41.1 Single Shell Commands

M-! (shell-command) reads a line of text using the minibuffer and executes it as a shell command in a subshell made just for that command. Standard input for the command comes from the null device. If the shell command produces any output, the output appears either in the echo area (if it is short), or in an Emacs buffer named ‘*Shell Command Output*’, which is displayed in another window but not selected (if the output is long).

For instance, one way to decompress a file ‘foo.gz’ from Emacs is to type M-! gunzip foo.gz <RET>. That shell command normally creates the file ‘foo’ and produces no terminal output.

A numeric argument, as in M-1 M-!, says to insert terminal output into the current buffer instead of a separate buffer. It puts point before the output, and sets the mark after the output. For instance, M-1 M-! gunzip < foo.gz <RET> would insert the uncompressed equivalent of ‘foo.gz’ into the current buffer.

If the shell command line ends in ‘&’, it runs asynchronously. For a synchronous shell command, shell-command returns the command's exit status (0 means success), when it is called from a Lisp program. You do not get any status information for an asynchronous command, since it hasn't finished yet when shell-command returns.

M-| (shell-command-on-region) is like M-! but passes the contents of the region as the standard input to the shell command, instead of no input. With a numeric argument, meaning insert the output in the current buffer, it deletes the old region and the output replaces it as the contents of the region. It returns the command's exit status, like M-!.

One use for M-| is to run gpg to see what keys are in the buffer. For instance, if the buffer contains a GPG key, type C-x h M-| gpg <RET> to feed the entire buffer contents to the gpg program. That program will ignore everything except the encoded keys, and will output a list of the keys the buffer contains.

Both M-! and M-| use shell-file-name to specify the shell to use. This variable is initialized based on your SHELL environment variable when Emacs is started. If the file name is relative, Emacs searches the directories in the list exec-path; this list is initialized based on the environment variable PATH when Emacs is started. Your ‘.emacs’ file can override either or both of these default initializations.

Both M-! and M-| wait for the shell command to complete, unless you end the command with ‘&’ to make it asynchronous. To stop waiting, type C-g to quit; that terminates the shell command with the signal SIGINT—the same signal that C-c normally generates in the shell. Emacs then waits until the command actually terminates. If the shell command doesn't stop (because it ignores the SIGINT signal), type C-g again; this sends the command a SIGKILL signal which is impossible to ignore.

Asynchronous commands ending in ‘&’ feed their output into the buffer ‘*Async Shell Command*’. Output arrives in that buffer regardless of whether it is visible in a window.

To specify a coding system for M-! or M-|, use the command C-x <RET> c immediately beforehand. See section Coding Systems for Interprocess Communication.

Error output from these commands is normally intermixed with the regular output. But if the variable shell-command-default-error-buffer has a string as value, and it's the name of a buffer, M-! and M-| insert error output before point in that buffer.