Various programs can invoke your choice of editor to edit a particular piece of text. For instance, version control programs invoke an editor to enter version control logs (see Version Control), and the Unix mail utility invokes an editor to enter a message to send. By convention, your choice of editor is specified by the environment variable EDITOR. If you set EDITOR to ‘emacs’, Emacs would be invoked, but in an inconvenient way—by starting a new Emacs process. This is inconvenient because the new Emacs process doesn't share buffers, a command history, or other kinds of information with any existing Emacs process.
You can solve this problem by setting up Emacs as an edit server, so that it “listens” for external edit requests and acts accordingly. There are two ways to start an Emacs server:
server-start in an existing Emacs process:
either type M-x server-start, or put the expression
(server-start) in your init file (see Init File). The
existing Emacs process is the server; when you exit Emacs, the server
dies with the Emacs process.
server-start after initialization, and returns control to
the calling terminal instead of opening an initial frame; it then
waits in the background, listening for edit requests.
Either way, once an Emacs server is started, you can use a shell command called emacsclient to connect to the Emacs process and tell it to visit a file. You can then set the EDITOR environment variable to ‘emacsclient’, so that external programs will use the existing Emacs process for editing.1
You can run multiple Emacs servers on the same machine by giving
each one a unique “server name”, using the variable
server-name. For example, M-x set-variable <RET>
server-name <RET> foo <RET> sets the server name to
‘foo’. The emacsclient program can specify a server by
name, using the ‘-s’ option (see emacsclient Options).
If you have defined a server by a unique server name, it is possible
to connect to the server from another Emacs instance and evaluate Lisp
expressions on the server, using the server-eval-at function.
For instance, (server-eval-at "foo" '(+ 1 2)) evaluates the
expression (+ 1 2) on the ‘foo’ server, and returns
3. (If there is no server with that name, an error is
signaled.) Currently, this feature is mainly useful for developers.
[1] Some programs use a different environment variable; for example, to make TeX use ‘emacsclient’, set the TEXEDIT environment variable to ‘emacsclient +%d %s’.