Like other projects, we also have some guidelines to keep to the
code.  For Git in general, three rough rules are:

 - Most importantly, we never say "It's in POSIX; we'll happily
   ignore your needs should your system not conform to it."
   We live in the real world.

 - However, we often say "Let's stay away from that construct,
   it's not even in POSIX".

 - In spite of the above two rules, we sometimes say "Although
   this is not in POSIX, it (is so convenient | makes the code
   much more readable | has other good characteristics) and
   practically all the platforms we care about support it, so
   let's use it".

   Again, we live in the real world, and it is sometimes a
   judgement call, the decision based more on real world
   constraints people face than what the paper standard says.

 - Fixing style violations while working on a real change as a
   preparatory clean-up step is good, but otherwise avoid useless code
   churn for the sake of conforming to the style.

   "Once it _is_ in the tree, it's not really worth the patch noise to
   go and fix it up."
   Cf. http://article.gmane.org/gmane.linux.kernel/943020

Make your code readable and sensible, and don't try to be clever.

As for more concrete guidelines, just imitate the existing code
(this is a good guideline, no matter which project you are
contributing to). It is always preferable to match the _local_
convention. New code added to Git suite is expected to match
the overall style of existing code. Modifications to existing
code is expected to match the style the surrounding code already
uses (even if it doesn't match the overall style of existing code).

But if you must have a list of rules, here they are.

For shell scripts specifically (not exhaustive):

 - We use tabs for indentation.

 - Case arms are indented at the same depth as case and esac lines,
   like this:

        case "$variable" in
        pattern1)
                do this
                ;;
        pattern2)
                do that
                ;;
        esac

 - Redirection operators should be written with space before, but no
   space after them.  In other words, write 'echo test >"$file"'
   instead of 'echo test> $file' or 'echo test > $file'.  Note that
   even though it is not required by POSIX to double-quote the
   redirection target in a variable (as shown above), our code does so
   because some versions of bash issue a warning without the quotes.

        (incorrect)
        cat hello > world < universe
        echo hello >$world

        (correct)
        cat hello >world <universe
        echo hello >"$world"

 - We prefer $( ... ) for command substitution; unlike ``, it
   properly nests.  It should have been the way Bourne spelled
   it from day one, but unfortunately isn't.

 - If you want to find out if a command is available on the user's
   $PATH, you should use 'type <command>', instead of 'which <command>'.
   The output of 'which' is not machine parseable and its exit code
   is not reliable across platforms.

 - We use POSIX compliant parameter substitutions and avoid bashisms;
   namely:

   - We use ${parameter-word} and its [-=?+] siblings, and their
     colon'ed "unset or null" form.

   - We use ${parameter#word} and its [#%] siblings, and their
     doubled "longest matching" form.

   - No "Substring Expansion" ${parameter:offset:length}.

   - No shell arrays.

   - No strlen ${#parameter}.

   - No pattern replacement ${parameter/pattern/string}.

 - We use Arithmetic Expansion $(( ... )).

 - Inside Arithmetic Expansion, spell shell variables with $ in front
   of them, as some shells do not grok $((x)) while accepting $(($x))
   just fine (e.g. dash older than 0.5.4).

 - We do not use Process Substitution <(list) or >(list).

 - Do not write control structures on a single line with semicolon.
   "then" should be on the next line for if statements, and "do"
   should be on the next line for "while" and "for".

        (incorrect)
        if test -f hello; then
                do this
        fi

        (correct)
        if test -f hello
        then
                do this
        fi

 - We prefer "test" over "[ ... ]".

 - We do not write the noiseword "function" in front of shell
   functions.

 - We prefer a space between the function name and the parentheses,
   and no space inside the parentheses. The opening "{" should also
   be on the same line.

        (incorrect)
        my_function(){
                ...

        (correct)
        my_function () {
                ...

 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
   [::], [==], or [..]) for portability.

   - We do not use \{m,n\};

   - We do not use -E;

   - We do not use ? or + (which are \{0,1\} and \{1,\}
     respectively in BRE) but that goes without saying as these
     are ERE elements not BRE (note that \? and \+ are not even part
     of BRE -- making them accessible from BRE is a GNU extension).

 - Use Git's gettext wrappers in git-sh-i18n to make the user
   interface translatable. See "Marking strings for translation" in
   po/README.

For C programs:

 - We use tabs to indent, and interpret tabs as taking up to
   8 spaces.

 - We try to keep to at most 80 characters per line.

 - We try to support a wide range of C compilers to compile Git with,
   including old ones. That means that you should not use C99
   initializers, even if a lot of compilers grok it.

 - Variables have to be declared at the beginning of the block.

 - NULL pointers shall be written as NULL, not as 0.

 - When declaring pointers, the star sides with the variable
   name, i.e. "char *string", not "char* string" or
   "char * string".  This makes it easier to understand code
   like "char *string, c;".

 - Use whitespace around operators and keywords, but not inside
   parentheses and not around functions. So:

        while (condition)
                func(bar + 1);

   and not:

        while( condition )
                func (bar+1);

 - We avoid using braces unnecessarily.  I.e.

        if (bla) {
                x = 1;
        }

   is frowned upon.  A gray area is when the statement extends
   over a few lines, and/or you have a lengthy comment atop of
   it.  Also, like in the Linux kernel, if there is a long list
   of "else if" statements, it can make sense to add braces to
   single line blocks.

 - We try to avoid assignments in the condition of an "if" statement.

 - Try to make your code understandable.  You may put comments
   in, but comments invariably tend to stale out when the code
   they were describing changes.  Often splitting a function
   into two makes the intention of the code much clearer.

 - Multi-line comments include their delimiters on separate lines from
   the text.  E.g.

        /*
         * A very long
         * multi-line comment.
         */

   Note however that a comment that explains a translatable string to
   translators uses a convention of starting with a magic token
   "TRANSLATORS: " immediately after the opening delimiter, even when
   it spans multiple lines.  We do not add an asterisk at the beginning
   of each line, either.  E.g.

        /* TRANSLATORS: here is a comment that explains the string
           to be translated, that follows immediately after it */
        _("Here is a translatable string explained by the above.");

 - Double negation is often harder to understand than no negation
   at all.

 - There are two schools of thought when it comes to comparison,
   especially inside a loop. Some people prefer to have the less stable
   value on the left hand side and the more stable value on the right hand
   side, e.g. if you have a loop that counts variable i down to the
   lower bound,

        while (i > lower_bound) {
                do something;
                i--;
        }

   Other people prefer to have the textual order of values match the
   actual order of values in their comparison, so that they can
   mentally draw a number line from left to right and place these
   values in order, i.e.

        while (lower_bound < i) {
                do something;
                i--;
        }

   Both are valid, and we use both.  However, the more "stable" the
   stable side becomes, the more we tend to prefer the former
   (comparison with a constant, "i > 0", is an extreme example).
   Just do not mix styles in the same part of the code and mimic
   existing styles in the neighbourhood.

 - Some clever tricks, like using the !! operator with arithmetic
   constructs, can be extremely confusing to others.  Avoid them,
   unless there is a compelling reason to use them.

 - Use the API.  No, really.  We have a strbuf (variable length
   string), several arrays with the ALLOC_GROW() macro, a
   string_list for sorted string lists, a hash map (mapping struct
   objects) named "struct decorate", amongst other things.

 - When you come up with an API, document it.

 - The first #include in C files, except in platform specific
   compat/ implementations, should be git-compat-util.h or another
   header file that includes it, such as cache.h or builtin.h.

 - If you are planning a new command, consider writing it in shell
   or perl first, so that changes in semantics can be easily
   changed and discussed.  Many Git commands started out like
   that, and a few are still scripts.

 - Avoid introducing a new dependency into Git. This means you
   usually should stay away from scripting languages not already
   used in the Git core command set (unless your command is clearly
   separate from it, such as an importer to convert random-scm-X
   repositories to Git).

 - When we pass <string, length> pair to functions, we should try to
   pass them in that order.

 - Use Git's gettext wrappers to make the user interface
   translatable. See "Marking strings for translation" in po/README.

For Perl programs:

 - Most of the C guidelines above apply.

 - We try to support Perl 5.8 and later ("use Perl 5.008").

 - use strict and use warnings are strongly preferred.

 - Don't overuse statement modifiers unless using them makes the
   result easier to follow.

        ... do something ...
        do_this() unless (condition);
        ... do something else ...

   is more readable than:

        ... do something ...
        unless (condition) {
                do_this();
        }
        ... do something else ...

   *only* when the condition is so rare that do_this() will be almost
   always called.

 - We try to avoid assignments inside "if ()" conditions.

 - Learn and use Git.pm if you need that functionality.

 - For Emacs, it's useful to put the following in
   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:

    ;; note the first part is useful for C editing, too
    ((nil . ((indent-tabs-mode . t)
                  (tab-width . 8)
                  (fill-column . 80)))
     (cperl-mode . ((cperl-indent-level . 8)
                    (cperl-extra-newline-before-brace . nil)
                    (cperl-merge-trailing-else . t))))

For Python scripts:

 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).

 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.

 - Where required libraries do not restrict us to Python 2, we try to
   also be compatible with Python 3.1 and later.

 - When you must differentiate between Unicode literals and byte string
   literals, it is OK to use the 'b' prefix.  Even though the Python
   documentation for version 2.6 does not mention this prefix, it has
   been supported since version 2.6.0.

Writing Documentation:

 Most (if not all) of the documentation pages are written in the
 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 processed into HTML and manpages (e.g. git.html and git.1 in the
 same directory).

 The documentation liberally mixes US and UK English (en_US/UK)
 norms for spelling and grammar, which is somewhat unfortunate.
 In an ideal world, it would have been better if it consistently
 used only one and not the other, and we would have picked en_US
 (if you wish to correct the English of some of the existing
 documentation, please see the documentation-related advice in the
 Documentation/SubmittingPatches file).

 Every user-visible change should be reflected in the documentation.
 The same general rule as for code applies -- imitate the existing
 conventions.

 A few commented examples follow to provide reference when writing or
 modifying command usage strings and synopsis sections in the manual
 pages:

 Placeholders are spelled in lowercase and enclosed in angle brackets:
   <file>
   --sort=<key>
   --abbrev[=<n>]

 Possibility of multiple occurrences is indicated by three dots:
   <file>...
   (One or more of <file>.)

 Optional parts are enclosed in square brackets:
   [<extra>]
   (Zero or one <extra>.)

   --exec-path[=<path>]
   (Option with an optional argument.  Note that the "=" is inside the
   brackets.)

   [<patch>...]
   (Zero or more of <patch>.  Note that the dots are inside, not
   outside the brackets.)

 Multiple alternatives are indicated with vertical bar:
   [-q | --quiet]
   [--utf8 | --no-utf8]

 Parentheses are used for grouping:
   [(<rev>|<range>)...]
   (Any number of either <rev> or <range>.  Parens are needed to make
   it clear that "..." pertains to both <rev> and <range>.)

   [(-p <parent>)...]
   (Any number of option -p, each with one <parent> argument.)

   git remote set-head <name> (-a | -d | <branch>)
   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
   brackets) be provided.)

 And a somewhat more contrived example:
   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
   Here "=" is outside the brackets, because "--diff-filter=" is a
   valid usage.  "*" has its own pair of brackets, because it can
   (optionally) be specified only when one or more of the letters is
   also provided.

  A note on notation:
   Use 'git' (all lowercase) when talking about commands i.e. something
   the user would type into a shell and use 'Git' (uppercase first letter)
   when talking about the version control system and its properties.

 A few commented examples follow to provide reference when writing or
 modifying paragraphs or option/command explanations that contain options
 or commands:

 Literal examples (e.g. use of command-line options, command names, and
 configuration variables) are typeset in monospace, and if you can use
 `backticks around word phrases`, do so.
   `--pretty=oneline`
   `git rev-list`
   `remote.pushdefault`

 Word phrases enclosed in `backtick characters` are rendered literally
 and will not be further expanded. The use of `backticks` to achieve the
 previous rule means that literal examples should not use AsciiDoc
 escapes.
   Correct:
      `--pretty=oneline`
   Incorrect:
      `\--pretty=oneline`

 If some place in the documentation needs to typeset a command usage
 example with inline substitutions, it is fine to use +monospaced and
 inline substituted text+ instead of `monospaced literal text`, and with
 the former, the part that should not get substituted must be
 quoted/escaped.