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 - Do not write control structures on a single line with semicolon. 80 "then" should be on the next line for if statements, and "do" 81 should be on the next line for "while" and "for". 82 83 - We prefer "test" over "[ ... ]". 84 85 - We do not write the noiseword "function" in front of shell 86 functions. 87 88 - We prefer a space between the function name and the parentheses. The 89 opening "{" should also be on the same line. 90 E.g.: my_function () { 91 92 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\}, 93 [::], [==], nor [..]) for portability. 94 95 - We do not use \{m,n\}; 96 97 - We do not use -E; 98 99 - We do not use ? nor + (which are \{0,1\} and \{1,\} 100 respectively in BRE) but that goes without saying as these 101 are ERE elements not BRE (note that \? and \+ are not even part 102 of BRE -- making them accessible from BRE is a GNU extension). 103 104 - Use Git's gettext wrappers in git-sh-i18n to make the user 105 interface translatable. See "Marking strings for translation" in 106 po/README. 107 108For C programs: 109 110 - We use tabs to indent, and interpret tabs as taking up to 111 8 spaces. 112 113 - We try to keep to at most 80 characters per line. 114 115 - We try to support a wide range of C compilers to compile Git with, 116 including old ones. That means that you should not use C99 117 initializers, even if a lot of compilers grok it. 118 119 - Variables have to be declared at the beginning of the block. 120 121 - NULL pointers shall be written as NULL, not as 0. 122 123 - When declaring pointers, the star sides with the variable 124 name, i.e. "char *string", not "char* string" or 125 "char * string". This makes it easier to understand code 126 like "char *string, c;". 127 128 - We avoid using braces unnecessarily. I.e. 129 130 if (bla) { 131 x = 1; 132 } 133 134 is frowned upon. A gray area is when the statement extends 135 over a few lines, and/or you have a lengthy comment atop of 136 it. Also, like in the Linux kernel, if there is a long list 137 of "else if" statements, it can make sense to add braces to 138 single line blocks. 139 140 - We try to avoid assignments inside if(). 141 142 - Try to make your code understandable. You may put comments 143 in, but comments invariably tend to stale out when the code 144 they were describing changes. Often splitting a function 145 into two makes the intention of the code much clearer. 146 147 - Double negation is often harder to understand than no negation 148 at all. 149 150 - Some clever tricks, like using the !! operator with arithmetic 151 constructs, can be extremely confusing to others. Avoid them, 152 unless there is a compelling reason to use them. 153 154 - Use the API. No, really. We have a strbuf (variable length 155 string), several arrays with the ALLOC_GROW() macro, a 156 string_list for sorted string lists, a hash map (mapping struct 157 objects) named "struct decorate", amongst other things. 158 159 - When you come up with an API, document it. 160 161 - The first #include in C files, except in platform specific 162 compat/ implementations, should be git-compat-util.h or another 163 header file that includes it, such as cache.h or builtin.h. 164 165 - If you are planning a new command, consider writing it in shell 166 or perl first, so that changes in semantics can be easily 167 changed and discussed. Many Git commands started out like 168 that, and a few are still scripts. 169 170 - Avoid introducing a new dependency into Git. This means you 171 usually should stay away from scripting languages not already 172 used in the Git core command set (unless your command is clearly 173 separate from it, such as an importer to convert random-scm-X 174 repositories to Git). 175 176 - When we pass <string, length> pair to functions, we should try to 177 pass them in that order. 178 179 - Use Git's gettext wrappers to make the user interface 180 translatable. See "Marking strings for translation" in po/README. 181 182For Python scripts: 183 184 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/). 185 186 - As a minimum, we aim to be compatible with Python 2.6 and 2.7. 187 188 - Where required libraries do not restrict us to Python 2, we try to 189 also be compatible with Python 3.1 and later. 190 191 - When you must differentiate between Unicode literals and byte string 192 literals, it is OK to use the 'b' prefix. Even though the Python 193 documentation for version 2.6 does not mention this prefix, it has 194 been supported since version 2.6.0. 195 196Writing Documentation: 197 198 Every user-visible change should be reflected in the documentation. 199 The same general rule as for code applies -- imitate the existing 200 conventions. A few commented examples follow to provide reference 201 when writing or modifying command usage strings and synopsis sections 202 in the manual pages: 203 204 Placeholders are spelled in lowercase and enclosed in angle brackets: 205 <file> 206 --sort=<key> 207 --abbrev[=<n>] 208 209 Possibility of multiple occurrences is indicated by three dots: 210 <file>... 211 (One or more of <file>.) 212 213 Optional parts are enclosed in square brackets: 214 [<extra>] 215 (Zero or one <extra>.) 216 217 --exec-path[=<path>] 218 (Option with an optional argument. Note that the "=" is inside the 219 brackets.) 220 221 [<patch>...] 222 (Zero or more of <patch>. Note that the dots are inside, not 223 outside the brackets.) 224 225 Multiple alternatives are indicated with vertical bar: 226 [-q | --quiet] 227 [--utf8 | --no-utf8] 228 229 Parentheses are used for grouping: 230 [(<rev>|<range>)...] 231 (Any number of either <rev> or <range>. Parens are needed to make 232 it clear that "..." pertains to both <rev> and <range>.) 233 234 [(-p <parent>)...] 235 (Any number of option -p, each with one <parent> argument.) 236 237 git remote set-head <name> (-a | -d | <branch>) 238 (One and only one of "-a", "-d" or "<branch>" _must_ (no square 239 brackets) be provided.) 240 241 And a somewhat more contrived example: 242 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]] 243 Here "=" is outside the brackets, because "--diff-filter=" is a 244 valid usage. "*" has its own pair of brackets, because it can 245 (optionally) be specified only when one or more of the letters is 246 also provided. 247 248 A note on notation: 249 Use 'git' (all lowercase) when talking about commands i.e. something 250 the user would type into a shell and use 'Git' (uppercase first letter) 251 when talking about the version control system and its properties.