There are two ways of canceling a command before it has finished: quitting with C-g, and aborting with C-] or M-x top-level. Quitting cancels a partially typed command, or one which is still running. Aborting exits a recursive editing level and cancels the command that invoked the recursive edit (see Recursive Edit).
Quitting with C-g is the way to get rid of a partially typed command, or a numeric argument that you don't want. Furthermore, if you are in the middle of a command that is running, C-g stops the command in a relatively safe way. For example, if you quit out of a kill command that is taking a long time, either your text will all still be in the buffer, or it will all be in the kill ring, or maybe both. If the region is active, C-g deactivates the mark, unless Transient Mark mode is off (see Disabled Transient Mark). If you are in the middle of an incremental search, C-g behaves specially; it may take two successive C-g characters to get out of a search. See Incremental Search, for details.
On MS-DOS, the character C-<Break> serves as a quit character like C-g. The reason is that it is not feasible, on MS-DOS, to recognize C-g while a command is running, between interactions with the user. By contrast, it is feasible to recognize C-<Break> at all times. See MS-DOS Keyboard.
C-g works by setting the variable
the instant C-g is typed; Emacs Lisp checks this variable
frequently, and quits if it is non-
nil. C-g is only
actually executed as a command if you type it while Emacs is waiting for
input. In that case, the command it runs is
On a text terminal, if you quit with C-g a second time before the first C-g is recognized, you activate the emergency-escape feature and return to the shell. See Emergency Escape.
There are some situations where you cannot quit. When Emacs is waiting for the operating system to do something, quitting is impossible unless special pains are taken for the particular system call within Emacs where the waiting occurs. We have done this for the system calls that users are likely to want to quit from, but it's possible you will encounter a case not handled. In one very common case—waiting for file input or output using NFS—Emacs itself knows how to quit, but many NFS implementations simply do not allow user programs to stop waiting for NFS when the NFS server is hung.
Aborting with C-] (
abort-recursive-edit) is used to get
out of a recursive editing level and cancel the command which invoked
it. Quitting with C-g does not do this, and could not do this,
because it is used to cancel a partially typed command within the
recursive editing level. Both operations are useful. For example, if
you are in a recursive edit and type C-u 8 to enter a numeric
argument, you can cancel that argument with C-g and remain in the
The sequence <ESC> <ESC> <ESC>
keyboard-escape-quit) can either quit or abort. (We defined
it this way because <ESC> means “get out” in many PC programs.)
It can cancel a prefix argument, clear a selected region, or get out
of a Query Replace, like C-g. It can get out of the minibuffer
or a recursive edit, like C-]. It can also get out of splitting
the frame into multiple windows, as with C-x 1. One thing it
cannot do, however, is stop a command that is running. That's because
it executes as an ordinary command, and Emacs doesn't notice it until
it is ready for the next command.
The command M-x top-level is equivalent to enough C-] commands to get you out of all the levels of recursive edits that you are in; it also exits the minibuffer if it is active. C-] gets you out one level at a time, but M-x top-level goes out all levels at once. Both C-] and M-x top-level are like all other commands, and unlike C-g, in that they take effect only when Emacs is ready for a command. C-] is an ordinary key and has its meaning only because of its binding in the keymap. See Recursive Edit.
undo) is not strictly speaking a way of canceling
a command, but you can think of it as canceling a command that already
finished executing. See Undo, for more information about the undo