20.1 Command Loop Overview

The first thing the command loop must do is read a key sequence, which is a sequence of input events that translates into a command. It does this by calling the function read-key-sequence. Lisp programs can also call this function (see Key Sequence Input). They can also read input at a lower level with read-key or read-event (see Reading One Event), or discard pending input with discard-input (see Event Input Misc).

The key sequence is translated into a command through the currently active keymaps. See Key Lookup, for information on how this is done. The result should be a keyboard macro or an interactively callable function. If the key is M-x, then it reads the name of another command, which it then calls. This is done by the command execute-extended-command (see Interactive Call).

Prior to executing the command, Emacs runs undo-boundary to create an undo boundary. See Maintaining Undo.

To execute a command, Emacs first reads its arguments by calling command-execute (see Interactive Call). For commands written in Lisp, the interactive specification says how to read the arguments. This may use the prefix argument (see Prefix Command Arguments) or may read with prompting in the minibuffer (see Minibuffers). For example, the command find-file has an interactive specification which says to read a file name using the minibuffer. The function body of find-file does not use the minibuffer, so if you call find-file as a function from Lisp code, you must supply the file name string as an ordinary Lisp function argument.

If the command is a keyboard macro (i.e., a string or vector), Emacs executes it using execute-kbd-macro (see Keyboard Macros).

Variable: pre-command-hook

This normal hook is run by the editor command loop before it executes each command. At that time, this-command contains the command that is about to run, and last-command describes the previous command. See Command Loop Info.

Variable: post-command-hook

This normal hook is run by the editor command loop after it executes each command (including commands terminated prematurely by quitting or by errors). At that time, this-command refers to the command that just ran, and last-command refers to the command before that.

This hook is also run when Emacs first enters the command loop (at which point this-command and last-command are both nil).

Quitting is suppressed while running pre-command-hook and post-command-hook. If an error happens while executing one of these hooks, it does not terminate execution of the hook; instead the error is silenced and the function in which the error occurred is removed from the hook.

A request coming into the Emacs server (see Emacs Server in The GNU Emacs Manual) runs these two hooks just as a keyboard command does.