CodingGuidelines: give an example for redirection
[git/git.git] / Documentation / CodingGuidelines
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6d0618a8 1Like other projects, we also have some guidelines to keep to the
2de9b711 2code. For Git in general, three rough rules are:
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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
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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.
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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
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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.
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40
41For shell scripts specifically (not exhaustive):
42
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43 - We use tabs for indentation.
44
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45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
f36a4fa8 56
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57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
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64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
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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
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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
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81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
6d0618a8 83
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84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
6d0618a8 86
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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
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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).
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103
104 - We do not use Process Substitution <(list) or >(list).
105
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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
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110 - We prefer "test" over "[ ... ]".
111
112 - We do not write the noiseword "function" in front of shell
113 functions.
114
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115 - We prefer a space between the function name and the parentheses. The
116 opening "{" should also be on the same line.
117 E.g.: my_function () {
118
009c98ee 119 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 120 [::], [==], or [..]) for portability.
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121
122 - We do not use \{m,n\};
123
124 - We do not use -E;
125
a58088ab 126 - We do not use ? or + (which are \{0,1\} and \{1,\}
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127 respectively in BRE) but that goes without saying as these
128 are ERE elements not BRE (note that \? and \+ are not even part
129 of BRE -- making them accessible from BRE is a GNU extension).
130
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131 - Use Git's gettext wrappers in git-sh-i18n to make the user
132 interface translatable. See "Marking strings for translation" in
133 po/README.
134
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135For C programs:
136
137 - We use tabs to indent, and interpret tabs as taking up to
138 8 spaces.
139
140 - We try to keep to at most 80 characters per line.
141
2de9b711 142 - We try to support a wide range of C compilers to compile Git with,
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143 including old ones. That means that you should not use C99
144 initializers, even if a lot of compilers grok it.
145
146 - Variables have to be declared at the beginning of the block.
147
148 - NULL pointers shall be written as NULL, not as 0.
149
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150 - When declaring pointers, the star sides with the variable
151 name, i.e. "char *string", not "char* string" or
152 "char * string". This makes it easier to understand code
153 like "char *string, c;".
154
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155 - Use whitespace around operators and keywords, but not inside
156 parentheses and not around functions. So:
157
158 while (condition)
159 func(bar + 1);
160
161 and not:
162
163 while( condition )
164 func (bar+1);
165
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166 - We avoid using braces unnecessarily. I.e.
167
168 if (bla) {
169 x = 1;
170 }
171
172 is frowned upon. A gray area is when the statement extends
173 over a few lines, and/or you have a lengthy comment atop of
174 it. Also, like in the Linux kernel, if there is a long list
175 of "else if" statements, it can make sense to add braces to
176 single line blocks.
177
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178 - We try to avoid assignments inside if().
179
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180 - Try to make your code understandable. You may put comments
181 in, but comments invariably tend to stale out when the code
182 they were describing changes. Often splitting a function
183 into two makes the intention of the code much clearer.
184
b75a6ca7 185 - Multi-line comments include their delimiters on separate lines from
186 the text. E.g.
187
188 /*
189 * A very long
190 * multi-line comment.
191 */
192
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193 Note however that a comment that explains a translatable string to
194 translators uses a convention of starting with a magic token
195 "TRANSLATORS: " immediately after the opening delimiter, even when
196 it spans multiple lines. We do not add an asterisk at the beginning
197 of each line, either. E.g.
198
199 /* TRANSLATORS: here is a comment that explains the string
200 to be translated, that follows immediately after it */
201 _("Here is a translatable string explained by the above.");
202
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203 - Double negation is often harder to understand than no negation
204 at all.
205
206 - Some clever tricks, like using the !! operator with arithmetic
207 constructs, can be extremely confusing to others. Avoid them,
208 unless there is a compelling reason to use them.
209
210 - Use the API. No, really. We have a strbuf (variable length
211 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 212 string_list for sorted string lists, a hash map (mapping struct
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213 objects) named "struct decorate", amongst other things.
214
215 - When you come up with an API, document it.
216
217 - The first #include in C files, except in platform specific
218 compat/ implementations, should be git-compat-util.h or another
219 header file that includes it, such as cache.h or builtin.h.
220
221 - If you are planning a new command, consider writing it in shell
222 or perl first, so that changes in semantics can be easily
2de9b711 223 changed and discussed. Many Git commands started out like
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224 that, and a few are still scripts.
225
2de9b711 226 - Avoid introducing a new dependency into Git. This means you
6d0618a8 227 usually should stay away from scripting languages not already
2de9b711 228 used in the Git core command set (unless your command is clearly
6d0618a8 229 separate from it, such as an importer to convert random-scm-X
2de9b711 230 repositories to Git).
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231
232 - When we pass <string, length> pair to functions, we should try to
233 pass them in that order.
c455bd89 234
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235 - Use Git's gettext wrappers to make the user interface
236 translatable. See "Marking strings for translation" in po/README.
237
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238For Perl programs:
239
240 - Most of the C guidelines above apply.
241
242 - We try to support Perl 5.8 and later ("use Perl 5.008").
243
244 - use strict and use warnings are strongly preferred.
245
246 - Don't overuse statement modifiers unless using them makes the
247 result easier to follow.
248
249 ... do something ...
250 do_this() unless (condition);
251 ... do something else ...
252
253 is more readable than:
254
255 ... do something ...
256 unless (condition) {
257 do_this();
258 }
259 ... do something else ...
260
261 *only* when the condition is so rare that do_this() will be almost
262 always called.
263
264 - We try to avoid assignments inside "if ()" conditions.
265
266 - Learn and use Git.pm if you need that functionality.
267
268 - For Emacs, it's useful to put the following in
269 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
270
271 ;; note the first part is useful for C editing, too
272 ((nil . ((indent-tabs-mode . t)
273 (tab-width . 8)
274 (fill-column . 80)))
275 (cperl-mode . ((cperl-indent-level . 8)
276 (cperl-extra-newline-before-brace . nil)
277 (cperl-merge-trailing-else . t))))
278
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279For Python scripts:
280
281 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
282
283 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
284
285 - Where required libraries do not restrict us to Python 2, we try to
286 also be compatible with Python 3.1 and later.
287
288 - When you must differentiate between Unicode literals and byte string
289 literals, it is OK to use the 'b' prefix. Even though the Python
290 documentation for version 2.6 does not mention this prefix, it has
291 been supported since version 2.6.0.
292
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293Writing Documentation:
294
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295 Most (if not all) of the documentation pages are written in the
296 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
297 processed into HTML and manpages (e.g. git.html and git.1 in the
298 same directory).
bb9f2aec 299
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300 The documentation liberally mixes US and UK English (en_US/UK)
301 norms for spelling and grammar, which is somewhat unfortunate.
302 In an ideal world, it would have been better if it consistently
303 used only one and not the other, and we would have picked en_US
304 (if you wish to correct the English of some of the existing
305 documentation, please see the documentation-related advice in the
306 Documentation/SubmittingPatches file).
307
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308 Every user-visible change should be reflected in the documentation.
309 The same general rule as for code applies -- imitate the existing
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310 conventions.
311
312 A few commented examples follow to provide reference when writing or
313 modifying command usage strings and synopsis sections in the manual
314 pages:
c455bd89 315
b1afe49d 316 Placeholders are spelled in lowercase and enclosed in angle brackets:
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317 <file>
318 --sort=<key>
319 --abbrev[=<n>]
320
469bfc96 321 Possibility of multiple occurrences is indicated by three dots:
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322 <file>...
323 (One or more of <file>.)
324
325 Optional parts are enclosed in square brackets:
326 [<extra>]
327 (Zero or one <extra>.)
328
329 --exec-path[=<path>]
330 (Option with an optional argument. Note that the "=" is inside the
331 brackets.)
332
333 [<patch>...]
334 (Zero or more of <patch>. Note that the dots are inside, not
335 outside the brackets.)
336
337 Multiple alternatives are indicated with vertical bar:
338 [-q | --quiet]
339 [--utf8 | --no-utf8]
340
341 Parentheses are used for grouping:
342 [(<rev>|<range>)...]
343 (Any number of either <rev> or <range>. Parens are needed to make
344 it clear that "..." pertains to both <rev> and <range>.)
345
346 [(-p <parent>)...]
347 (Any number of option -p, each with one <parent> argument.)
348
349 git remote set-head <name> (-a | -d | <branch>)
350 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
351 brackets) be provided.)
352
353 And a somewhat more contrived example:
354 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
355 Here "=" is outside the brackets, because "--diff-filter=" is a
356 valid usage. "*" has its own pair of brackets, because it can
357 (optionally) be specified only when one or more of the letters is
358 also provided.
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359
360 A note on notation:
361 Use 'git' (all lowercase) when talking about commands i.e. something
362 the user would type into a shell and use 'Git' (uppercase first letter)
363 when talking about the version control system and its properties.
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364
365 A few commented examples follow to provide reference when writing or
366 modifying paragraphs or option/command explanations that contain options
367 or commands:
368
369 Literal examples (e.g. use of command-line options, command names, and
370 configuration variables) are typeset in monospace, and if you can use
371 `backticks around word phrases`, do so.
372 `--pretty=oneline`
373 `git rev-list`
374 `remote.pushdefault`
375
376 Word phrases enclosed in `backtick characters` are rendered literally
377 and will not be further expanded. The use of `backticks` to achieve the
378 previous rule means that literal examples should not use AsciiDoc
379 escapes.
380 Correct:
381 `--pretty=oneline`
382 Incorrect:
383 `\--pretty=oneline`
384
385 If some place in the documentation needs to typeset a command usage
386 example with inline substitutions, it is fine to use +monospaced and
387 inline substituted text+ instead of `monospaced literal text`, and with
388 the former, the part that should not get substituted must be
389 quoted/escaped.