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