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 prefer $( ... ) for command substitution; unlike ``, it 35 properly nests. It should have been the way Bourne spelled 36 it from day one, but unfortunately isn't. 37 38 - We use POSIX compliant parameter substitutions and avoid bashisms; 39 namely: 40 41 - We use ${parameter-word} and its [-=?+] siblings, and their 42 colon'ed "unset or null" form. 43 44 - We use ${parameter#word} and its [#%] siblings, and their 45 doubled "longest matching" form. 46 47 - No "Substring Expansion" ${parameter:offset:length}. 48 49 - No shell arrays. 50 51 - No strlen ${#parameter}. 52 53 - No pattern replacement ${parameter/pattern/string}. 54 55 - We use Arithmetic Expansion $(( ... )). 56 57 - Inside Arithmetic Expansion, spell shell variables with $ in front 58 of them, as some shells do not grok $((x)) while accepting $(($x)) 59 just fine (e.g. dash older than 0.5.4). 60 61 - We do not use Process Substitution <(list) or >(list). 62 63 - We prefer "test" over "[ ... ]". 64 65 - We do not write the noiseword "function" in front of shell 66 functions. 67 68 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\}, 69 [::], [==], nor [..]) for portability. 70 71 - We do not use \{m,n\}; 72 73 - We do not use -E; 74 75 - We do not use ? nor + (which are \{0,1\} and \{1,\} 76 respectively in BRE) but that goes without saying as these 77 are ERE elements not BRE (note that \? and \+ are not even part 78 of BRE -- making them accessible from BRE is a GNU extension). 79 80For C programs: 81 82 - We use tabs to indent, and interpret tabs as taking up to 83 8 spaces. 84 85 - We try to keep to at most 80 characters per line. 86 87 - When declaring pointers, the star sides with the variable 88 name, i.e. "char *string", not "char* string" or 89 "char * string". This makes it easier to understand code 90 like "char *string, c;". 91 92 - We avoid using braces unnecessarily. I.e. 93 94 if (bla) { 95 x = 1; 96 } 97 98 is frowned upon. A gray area is when the statement extends 99 over a few lines, and/or you have a lengthy comment atop of 100 it. Also, like in the Linux kernel, if there is a long list 101 of "else if" statements, it can make sense to add braces to 102 single line blocks. 103 104 - We try to avoid assignments inside if(). 105 106 - Try to make your code understandable. You may put comments 107 in, but comments invariably tend to stale out when the code 108 they were describing changes. Often splitting a function 109 into two makes the intention of the code much clearer. 110 111 - Double negation is often harder to understand than no negation 112 at all. 113 114 - Some clever tricks, like using the !! operator with arithmetic 115 constructs, can be extremely confusing to others. Avoid them, 116 unless there is a compelling reason to use them. 117 118 - Use the API. No, really. We have a strbuf (variable length 119 string), several arrays with the ALLOC_GROW() macro, a 120 string_list for sorted string lists, a hash map (mapping struct 121 objects) named "struct decorate", amongst other things. 122 123 - When you come up with an API, document it. 124 125 - The first #include in C files, except in platform specific 126 compat/ implementations, should be git-compat-util.h or another 127 header file that includes it, such as cache.h or builtin.h. 128 129 - If you are planning a new command, consider writing it in shell 130 or perl first, so that changes in semantics can be easily 131 changed and discussed. Many git commands started out like 132 that, and a few are still scripts. 133 134 - Avoid introducing a new dependency into git. This means you 135 usually should stay away from scripting languages not already 136 used in the git core command set (unless your command is clearly 137 separate from it, such as an importer to convert random-scm-X 138 repositories to git). 139 140 - When we pass <string, length> pair to functions, we should try to 141 pass them in that order.