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Disabling a command means that invoking it interactively asks for confirmation from the user. The purpose of disabling a command is to prevent users from executing it by accident; we do this for commands that might be confusing to the uninitiated.
Attempting to invoke a disabled command interactively in Emacs displays a window containing the command's name, its documentation, and some instructions on what to do immediately; then Emacs asks for input saying whether to execute the command as requested, enable it and execute it, or cancel. If you decide to enable the command, you must then answer another question—whether to do this permanently, or just for the current session. (Enabling permanently works by automatically editing your initialization file.) You can also type ! to enable all commands, for the current session only.
The direct mechanism for disabling a command is to put a
disabled property on the Lisp symbol for the
command. Here is the Lisp program to do this:
(put 'delete-region 'disabled t)
If the value of the
disabled property is a string, that string
is included in the message displayed when the command is used:
(put 'delete-region 'disabled "It's better to use `kill-region' instead.\n")
You can make a command disabled either by editing the initialization file directly, or with the command M-x disable-command, which edits the initialization file for you. Likewise, M-x enable-command edits the initialization file to enable a command permanently. See Init File.
If Emacs was invoked with the -q or --no-init-file options (see Initial Options), it will not edit your initialization file. Doing so could lose information because Emacs has not read your initialization file.
Whether a command is disabled is independent of what key is used to invoke it; disabling also applies if the command is invoked using M-x. However, disabling a command has no effect on calling it as a function from Lisp programs.