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 84 - Use Git's gettext wrappers in git-sh-i18n to make the user 85 interface translatable. See "Marking strings for translation" in 86 po/README. 87 88For C programs: 89 90 - We use tabs to indent, and interpret tabs as taking up to 91 8 spaces. 92 93 - We try to keep to at most 80 characters per line. 94 95 - When declaring pointers, the star sides with the variable 96 name, i.e. "char *string", not "char* string" or 97 "char * string". This makes it easier to understand code 98 like "char *string, c;". 99 100 - We avoid using braces unnecessarily. I.e. 101 102 if (bla) { 103 x = 1; 104 } 105 106 is frowned upon. A gray area is when the statement extends 107 over a few lines, and/or you have a lengthy comment atop of 108 it. Also, like in the Linux kernel, if there is a long list 109 of "else if" statements, it can make sense to add braces to 110 single line blocks. 111 112 - We try to avoid assignments inside if(). 113 114 - Try to make your code understandable. You may put comments 115 in, but comments invariably tend to stale out when the code 116 they were describing changes. Often splitting a function 117 into two makes the intention of the code much clearer. 118 119 - Double negation is often harder to understand than no negation 120 at all. 121 122 - Some clever tricks, like using the !! operator with arithmetic 123 constructs, can be extremely confusing to others. Avoid them, 124 unless there is a compelling reason to use them. 125 126 - Use the API. No, really. We have a strbuf (variable length 127 string), several arrays with the ALLOC_GROW() macro, a 128 string_list for sorted string lists, a hash map (mapping struct 129 objects) named "struct decorate", amongst other things. 130 131 - When you come up with an API, document it. 132 133 - The first #include in C files, except in platform specific 134 compat/ implementations, should be git-compat-util.h or another 135 header file that includes it, such as cache.h or builtin.h. 136 137 - If you are planning a new command, consider writing it in shell 138 or perl first, so that changes in semantics can be easily 139 changed and discussed. Many git commands started out like 140 that, and a few are still scripts. 141 142 - Avoid introducing a new dependency into git. This means you 143 usually should stay away from scripting languages not already 144 used in the git core command set (unless your command is clearly 145 separate from it, such as an importer to convert random-scm-X 146 repositories to git). 147 148 - When we pass <string, length> pair to functions, we should try to 149 pass them in that order. 150 151 - Use Git's gettext wrappers to make the user interface 152 translatable. See "Marking strings for translation" in po/README. 153 154Writing Documentation: 155 156 Every user-visible change should be reflected in the documentation. 157 The same general rule as for code applies -- imitate the existing 158 conventions. A few commented examples follow to provide reference 159 when writing or modifying command usage strings and synopsis sections 160 in the manual pages: 161 162 Placeholders are spelled in lowercase and enclosed in angle brackets: 163 <file> 164 --sort=<key> 165 --abbrev[=<n>] 166 167 Possibility of multiple occurrences is indicated by three dots: 168 <file>... 169 (One or more of <file>.) 170 171 Optional parts are enclosed in square brackets: 172 [<extra>] 173 (Zero or one <extra>.) 174 175 --exec-path[=<path>] 176 (Option with an optional argument. Note that the "=" is inside the 177 brackets.) 178 179 [<patch>...] 180 (Zero or more of <patch>. Note that the dots are inside, not 181 outside the brackets.) 182 183 Multiple alternatives are indicated with vertical bar: 184 [-q | --quiet] 185 [--utf8 | --no-utf8] 186 187 Parentheses are used for grouping: 188 [(<rev>|<range>)...] 189 (Any number of either <rev> or <range>. Parens are needed to make 190 it clear that "..." pertains to both <rev> and <range>.) 191 192 [(-p <parent>)...] 193 (Any number of option -p, each with one <parent> argument.) 194 195 git remote set-head <name> (-a | -d | <branch>) 196 (One and only one of "-a", "-d" or "<branch>" _must_ (no square 197 brackets) be provided.) 198 199 And a somewhat more contrived example: 200 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]] 201 Here "=" is outside the brackets, because "--diff-filter=" is a 202 valid usage. "*" has its own pair of brackets, because it can 203 (optionally) be specified only when one or more of the letters is 204 also provided.