41.1 Single Shell Commands
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.
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. If a numeric argument is used, meaning
insert the output in the current buffer, then the old region is deleted
first and the output replaces it as the contents of the region. It
returns the command's exit status when it is called from a Lisp program.
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
gpg program. That program will ignore everything except
the encoded keys, and will output a list of the keys it 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 does not
specify a directory, the directories in the list
searched; 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 waits until the command
actually terminates. If the shell command doesn't stop (because it
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 Specify Coding.
Error output from the command is normally intermixed with the regular
output. If you set the variable
shell-command-default-error-buffer to a string, which is a buffer
name, error output is inserted before point in the buffer of that name.