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abort-recursive-edit).
keyboard-escape-quit).
undo).
There are two ways of canceling commands which are not finished executing: quitting with C-g, and aborting with C-] or M-x top-level. Quitting cancels a partially typed command or one which is already running. Aborting exits a recursive editing level and cancels the command that invoked the recursive edit. (See Recursive Edit.)
Quitting with C-g is used for getting rid of a partially typed command, or a numeric argument that you don't want. It also stops a running command in the middle in a relatively safe way, so you can use it if you accidentally give a command which takes a long time. In particular, it is safe to quit out of killing; either your text will all still be in the buffer, or it will all be in the kill ring (or maybe both). Quitting an incremental search does special things documented under searching; in general, it may take two successive C-g characters to get out of a search (see Incremental Search).
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 quit-flag to t
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 keyboard-quit.
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 may be times when 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 find another. 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
recursive edit.
The command <ESC> <ESC> <ESC>
(keyboard-escape-quit) can either quit or abort. This key was
defined because <ESC> is used to “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, like 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 a 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. 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.
C-x u (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 facility.