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 22As for more concrete guidelines, just imitate the existing code 23(this is a good guideline, no matter which project you are 24contributing to). It is always preferable to match the _local_ 25convention. New code added to git suite is expected to match 26the overall style of existing code. Modifications to existing 27code is expected to match the style the surrounding code already 28uses (even if it doesn't match the overall style of existing code). 29 30But if you must have a list of rules, here they are. 31 32For shell scripts specifically (not exhaustive): 33 34 - We use tabs for indentation. 35 36 - Case arms are indented at the same depth as case and esac lines. 37 38 - Redirection operators should be written with space before, but no 39 space after them. In other words, write 'echo test >"$file"' 40 instead of 'echo test> $file' or 'echo test > $file'. Note that 41 even though it is not required by POSIX to double-quote the 42 redirection target in a variable (as shown above), our code does so 43 because some versions of bash issue a warning without the quotes. 44 45 - We prefer $( ... ) for command substitution; unlike ``, it 46 properly nests. It should have been the way Bourne spelled 47 it from day one, but unfortunately isn't. 48 49 - If you want to find out if a command is available on the user's 50 $PATH, you should use 'type <command>', instead of 'which <command>'. 51 The output of 'which' is not machine parseable and its exit code 52 is not reliable across platforms. 53 54 - We use POSIX compliant parameter substitutions and avoid bashisms; 55 namely: 56 57 - We use ${parameter-word} and its [-=?+] siblings, and their 58 colon'ed "unset or null" form. 59 60 - We use ${parameter#word} and its [#%] siblings, and their 61 doubled "longest matching" form. 62 63 - No "Substring Expansion" ${parameter:offset:length}. 64 65 - No shell arrays. 66 67 - No strlen ${#parameter}. 68 69 - No pattern replacement ${parameter/pattern/string}. 70 71 - We use Arithmetic Expansion $(( ... )). 72 73 - Inside Arithmetic Expansion, spell shell variables with $ in front 74 of them, as some shells do not grok $((x)) while accepting $(($x)) 75 just fine (e.g. dash older than 0.5.4). 76 77 - We do not use Process Substitution <(list) or >(list). 78 79 - We prefer "test" over "[ ... ]". 80 81 - We do not write the noiseword "function" in front of shell 82 functions. 83 84 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\}, 85 [::], [==], nor [..]) for portability. 86 87 - We do not use \{m,n\}; 88 89 - We do not use -E; 90 91 - We do not use ? nor + (which are \{0,1\} and \{1,\} 92 respectively in BRE) but that goes without saying as these 93 are ERE elements not BRE (note that \? and \+ are not even part 94 of BRE -- making them accessible from BRE is a GNU extension). 95 96 - Use Git's gettext wrappers in git-sh-i18n to make the user 97 interface translatable. See "Marking strings for translation" in 98 po/README. 99 100For C programs: 101 102 - We use tabs to indent, and interpret tabs as taking up to 103 8 spaces. 104 105 - We try to keep to at most 80 characters per line. 106 107 - When declaring pointers, the star sides with the variable 108 name, i.e. "char *string", not "char* string" or 109 "char * string". This makes it easier to understand code 110 like "char *string, c;". 111 112 - We avoid using braces unnecessarily. I.e. 113 114 if (bla) { 115 x = 1; 116 } 117 118 is frowned upon. A gray area is when the statement extends 119 over a few lines, and/or you have a lengthy comment atop of 120 it. Also, like in the Linux kernel, if there is a long list 121 of "else if" statements, it can make sense to add braces to 122 single line blocks. 123 124 - We try to avoid assignments inside if(). 125 126 - Try to make your code understandable. You may put comments 127 in, but comments invariably tend to stale out when the code 128 they were describing changes. Often splitting a function 129 into two makes the intention of the code much clearer. 130 131 - Double negation is often harder to understand than no negation 132 at all. 133 134 - Some clever tricks, like using the !! operator with arithmetic 135 constructs, can be extremely confusing to others. Avoid them, 136 unless there is a compelling reason to use them. 137 138 - Use the API. No, really. We have a strbuf (variable length 139 string), several arrays with the ALLOC_GROW() macro, a 140 string_list for sorted string lists, a hash map (mapping struct 141 objects) named "struct decorate", amongst other things. 142 143 - When you come up with an API, document it. 144 145 - The first #include in C files, except in platform specific 146 compat/ implementations, should be git-compat-util.h or another 147 header file that includes it, such as cache.h or builtin.h. 148 149 - If you are planning a new command, consider writing it in shell 150 or perl first, so that changes in semantics can be easily 151 changed and discussed. Many git commands started out like 152 that, and a few are still scripts. 153 154 - Avoid introducing a new dependency into git. This means you 155 usually should stay away from scripting languages not already 156 used in the git core command set (unless your command is clearly 157 separate from it, such as an importer to convert random-scm-X 158 repositories to git). 159 160 - When we pass <string, length> pair to functions, we should try to 161 pass them in that order. 162 163 - Use Git's gettext wrappers to make the user interface 164 translatable. See "Marking strings for translation" in po/README. 165 166Writing Documentation: 167 168 Every user-visible change should be reflected in the documentation. 169 The same general rule as for code applies -- imitate the existing 170 conventions. A few commented examples follow to provide reference 171 when writing or modifying command usage strings and synopsis sections 172 in the manual pages: 173 174 Placeholders are spelled in lowercase and enclosed in angle brackets: 175 <file> 176 --sort=<key> 177 --abbrev[=<n>] 178 179 Possibility of multiple occurrences is indicated by three dots: 180 <file>... 181 (One or more of <file>.) 182 183 Optional parts are enclosed in square brackets: 184 [<extra>] 185 (Zero or one <extra>.) 186 187 --exec-path[=<path>] 188 (Option with an optional argument. Note that the "=" is inside the 189 brackets.) 190 191 [<patch>...] 192 (Zero or more of <patch>. Note that the dots are inside, not 193 outside the brackets.) 194 195 Multiple alternatives are indicated with vertical bar: 196 [-q | --quiet] 197 [--utf8 | --no-utf8] 198 199 Parentheses are used for grouping: 200 [(<rev>|<range>)...] 201 (Any number of either <rev> or <range>. Parens are needed to make 202 it clear that "..." pertains to both <rev> and <range>.) 203 204 [(-p <parent>)...] 205 (Any number of option -p, each with one <parent> argument.) 206 207 git remote set-head <name> (-a | -d | <branch>) 208 (One and only one of "-a", "-d" or "<branch>" _must_ (no square 209 brackets) be provided.) 210 211 And a somewhat more contrived example: 212 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]] 213 Here "=" is outside the brackets, because "--diff-filter=" is a 214 valid usage. "*" has its own pair of brackets, because it can 215 (optionally) be specified only when one or more of the letters is 216 also provided.