21.1 Command Loop Overview

The first thing the command loop must do is read a key sequence, which is a sequence of events that translates into a command. It does this by calling the function read-key-sequence. Your Lisp code can also call this function (see section 21.7.1 Key Sequence Input). Lisp programs can also do input at a lower level with read-event (see section 21.7.2 Reading One Event) or discard pending input with discard-input (see section 21.7.5 Miscellaneous Event Input Features).

The key sequence is translated into a command through the currently active keymaps. See section 22.7 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 section 21.3 Interactive Call).

To execute a command requires first reading the arguments for it. This is done by calling command-execute (see section 21.3 Interactive Call). For commands written in Lisp, the interactive specification says how to read the arguments. This may use the prefix argument (see section 21.11 Prefix Command Arguments) or may read with prompting in the minibuffer (see section 20. Minibuffers). For example, the command find-file has an interactive specification which says to read a file name using the minibuffer. The command's function body does not use the minibuffer; if you call this command from Lisp code as a function, you must supply the file name string as an ordinary Lisp function argument.

If the command is a string or vector (i.e., a keyboard macro) then execute-kbd-macro is used to execute it. You can call this function yourself (see section 21.15 Keyboard Macros).

To terminate the execution of a running command, type C-g. This character causes quitting (see section 21.10 Quitting).

Variable: pre-command-hook

The editor command loop runs this normal hook before each command. At that time, this-command contains the command that is about to run, and last-command describes the previous command. See section 23.6 Hooks.

Variable: post-command-hook

The editor command loop runs this normal hook after each command (including commands terminated prematurely by quitting or by errors), and also when the command loop is first entered. At that time, this-command describes the command that just ran, and last-command describes the command before that. See section 23.6 Hooks.

Quitting is suppressed while running pre-command-hook and post-command-hook. If an error happens while executing one of these hooks, it terminates execution of the hook, and clears the hook variable to nil so as to prevent an infinite loop of errors.