.
499
500 A few commented examples follow to provide reference when writing or
501 modifying command usage strings and synopsis sections in the manual
502 pages:
c455bd89 503
b1afe49d 504 Placeholders are spelled in lowercase and enclosed in angle brackets:
c455bd89
ŠN
505 <file>
506 --sort=<key>
507 --abbrev[=<n>]
508
9c9b4f2f
AH
509 If a placeholder has multiple words, they are separated by dashes:
510 <new-branch-name>
511 --template=<template-directory>
512
469bfc96 513 Possibility of multiple occurrences is indicated by three dots:
c455bd89
ŠN
514 <file>...
515 (One or more of <file>.)
516
517 Optional parts are enclosed in square brackets:
518 [<extra>]
519 (Zero or one <extra>.)
520
521 --exec-path[=<path>]
522 (Option with an optional argument. Note that the "=" is inside the
523 brackets.)
524
525 [<patch>...]
526 (Zero or more of <patch>. Note that the dots are inside, not
527 outside the brackets.)
528
9c9b4f2f 529 Multiple alternatives are indicated with vertical bars:
c455bd89
ŠN
530 [-q | --quiet]
531 [--utf8 | --no-utf8]
532
533 Parentheses are used for grouping:
9c9b4f2f 534 [(<rev> | <range>)...]
c455bd89
ŠN
535 (Any number of either <rev> or <range>. Parens are needed to make
536 it clear that "..." pertains to both <rev> and <range>.)
537
538 [(-p <parent>)...]
539 (Any number of option -p, each with one <parent> argument.)
540
541 git remote set-head <name> (-a | -d | <branch>)
542 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
543 brackets) be provided.)
544
545 And a somewhat more contrived example:
546 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
547 Here "=" is outside the brackets, because "--diff-filter=" is a
548 valid usage. "*" has its own pair of brackets, because it can
549 (optionally) be specified only when one or more of the letters is
550 also provided.
48a8c26c
TA
551
552 A note on notation:
553 Use 'git' (all lowercase) when talking about commands i.e. something
554 the user would type into a shell and use 'Git' (uppercase first letter)
555 when talking about the version control system and its properties.
ca03c368
JSJ
556
557 A few commented examples follow to provide reference when writing or
558 modifying paragraphs or option/command explanations that contain options
559 or commands:
560
41f5b21f 561 Literal examples (e.g. use of command-line options, command names,
57103dbf
MM
562 branch names, configuration and environment variables) must be
563 typeset in monospace (i.e. wrapped with backticks):
ca03c368
JSJ
564 `--pretty=oneline`
565 `git rev-list`
da0005b8 566 `remote.pushDefault`
41f5b21f 567 `GIT_DIR`
57103dbf 568 `HEAD`
41f5b21f
TR
569
570 An environment variable must be prefixed with "$" only when referring to its
571 value and not when referring to the variable itself, in this case there is
572 nothing to add except the backticks:
573 `GIT_DIR` is specified
574 `$GIT_DIR/hooks/pre-receive`
ca03c368
JSJ
575
576 Word phrases enclosed in `backtick characters` are rendered literally
577 and will not be further expanded. The use of `backticks` to achieve the
578 previous rule means that literal examples should not use AsciiDoc
579 escapes.
580 Correct:
581 `--pretty=oneline`
582 Incorrect:
583 `\--pretty=oneline`
584
585 If some place in the documentation needs to typeset a command usage
586 example with inline substitutions, it is fine to use +monospaced and
587 inline substituted text+ instead of `monospaced literal text`, and with
588 the former, the part that should not get substituted must be
589 quoted/escaped.