42 Using Emacs as a Server
Various programs such as mail can invoke your choice of editor
to edit a particular piece of text, such as a message that you are
sending. By convention, most of these programs use the environment
variable EDITOR to specify which editor to run. If you set
EDITOR to ‘emacs’, they invoke Emacs—but in an
inconvenient fashion, by starting a new, separate Emacs process. This
is inconvenient because it takes time and because the new Emacs process
doesn't share the buffers in any existing Emacs process.
You can arrange to use your existing Emacs process as the editor for
programs like mail by using the Emacs client and Emacs server
programs. Here is how.
First, the preparation. Within Emacs, call the function
server-start. (Your .emacs file can do this automatically
if you add the expression (server-start) to it.) Then, outside
Emacs, set the EDITOR environment variable to ‘emacsclient’.
(Note that some programs use a different environment variable; for
example, to make TeX use ‘emacsclient’, you should set the
TEXEDIT environment variable to ‘emacsclient +%d %s’.)
Then, whenever any program invokes your specified EDITOR
program, the effect is to send a message to your principal Emacs telling
it to visit a file. (That's what the program emacsclient does.)
Emacs displays the buffer immediately and you can immediately begin
editing it.
When you've finished editing that buffer, type C-x #
(server-edit). This saves the file and sends a message back to
the emacsclient program telling it to exit. The programs that
use EDITOR wait for the “editor” (actually, emacsclient)
to exit. C-x # also checks for other pending external requests
to edit various files, and selects the next such file.
You can switch to a server buffer manually if you wish; you don't
have to arrive at it with C-x #. But C-x # is the way to
say that you are finished with one.
Finishing with a server buffer also kills the buffer, unless it
already existed in the Emacs session before the server asked to create
it. However, if you set server-kill-new-buffers to nil,
then a different criterion is used: finishing with a server buffer
kills it if the file name matches the regular expression
server-temp-file-regexp. This is set up to distinguish certain
“temporary” files.
If you set the variable server-window to a window or a frame,
C-x # displays the server buffer in that window or in that frame.
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 visit a server by name
using the ‘-s’ option. See Invoking emacsclient.
While mail or another application is waiting for
emacsclient to finish, emacsclient does not read terminal
input. So the terminal that mail was using is effectively
blocked for the duration. In order to edit with your principal Emacs,
you need to be able to use it without using that terminal. There are
three ways to do this:
- Using a window system, run
mail and the principal Emacs in two
separate windows. While mail is waiting for emacsclient,
the window where it was running is blocked, but you can use Emacs by
switching windows.
- Using virtual terminals, run
mail in one virtual terminal
and run Emacs in another.
- Use Shell mode or Term mode in Emacs to run the other program such as
mail; then, emacsclient blocks only the subshell under
Emacs, and you can still use Emacs to edit the file.
If you run emacsclient with the option ‘--no-wait’, it
returns immediately without waiting for you to “finish” the buffer
in Emacs. Note that server buffers created in this way are not killed
automatically when you finish with them.
