38.2.2 Suspending Emacs

On text terminals, it is possible to suspend Emacs, which means stopping Emacs temporarily and returning control to its superior process, which is usually the shell. This allows you to resume editing later in the same Emacs process, with the same buffers, the same kill ring, the same undo history, and so on. To resume Emacs, use the appropriate command in the parent shell—most likely fg.

Suspending works only on a terminal device from which the Emacs session was started. We call that device the controlling terminal of the session. Suspending is not allowed if the controlling terminal is a graphical terminal. Suspending is usually not relevant in graphical environments, since you can simply switch to another application without doing anything special to Emacs.

Some operating systems (those without SIGTSTP, or MS-DOS) do not support suspension of jobs; on these systems, “suspension” actually creates a new shell temporarily as a subprocess of Emacs. Then you would exit the shell to return to Emacs.

Command: suspend-emacs &optional string

This function stops Emacs and returns control to the superior process. If and when the superior process resumes Emacs, suspend-emacs returns nil to its caller in Lisp.

This function works only on the controlling terminal of the Emacs session; to relinquish control of other tty devices, use suspend-tty (see below). If the Emacs session uses more than one terminal, you must delete the frames on all the other terminals before suspending Emacs, or this function signals an error. See Multiple Terminals.

If string is non-nil, its characters are sent to Emacs’s superior shell, to be read as terminal input. The characters in string are not echoed by the superior shell; only the results appear.

Before suspending, suspend-emacs runs the normal hook suspend-hook. After the user resumes Emacs, suspend-emacs runs the normal hook suspend-resume-hook. See Hooks.

The next redisplay after resumption will redraw the entire screen, unless the variable no-redraw-on-reenter is non-nil. See Refresh Screen.

Here is an example of how you could use these hooks:

(add-hook 'suspend-hook
          (lambda () (or (y-or-n-p "Really suspend? ")
                         (error "Suspend canceled"))))

(add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
                                 (sit-for 2)))

Here is what you would see upon evaluating (suspend-emacs "pwd"):

---------- Buffer: Minibuffer ----------
Really suspend? y
---------- Buffer: Minibuffer ----------

---------- Parent Shell ----------
bash$ /home/username
bash$ fg

---------- Echo Area ----------
Resumed!

Note that ‘pwd’ is not echoed after Emacs is suspended. But it is read and executed by the shell.

Variable: suspend-hook

This variable is a normal hook that Emacs runs before suspending.

Variable: suspend-resume-hook

This variable is a normal hook that Emacs runs on resuming after a suspension.

Function: suspend-tty &optional tty

If tty specifies a terminal device used by Emacs, this function relinquishes the device and restores it to its prior state. Frames that used the device continue to exist, but are not updated and Emacs doesn’t read input from them. tty can be a terminal object, a frame (meaning the terminal for that frame), or nil (meaning the terminal for the selected frame). See Multiple Terminals.

If tty is already suspended, this function does nothing.

This function runs the hook suspend-tty-functions, passing the terminal object as an argument to each function.

Function: resume-tty &optional tty

This function resumes the previously suspended terminal device tty; where tty has the same possible values as it does for suspend-tty.

This function reopens the terminal device, re-initializes it, and redraws it with that terminal’s selected frame. It then runs the hook resume-tty-functions, passing the terminal object as an argument to each function.

If the same device is already used by another Emacs terminal, this function signals an error. If tty is not suspended, this function does nothing.

Function: controlling-tty-p &optional tty

This function returns non-nil if tty is the controlling terminal of the Emacs session; tty can be a terminal object, a frame (meaning the terminal for that frame), or nil (meaning the terminal for the selected frame).

Command: suspend-frame

This command suspends a frame. For GUI frames, it calls iconify-frame (see Visibility of Frames); for frames on text terminals, it calls either suspend-emacs or suspend-tty, depending on whether the frame is displayed on the controlling terminal device or not.