CodingGuidelines: give an example for case/esac statement
[git/git.git] / Documentation / CodingGuidelines
CommitLineData
6d0618a8 1Like other projects, we also have some guidelines to keep to the
2de9b711 2code. For Git in general, three 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
6d0618a8
JS
64 - We prefer $( ... ) for command substitution; unlike ``, it
65 properly nests. It should have been the way Bourne spelled
66 it from day one, but unfortunately isn't.
67
860f70f9
TH
68 - If you want to find out if a command is available on the user's
69 $PATH, you should use 'type <command>', instead of 'which <command>'.
70 The output of 'which' is not machine parseable and its exit code
71 is not reliable across platforms.
72
bc979945
JH
73 - We use POSIX compliant parameter substitutions and avoid bashisms;
74 namely:
6d0618a8 75
bc979945
JH
76 - We use ${parameter-word} and its [-=?+] siblings, and their
77 colon'ed "unset or null" form.
6d0618a8 78
bc979945
JH
79 - We use ${parameter#word} and its [#%] siblings, and their
80 doubled "longest matching" form.
6d0618a8 81
bc979945 82 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 83
bc979945 84 - No shell arrays.
6d0618a8 85
bc979945 86 - No strlen ${#parameter}.
6d0618a8 87
bc979945 88 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 89
bc979945
JH
90 - We use Arithmetic Expansion $(( ... )).
91
92 - Inside Arithmetic Expansion, spell shell variables with $ in front
93 of them, as some shells do not grok $((x)) while accepting $(($x))
94 just fine (e.g. dash older than 0.5.4).
6d0618a8
JS
95
96 - We do not use Process Substitution <(list) or >(list).
97
03b05c7d
HV
98 - Do not write control structures on a single line with semicolon.
99 "then" should be on the next line for if statements, and "do"
100 should be on the next line for "while" and "for".
101
6d0618a8
JS
102 - We prefer "test" over "[ ... ]".
103
104 - We do not write the noiseword "function" in front of shell
105 functions.
106
03b05c7d
HV
107 - We prefer a space between the function name and the parentheses. The
108 opening "{" should also be on the same line.
109 E.g.: my_function () {
110
009c98ee 111 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 112 [::], [==], or [..]) for portability.
009c98ee
JH
113
114 - We do not use \{m,n\};
115
116 - We do not use -E;
117
a58088ab 118 - We do not use ? or + (which are \{0,1\} and \{1,\}
009c98ee
JH
119 respectively in BRE) but that goes without saying as these
120 are ERE elements not BRE (note that \? and \+ are not even part
121 of BRE -- making them accessible from BRE is a GNU extension).
122
5e9637c6
ÆAB
123 - Use Git's gettext wrappers in git-sh-i18n to make the user
124 interface translatable. See "Marking strings for translation" in
125 po/README.
126
6d0618a8
JS
127For C programs:
128
129 - We use tabs to indent, and interpret tabs as taking up to
130 8 spaces.
131
132 - We try to keep to at most 80 characters per line.
133
2de9b711 134 - We try to support a wide range of C compilers to compile Git with,
a26fd033
AS
135 including old ones. That means that you should not use C99
136 initializers, even if a lot of compilers grok it.
137
138 - Variables have to be declared at the beginning of the block.
139
140 - NULL pointers shall be written as NULL, not as 0.
141
6d0618a8
JS
142 - When declaring pointers, the star sides with the variable
143 name, i.e. "char *string", not "char* string" or
144 "char * string". This makes it easier to understand code
145 like "char *string, c;".
146
f57b6cfd
JK
147 - Use whitespace around operators and keywords, but not inside
148 parentheses and not around functions. So:
149
150 while (condition)
151 func(bar + 1);
152
153 and not:
154
155 while( condition )
156 func (bar+1);
157
6d0618a8
JS
158 - We avoid using braces unnecessarily. I.e.
159
160 if (bla) {
161 x = 1;
162 }
163
164 is frowned upon. A gray area is when the statement extends
165 over a few lines, and/or you have a lengthy comment atop of
166 it. Also, like in the Linux kernel, if there is a long list
167 of "else if" statements, it can make sense to add braces to
168 single line blocks.
169
0b0b8cd7
MV
170 - We try to avoid assignments inside if().
171
6d0618a8
JS
172 - Try to make your code understandable. You may put comments
173 in, but comments invariably tend to stale out when the code
174 they were describing changes. Often splitting a function
175 into two makes the intention of the code much clearer.
176
b75a6ca7 177 - Multi-line comments include their delimiters on separate lines from
178 the text. E.g.
179
180 /*
181 * A very long
182 * multi-line comment.
183 */
184
cbcfd4e3
JH
185 Note however that a comment that explains a translatable string to
186 translators uses a convention of starting with a magic token
187 "TRANSLATORS: " immediately after the opening delimiter, even when
188 it spans multiple lines. We do not add an asterisk at the beginning
189 of each line, either. E.g.
190
191 /* TRANSLATORS: here is a comment that explains the string
192 to be translated, that follows immediately after it */
193 _("Here is a translatable string explained by the above.");
194
6d0618a8
JS
195 - Double negation is often harder to understand than no negation
196 at all.
197
198 - Some clever tricks, like using the !! operator with arithmetic
199 constructs, can be extremely confusing to others. Avoid them,
200 unless there is a compelling reason to use them.
201
202 - Use the API. No, really. We have a strbuf (variable length
203 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 204 string_list for sorted string lists, a hash map (mapping struct
6d0618a8
JS
205 objects) named "struct decorate", amongst other things.
206
207 - When you come up with an API, document it.
208
209 - The first #include in C files, except in platform specific
210 compat/ implementations, should be git-compat-util.h or another
211 header file that includes it, such as cache.h or builtin.h.
212
213 - If you are planning a new command, consider writing it in shell
214 or perl first, so that changes in semantics can be easily
2de9b711 215 changed and discussed. Many Git commands started out like
6d0618a8
JS
216 that, and a few are still scripts.
217
2de9b711 218 - Avoid introducing a new dependency into Git. This means you
6d0618a8 219 usually should stay away from scripting languages not already
2de9b711 220 used in the Git core command set (unless your command is clearly
6d0618a8 221 separate from it, such as an importer to convert random-scm-X
2de9b711 222 repositories to Git).
57199892
KB
223
224 - When we pass <string, length> pair to functions, we should try to
225 pass them in that order.
c455bd89 226
5e9637c6
ÆAB
227 - Use Git's gettext wrappers to make the user interface
228 translatable. See "Marking strings for translation" in po/README.
229
c5e366b1
TZ
230For Perl programs:
231
232 - Most of the C guidelines above apply.
233
234 - We try to support Perl 5.8 and later ("use Perl 5.008").
235
236 - use strict and use warnings are strongly preferred.
237
238 - Don't overuse statement modifiers unless using them makes the
239 result easier to follow.
240
241 ... do something ...
242 do_this() unless (condition);
243 ... do something else ...
244
245 is more readable than:
246
247 ... do something ...
248 unless (condition) {
249 do_this();
250 }
251 ... do something else ...
252
253 *only* when the condition is so rare that do_this() will be almost
254 always called.
255
256 - We try to avoid assignments inside "if ()" conditions.
257
258 - Learn and use Git.pm if you need that functionality.
259
260 - For Emacs, it's useful to put the following in
261 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
262
263 ;; note the first part is useful for C editing, too
264 ((nil . ((indent-tabs-mode . t)
265 (tab-width . 8)
266 (fill-column . 80)))
267 (cperl-mode . ((cperl-indent-level . 8)
268 (cperl-extra-newline-before-brace . nil)
269 (cperl-merge-trailing-else . t))))
270
9ef43dd7
JK
271For Python scripts:
272
273 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
274
275 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
276
277 - Where required libraries do not restrict us to Python 2, we try to
278 also be compatible with Python 3.1 and later.
279
280 - When you must differentiate between Unicode literals and byte string
281 literals, it is OK to use the 'b' prefix. Even though the Python
282 documentation for version 2.6 does not mention this prefix, it has
283 been supported since version 2.6.0.
284
c455bd89
ŠN
285Writing Documentation:
286
48bc1755
DW
287 Most (if not all) of the documentation pages are written in the
288 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
289 processed into HTML and manpages (e.g. git.html and git.1 in the
290 same directory).
bb9f2aec 291
42e0fae9
MB
292 The documentation liberally mixes US and UK English (en_US/UK)
293 norms for spelling and grammar, which is somewhat unfortunate.
294 In an ideal world, it would have been better if it consistently
295 used only one and not the other, and we would have picked en_US
296 (if you wish to correct the English of some of the existing
297 documentation, please see the documentation-related advice in the
298 Documentation/SubmittingPatches file).
299
c455bd89
ŠN
300 Every user-visible change should be reflected in the documentation.
301 The same general rule as for code applies -- imitate the existing
ca03c368
JSJ
302 conventions.
303
304 A few commented examples follow to provide reference when writing or
305 modifying command usage strings and synopsis sections in the manual
306 pages:
c455bd89 307
b1afe49d 308 Placeholders are spelled in lowercase and enclosed in angle brackets:
c455bd89
ŠN
309 <file>
310 --sort=<key>
311 --abbrev[=<n>]
312
469bfc96 313 Possibility of multiple occurrences is indicated by three dots:
c455bd89
ŠN
314 <file>...
315 (One or more of <file>.)
316
317 Optional parts are enclosed in square brackets:
318 [<extra>]
319 (Zero or one <extra>.)
320
321 --exec-path[=<path>]
322 (Option with an optional argument. Note that the "=" is inside the
323 brackets.)
324
325 [<patch>...]
326 (Zero or more of <patch>. Note that the dots are inside, not
327 outside the brackets.)
328
329 Multiple alternatives are indicated with vertical bar:
330 [-q | --quiet]
331 [--utf8 | --no-utf8]
332
333 Parentheses are used for grouping:
334 [(<rev>|<range>)...]
335 (Any number of either <rev> or <range>. Parens are needed to make
336 it clear that "..." pertains to both <rev> and <range>.)
337
338 [(-p <parent>)...]
339 (Any number of option -p, each with one <parent> argument.)
340
341 git remote set-head <name> (-a | -d | <branch>)
342 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
343 brackets) be provided.)
344
345 And a somewhat more contrived example:
346 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
347 Here "=" is outside the brackets, because "--diff-filter=" is a
348 valid usage. "*" has its own pair of brackets, because it can
349 (optionally) be specified only when one or more of the letters is
350 also provided.
48a8c26c
TA
351
352 A note on notation:
353 Use 'git' (all lowercase) when talking about commands i.e. something
354 the user would type into a shell and use 'Git' (uppercase first letter)
355 when talking about the version control system and its properties.
ca03c368
JSJ
356
357 A few commented examples follow to provide reference when writing or
358 modifying paragraphs or option/command explanations that contain options
359 or commands:
360
361 Literal examples (e.g. use of command-line options, command names, and
362 configuration variables) are typeset in monospace, and if you can use
363 `backticks around word phrases`, do so.
364 `--pretty=oneline`
365 `git rev-list`
366 `remote.pushdefault`
367
368 Word phrases enclosed in `backtick characters` are rendered literally
369 and will not be further expanded. The use of `backticks` to achieve the
370 previous rule means that literal examples should not use AsciiDoc
371 escapes.
372 Correct:
373 `--pretty=oneline`
374 Incorrect:
375 `\--pretty=oneline`
376
377 If some place in the documentation needs to typeset a command usage
378 example with inline substitutions, it is fine to use +monospaced and
379 inline substituted text+ instead of `monospaced literal text`, and with
380 the former, the part that should not get substituted must be
381 quoted/escaped.