CodingGuidelines: do not call the conditional statement "if()"
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1Like other projects, we also have some guidelines to keep to the
2code. For Git in general, three rough rules are:
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
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
29Make your code readable and sensible, and don't try to be clever.
30
31As for more concrete guidelines, just imitate the existing code
32(this is a good guideline, no matter which project you are
33contributing to). It is always preferable to match the _local_
34convention. New code added to Git suite is expected to match
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.
40
41For shell scripts specifically (not exhaustive):
42
43 - We use tabs for indentation.
44
45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
56
57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
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
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
81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
83
84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
86
87 - We use ${parameter#word} and its [#%] siblings, and their
88 doubled "longest matching" form.
89
90 - No "Substring Expansion" ${parameter:offset:length}.
91
92 - No shell arrays.
93
94 - No strlen ${#parameter}.
95
96 - No pattern replacement ${parameter/pattern/string}.
97
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).
103
104 - We do not use Process Substitution <(list) or >(list).
105
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
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
121 - We prefer "test" over "[ ... ]".
122
123 - We do not write the noiseword "function" in front of shell
124 functions.
125
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 ...
137
138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
139 [::], [==], or [..]) for portability.
140
141 - We do not use \{m,n\};
142
143 - We do not use -E;
144
145 - We do not use ? or + (which are \{0,1\} and \{1,\}
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
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
154For C programs:
155
156 - We use tabs to indent, and interpret tabs as taking up to
157 8 spaces.
158
159 - We try to keep to at most 80 characters per line.
160
161 - We try to support a wide range of C compilers to compile Git with,
162 including old ones. That means that you should not use C99
163 initializers, even if a lot of compilers grok it.
164
165 - Variables have to be declared at the beginning of the block.
166
167 - NULL pointers shall be written as NULL, not as 0.
168
169 - When declaring pointers, the star sides with the variable
170 name, i.e. "char *string", not "char* string" or
171 "char * string". This makes it easier to understand code
172 like "char *string, c;".
173
174 - Use whitespace around operators and keywords, but not inside
175 parentheses and not around functions. So:
176
177 while (condition)
178 func(bar + 1);
179
180 and not:
181
182 while( condition )
183 func (bar+1);
184
185 - We avoid using braces unnecessarily. I.e.
186
187 if (bla) {
188 x = 1;
189 }
190
191 is frowned upon. A gray area is when the statement extends
192 over a few lines, and/or you have a lengthy comment atop of
193 it. Also, like in the Linux kernel, if there is a long list
194 of "else if" statements, it can make sense to add braces to
195 single line blocks.
196
197 - We try to avoid assignments in the condition of an "if" statement.
198
199 - Try to make your code understandable. You may put comments
200 in, but comments invariably tend to stale out when the code
201 they were describing changes. Often splitting a function
202 into two makes the intention of the code much clearer.
203
204 - Multi-line comments include their delimiters on separate lines from
205 the text. E.g.
206
207 /*
208 * A very long
209 * multi-line comment.
210 */
211
212 Note however that a comment that explains a translatable string to
213 translators uses a convention of starting with a magic token
214 "TRANSLATORS: " immediately after the opening delimiter, even when
215 it spans multiple lines. We do not add an asterisk at the beginning
216 of each line, either. E.g.
217
218 /* TRANSLATORS: here is a comment that explains the string
219 to be translated, that follows immediately after it */
220 _("Here is a translatable string explained by the above.");
221
222 - Double negation is often harder to understand than no negation
223 at all.
224
225 - Some clever tricks, like using the !! operator with arithmetic
226 constructs, can be extremely confusing to others. Avoid them,
227 unless there is a compelling reason to use them.
228
229 - Use the API. No, really. We have a strbuf (variable length
230 string), several arrays with the ALLOC_GROW() macro, a
231 string_list for sorted string lists, a hash map (mapping struct
232 objects) named "struct decorate", amongst other things.
233
234 - When you come up with an API, document it.
235
236 - The first #include in C files, except in platform specific
237 compat/ implementations, should be git-compat-util.h or another
238 header file that includes it, such as cache.h or builtin.h.
239
240 - If you are planning a new command, consider writing it in shell
241 or perl first, so that changes in semantics can be easily
242 changed and discussed. Many Git commands started out like
243 that, and a few are still scripts.
244
245 - Avoid introducing a new dependency into Git. This means you
246 usually should stay away from scripting languages not already
247 used in the Git core command set (unless your command is clearly
248 separate from it, such as an importer to convert random-scm-X
249 repositories to Git).
250
251 - When we pass <string, length> pair to functions, we should try to
252 pass them in that order.
253
254 - Use Git's gettext wrappers to make the user interface
255 translatable. See "Marking strings for translation" in po/README.
256
257For Perl programs:
258
259 - Most of the C guidelines above apply.
260
261 - We try to support Perl 5.8 and later ("use Perl 5.008").
262
263 - use strict and use warnings are strongly preferred.
264
265 - Don't overuse statement modifiers unless using them makes the
266 result easier to follow.
267
268 ... do something ...
269 do_this() unless (condition);
270 ... do something else ...
271
272 is more readable than:
273
274 ... do something ...
275 unless (condition) {
276 do_this();
277 }
278 ... do something else ...
279
280 *only* when the condition is so rare that do_this() will be almost
281 always called.
282
283 - We try to avoid assignments inside "if ()" conditions.
284
285 - Learn and use Git.pm if you need that functionality.
286
287 - For Emacs, it's useful to put the following in
288 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
289
290 ;; note the first part is useful for C editing, too
291 ((nil . ((indent-tabs-mode . t)
292 (tab-width . 8)
293 (fill-column . 80)))
294 (cperl-mode . ((cperl-indent-level . 8)
295 (cperl-extra-newline-before-brace . nil)
296 (cperl-merge-trailing-else . t))))
297
298For Python scripts:
299
300 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
301
302 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
303
304 - Where required libraries do not restrict us to Python 2, we try to
305 also be compatible with Python 3.1 and later.
306
307 - When you must differentiate between Unicode literals and byte string
308 literals, it is OK to use the 'b' prefix. Even though the Python
309 documentation for version 2.6 does not mention this prefix, it has
310 been supported since version 2.6.0.
311
312Writing Documentation:
313
314 Most (if not all) of the documentation pages are written in the
315 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
316 processed into HTML and manpages (e.g. git.html and git.1 in the
317 same directory).
318
319 The documentation liberally mixes US and UK English (en_US/UK)
320 norms for spelling and grammar, which is somewhat unfortunate.
321 In an ideal world, it would have been better if it consistently
322 used only one and not the other, and we would have picked en_US
323 (if you wish to correct the English of some of the existing
324 documentation, please see the documentation-related advice in the
325 Documentation/SubmittingPatches file).
326
327 Every user-visible change should be reflected in the documentation.
328 The same general rule as for code applies -- imitate the existing
329 conventions.
330
331 A few commented examples follow to provide reference when writing or
332 modifying command usage strings and synopsis sections in the manual
333 pages:
334
335 Placeholders are spelled in lowercase and enclosed in angle brackets:
336 <file>
337 --sort=<key>
338 --abbrev[=<n>]
339
340 Possibility of multiple occurrences is indicated by three dots:
341 <file>...
342 (One or more of <file>.)
343
344 Optional parts are enclosed in square brackets:
345 [<extra>]
346 (Zero or one <extra>.)
347
348 --exec-path[=<path>]
349 (Option with an optional argument. Note that the "=" is inside the
350 brackets.)
351
352 [<patch>...]
353 (Zero or more of <patch>. Note that the dots are inside, not
354 outside the brackets.)
355
356 Multiple alternatives are indicated with vertical bar:
357 [-q | --quiet]
358 [--utf8 | --no-utf8]
359
360 Parentheses are used for grouping:
361 [(<rev>|<range>)...]
362 (Any number of either <rev> or <range>. Parens are needed to make
363 it clear that "..." pertains to both <rev> and <range>.)
364
365 [(-p <parent>)...]
366 (Any number of option -p, each with one <parent> argument.)
367
368 git remote set-head <name> (-a | -d | <branch>)
369 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
370 brackets) be provided.)
371
372 And a somewhat more contrived example:
373 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
374 Here "=" is outside the brackets, because "--diff-filter=" is a
375 valid usage. "*" has its own pair of brackets, because it can
376 (optionally) be specified only when one or more of the letters is
377 also provided.
378
379 A note on notation:
380 Use 'git' (all lowercase) when talking about commands i.e. something
381 the user would type into a shell and use 'Git' (uppercase first letter)
382 when talking about the version control system and its properties.
383
384 A few commented examples follow to provide reference when writing or
385 modifying paragraphs or option/command explanations that contain options
386 or commands:
387
388 Literal examples (e.g. use of command-line options, command names, and
389 configuration variables) are typeset in monospace, and if you can use
390 `backticks around word phrases`, do so.
391 `--pretty=oneline`
392 `git rev-list`
393 `remote.pushdefault`
394
395 Word phrases enclosed in `backtick characters` are rendered literally
396 and will not be further expanded. The use of `backticks` to achieve the
397 previous rule means that literal examples should not use AsciiDoc
398 escapes.
399 Correct:
400 `--pretty=oneline`
401 Incorrect:
402 `\--pretty=oneline`
403
404 If some place in the documentation needs to typeset a command usage
405 example with inline substitutions, it is fine to use +monospaced and
406 inline substituted text+ instead of `monospaced literal text`, and with
407 the former, the part that should not get substituted must be
408 quoted/escaped.