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