Emacs server
Emacs includes an optional client-server architecture.
Starting the Server
With server-start
To start a server in an existing Emacs session, run M-x server-start. A server started in this manner will close when the last visible Emacs frame closes. If you want Emacs to automatically run a server on startup, add the following to your configuration file.
(server-start)
With --daemon
Emacs 23 and higher provide the --daemon command-line argument, which will cause Emacs to immediately start a server and fork into the background. A server started in this manner is fully daemonized and will remain running even with no visible Emacs frames and after its parent terminal has closed (for example, after an SSH session has disconnected).
Note that, if using --daemon, your configuration file will be run while Emacs is still in a terminal. This may affect your theming if you usually use graphical Emacs.
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Using Clients
Once the server is started you may use the emacsclient command outside of Emacs to send a file to Emacs for editing, e.g. emacsclient ~/.emacs.d/init.el to modify your init file. On a GNU/Linux system it might be a good idea to set your EDITOR environment variable to emacsclient or emacsclient -t. Using emacsclient without any arguments will cause the calling process to simply pause and wait for the Emacs server to report that it's done editing. Using emacsclient -t will cause the calling process to put up a text-mode frame in the same terminal.
If the server was started using --daemon, you should run emacsclient -t or emacsclient -nc to create a visible frame to interact with.
Finishing Up
Once you have finished with a buffer that was opened via emacsclient you should use the [C-x #] (or M-x server-edit) to properly close the loop. Killing the buffer normally will not send the signal to emacsclient that Emacs has finished with the file.
If you're in an Emacs session that was started using --daemon, [C-x C-c] will only close the frame that it was invoked in. It will also signal to the client that owns the frame that its editing job is done, so emacsclient -c or emacsclient -t as EDITOR will behave much like a full emacs or emacs -nw.
To entirely close a server started with --daemon, run M-x save-buffers-kill-emacs in a frame.