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 21Make your code readable and sensible, and don't try to be clever. 22 23As for more concrete guidelines, just imitate the existing code 24(this is a good guideline, no matter which project you are 25contributing to). It is always preferable to match the _local_ 26convention. New code added to Git suite is expected to match 27the overall style of existing code. Modifications to existing 28code is expected to match the style the surrounding code already 29uses (even if it doesn't match the overall style of existing code). 30 31But if you must have a list of rules, here they are. 32 33For shell scripts specifically (not exhaustive): 34 35 - We use tabs for indentation. 36 37 - Case arms are indented at the same depth as case and esac lines. 38 39 - Redirection operators should be written with space before, but no 40 space after them. In other words, write 'echo test >"$file"' 41 instead of 'echo test> $file' or 'echo test > $file'. Note that 42 even though it is not required by POSIX to double-quote the 43 redirection target in a variable (as shown above), our code does so 44 because some versions of bash issue a warning without the quotes. 45 46 - We prefer $( ... ) for command substitution; unlike ``, it 47 properly nests. It should have been the way Bourne spelled 48 it from day one, but unfortunately isn't. 49 50 - If you want to find out if a command is available on the user's 51 $PATH, you should use 'type <command>', instead of 'which <command>'. 52 The output of 'which' is not machine parseable and its exit code 53 is not reliable across platforms. 54 55 - We use POSIX compliant parameter substitutions and avoid bashisms; 56 namely: 57 58 - We use ${parameter-word} and its [-=?+] siblings, and their 59 colon'ed "unset or null" form. 60 61 - We use ${parameter#word} and its [#%] siblings, and their 62 doubled "longest matching" form. 63 64 - No "Substring Expansion" ${parameter:offset:length}. 65 66 - No shell arrays. 67 68 - No strlen ${#parameter}. 69 70 - No pattern replacement ${parameter/pattern/string}. 71 72 - We use Arithmetic Expansion $(( ... )). 73 74 - Inside Arithmetic Expansion, spell shell variables with $ in front 75 of them, as some shells do not grok $((x)) while accepting $(($x)) 76 just fine (e.g. dash older than 0.5.4). 77 78 - We do not use Process Substitution <(list) or >(list). 79 80 - Do not write control structures on a single line with semicolon. 81 "then" should be on the next line for if statements, and "do" 82 should be on the next line for "while" and "for". 83 84 - We prefer "test" over "[ ... ]". 85 86 - We do not write the noiseword "function" in front of shell 87 functions. 88 89 - We prefer a space between the function name and the parentheses. The 90 opening "{" should also be on the same line. 91 E.g.: my_function () { 92 93 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\}, 94 [::], [==], or [..]) for portability. 95 96 - We do not use \{m,n\}; 97 98 - We do not use -E; 99 100 - We do not use ? or + (which are \{0,1\} and \{1,\} 101 respectively in BRE) but that goes without saying as these 102 are ERE elements not BRE (note that \? and \+ are not even part 103 of BRE -- making them accessible from BRE is a GNU extension). 104 105 - Use Git's gettext wrappers in git-sh-i18n to make the user 106 interface translatable. See "Marking strings for translation" in 107 po/README. 108 109For C programs: 110 111 - We use tabs to indent, and interpret tabs as taking up to 112 8 spaces. 113 114 - We try to keep to at most 80 characters per line. 115 116 - We try to support a wide range of C compilers to compile Git with, 117 including old ones. That means that you should not use C99 118 initializers, even if a lot of compilers grok it. 119 120 - Variables have to be declared at the beginning of the block. 121 122 - NULL pointers shall be written as NULL, not as 0. 123 124 - When declaring pointers, the star sides with the variable 125 name, i.e. "char *string", not "char* string" or 126 "char * string". This makes it easier to understand code 127 like "char *string, c;". 128 129 - Use whitespace around operators and keywords, but not inside 130 parentheses and not around functions. So: 131 132 while (condition) 133 func(bar + 1); 134 135 and not: 136 137 while( condition ) 138 func (bar+1); 139 140 - We avoid using braces unnecessarily. I.e. 141 142 if (bla) { 143 x = 1; 144 } 145 146 is frowned upon. A gray area is when the statement extends 147 over a few lines, and/or you have a lengthy comment atop of 148 it. Also, like in the Linux kernel, if there is a long list 149 of "else if" statements, it can make sense to add braces to 150 single line blocks. 151 152 - We try to avoid assignments inside if(). 153 154 - Try to make your code understandable. You may put comments 155 in, but comments invariably tend to stale out when the code 156 they were describing changes. Often splitting a function 157 into two makes the intention of the code much clearer. 158 159 - Multi-line comments include their delimiters on separate lines from 160 the text. E.g. 161 162 /* 163 * A very long 164 * multi-line comment. 165 */ 166 167 Note however that a comment that explains a translatable string to 168 translators uses a convention of starting with a magic token 169 "TRANSLATORS: " immediately after the opening delimiter, even when 170 it spans multiple lines. We do not add an asterisk at the beginning 171 of each line, either. E.g. 172 173 /* TRANSLATORS: here is a comment that explains the string 174 to be translated, that follows immediately after it */ 175 _("Here is a translatable string explained by the above."); 176 177 - Double negation is often harder to understand than no negation 178 at all. 179 180 - Some clever tricks, like using the !! operator with arithmetic 181 constructs, can be extremely confusing to others. Avoid them, 182 unless there is a compelling reason to use them. 183 184 - Use the API. No, really. We have a strbuf (variable length 185 string), several arrays with the ALLOC_GROW() macro, a 186 string_list for sorted string lists, a hash map (mapping struct 187 objects) named "struct decorate", amongst other things. 188 189 - When you come up with an API, document it. 190 191 - The first #include in C files, except in platform specific 192 compat/ implementations, should be git-compat-util.h or another 193 header file that includes it, such as cache.h or builtin.h. 194 195 - If you are planning a new command, consider writing it in shell 196 or perl first, so that changes in semantics can be easily 197 changed and discussed. Many Git commands started out like 198 that, and a few are still scripts. 199 200 - Avoid introducing a new dependency into Git. This means you 201 usually should stay away from scripting languages not already 202 used in the Git core command set (unless your command is clearly 203 separate from it, such as an importer to convert random-scm-X 204 repositories to Git). 205 206 - When we pass <string, length> pair to functions, we should try to 207 pass them in that order. 208 209 - Use Git's gettext wrappers to make the user interface 210 translatable. See "Marking strings for translation" in po/README. 211 212For Perl programs: 213 214 - Most of the C guidelines above apply. 215 216 - We try to support Perl 5.8 and later ("use Perl 5.008"). 217 218 - use strict and use warnings are strongly preferred. 219 220 - Don't overuse statement modifiers unless using them makes the 221 result easier to follow. 222 223 ... do something ... 224 do_this() unless (condition); 225 ... do something else ... 226 227 is more readable than: 228 229 ... do something ... 230 unless (condition) { 231 do_this(); 232 } 233 ... do something else ... 234 235 *only* when the condition is so rare that do_this() will be almost 236 always called. 237 238 - We try to avoid assignments inside "if ()" conditions. 239 240 - Learn and use Git.pm if you need that functionality. 241 242 - For Emacs, it's useful to put the following in 243 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode: 244 245 ;; note the first part is useful for C editing, too 246 ((nil . ((indent-tabs-mode . t) 247 (tab-width . 8) 248 (fill-column . 80))) 249 (cperl-mode . ((cperl-indent-level . 8) 250 (cperl-extra-newline-before-brace . nil) 251 (cperl-merge-trailing-else . t)))) 252 253For Python scripts: 254 255 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/). 256 257 - As a minimum, we aim to be compatible with Python 2.6 and 2.7. 258 259 - Where required libraries do not restrict us to Python 2, we try to 260 also be compatible with Python 3.1 and later. 261 262 - When you must differentiate between Unicode literals and byte string 263 literals, it is OK to use the 'b' prefix. Even though the Python 264 documentation for version 2.6 does not mention this prefix, it has 265 been supported since version 2.6.0. 266 267Error Messages 268 269 - Do not end error messages with a full stop. 270 271 - Do not capitalize ("unable to open %s", not "Unable to open %s") 272 273 - Say what the error is first ("cannot open %s", not "%s: cannot open") 274 275 276Writing Documentation: 277 278 Most (if not all) of the documentation pages are written in the 279 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and 280 processed into HTML and manpages (e.g. git.html and git.1 in the 281 same directory). 282 283 The documentation liberally mixes US and UK English (en_US/UK) 284 norms for spelling and grammar, which is somewhat unfortunate. 285 In an ideal world, it would have been better if it consistently 286 used only one and not the other, and we would have picked en_US 287 (if you wish to correct the English of some of the existing 288 documentation, please see the documentation-related advice in the 289 Documentation/SubmittingPatches file). 290 291 Every user-visible change should be reflected in the documentation. 292 The same general rule as for code applies -- imitate the existing 293 conventions. 294 295 A few commented examples follow to provide reference when writing or 296 modifying command usage strings and synopsis sections in the manual 297 pages: 298 299 Placeholders are spelled in lowercase and enclosed in angle brackets: 300 <file> 301 --sort=<key> 302 --abbrev[=<n>] 303 304 Possibility of multiple occurrences is indicated by three dots: 305 <file>... 306 (One or more of <file>.) 307 308 Optional parts are enclosed in square brackets: 309 [<extra>] 310 (Zero or one <extra>.) 311 312 --exec-path[=<path>] 313 (Option with an optional argument. Note that the "=" is inside the 314 brackets.) 315 316 [<patch>...] 317 (Zero or more of <patch>. Note that the dots are inside, not 318 outside the brackets.) 319 320 Multiple alternatives are indicated with vertical bar: 321 [-q | --quiet] 322 [--utf8 | --no-utf8] 323 324 Parentheses are used for grouping: 325 [(<rev>|<range>)...] 326 (Any number of either <rev> or <range>. Parens are needed to make 327 it clear that "..." pertains to both <rev> and <range>.) 328 329 [(-p <parent>)...] 330 (Any number of option -p, each with one <parent> argument.) 331 332 git remote set-head <name> (-a | -d | <branch>) 333 (One and only one of "-a", "-d" or "<branch>" _must_ (no square 334 brackets) be provided.) 335 336 And a somewhat more contrived example: 337 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]] 338 Here "=" is outside the brackets, because "--diff-filter=" is a 339 valid usage. "*" has its own pair of brackets, because it can 340 (optionally) be specified only when one or more of the letters is 341 also provided. 342 343 A note on notation: 344 Use 'git' (all lowercase) when talking about commands i.e. something 345 the user would type into a shell and use 'Git' (uppercase first letter) 346 when talking about the version control system and its properties. 347 348 A few commented examples follow to provide reference when writing or 349 modifying paragraphs or option/command explanations that contain options 350 or commands: 351 352 Literal examples (e.g. use of command-line options, command names, and 353 configuration variables) are typeset in monospace, and if you can use 354 `backticks around word phrases`, do so. 355 `--pretty=oneline` 356 `git rev-list` 357 `remote.pushdefault` 358 359 Word phrases enclosed in `backtick characters` are rendered literally 360 and will not be further expanded. The use of `backticks` to achieve the 361 previous rule means that literal examples should not use AsciiDoc 362 escapes. 363 Correct: 364 `--pretty=oneline` 365 Incorrect: 366 `\--pretty=oneline` 367 368 If some place in the documentation needs to typeset a command usage 369 example with inline substitutions, it is fine to use +monospaced and 370 inline substituted text+ instead of `monospaced literal text`, and with 371 the former, the part that should not get substituted must be 372 quoted/escaped.