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AC.15.1 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.

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. 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 uudecode. For instance, if the buffer contains uuencoded text, type C-x h M-| uudecode RET to feed the entire buffer contents to the uudecode program. That program will ignore everything except the encoded text, and will store the decoded output into the file whose name is specified in the encoded text.

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 exec-path are 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. 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 ignores the SIGINT signal), type C-g again; this sends the command a SIGKILL signal which is impossible to ignore.

To specify a coding system for M-! or M-|, use the command C-x RET c immediately beforehand. See section Q.9 Specifying a Coding System.

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.

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