Documentation: how to add a new command

This document contains no new policies or proposals; it attempts to
document established practices and interface requirements.

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/technical/api-command.txt b/Documentation/technical/api-command.txt
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+Integrating new subcommands
+===========================
+
+This is how-to documentation for people who want to add extension
+commands to git.  It should be read alongside api-builtin.txt.
+
+Runtime environment
+-------------------
+
+git subcommands are standalone executables that live in the git exec
+path, normally /usr/lib/git-core.  The git executable itself is a
+thin wrapper that knows where the subcommands live, and runs them by
+passing command-line arguments to them.
+
+(If "git foo" is not found in the git exec path, the wrapper
+will look in the rest of your $PATH for it.  Thus, it's possible
+to write local git extensions that don't live in system space.)
+
+Implementation languages
+------------------------
+
+Most subcommands are written in C or shell.  A few are written in
+Perl.
+
+While we strongly encourage coding in portable C for portability,
+these specific scripting languages are also acceptable.  We won't
+accept more without a very strong technical case, as we don't want
+to broaden the git suite's required dependencies.  Import utilities,
+surgical tools, remote helpers and other code at the edges of the
+git suite are more lenient and we allow Python (and even Tcl/tk),
+but they should not be used for core functions.
+
+This may change in the future.  Especially Python is not allowed in
+core because we need better Python integration in the git Windows
+installer before we can be confident people in that environment
+won't experience an unacceptably large loss of capability.
+
+C commands are normally written as single modules, named after the
+command, that link a collection of functions called libgit.  Thus,
+your command 'git-foo' would normally be implemented as a single
+"git-foo.c" (or "builtin/foo.c" if it is to be linked to the main
+binary); this organization makes it easy for people reading the code
+to find things.
+
+See the CodingGuidelines document for other guidance on what we consider
+good practice in C and shell, and api-builtin.txt for the support
+functions available to built-in commands written in C.
+
+What every extension command needs
+----------------------------------
+
+You must have a man page, written in asciidoc (this is what git help
+followed by your subcommand name will display).  Be aware that there is
+a local asciidoc configuration and macros which you should use.  It's
+often helpful to start by cloning an existing page and replacing the
+text content.
+
+You must have a test, written to report in TAP (Test Anything Protocol).
+Tests are executables (usually shell scripts) that live in the 't'
+subdirectory of the tree.  Each test name begins with 't' and a sequence
+number that controls where in the test sequence it will be executed;
+conventionally the rest of the name stem is that of the command
+being tested.
+
+Read the file t/README to learn more about the conventions to be used
+in writing tests, and the test support library.
+
+Integrating a command
+---------------------
+
+Here are the things you need to do when you want to merge a new
+subcommand into the git tree.
+
+0. Don't forget to sign off your patch!
+
+1. Append your command name to one of the variables BUILTIN_OBJS,
+EXTRA_PROGRAMS, SCRIPT_SH, SCRIPT_PERL or SCRIPT_PYTHON.
+
+2. Drop its test in the t directory.
+
+3. If your command is implemented in an interpreted language with a
+p-code intermediate form, make sure .gitignore in the main directory
+includes a pattern entry that ignores such files.  Python .pyc and
+.pyo files will already be covered.
+
+4. If your command has any dependency on a particular version of
+your language, document it in the INSTALL file.
+
+5. There is a file command-list.txt in the distribution main directory
+that categorizes commands by type, so they can be listed in appropriate
+subsections in the documentation's summary command list.  Add an entry
+for yours.  To understand the categories, look at git-cmmands.txt
+in the main directory.
+
+6. Give the maintainer one paragraph to include in the RelNotes file
+to describe the new feature; a good place to do so is in the cover
+letter [PATCH 0/n].
+
+That's all there is to it.