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37.1 Functions that Create Subprocesses

There are three primitives that create a new subprocess in which to run a program. One of them, start-process, creates an asynchronous process and returns a process object (see Asynchronous Processes). The other two, call-process and call-process-region, create a synchronous process and do not return a process object (see Synchronous Processes). There are various higher-level functions that make use of these primitives to run particular types of process.

Synchronous and asynchronous processes are explained in the following sections. Since the three functions are all called in a similar fashion, their common arguments are described here.

In all cases, the function's program argument specifies the program to be run. An error is signaled if the file is not found or cannot be executed. If the file name is relative, the variable exec-path contains a list of directories to search. Emacs initializes exec-path when it starts up, based on the value of the environment variable PATH. The standard file name constructs, ‘~’, ‘.’, and ‘..’, are interpreted as usual in exec-path, but environment variable substitutions (‘$HOME’, etc.) are not recognized; use substitute-in-file-name to perform them (see File Name Expansion). nil in this list refers to default-directory.

Executing a program can also try adding suffixes to the specified name:

— User Option: exec-suffixes

This variable is a list of suffixes (strings) to try adding to the specified program file name. The list should include "" if you want the name to be tried exactly as specified. The default value is system-dependent.

Please note: The argument program contains only the name of the program; it may not contain any command-line arguments. You must use a separate argument, args, to provide those, as described below.

Each of the subprocess-creating functions has a buffer-or-name argument that specifies where the standard output from the program will go. It should be a buffer or a buffer name; if it is a buffer name, that will create the buffer if it does not already exist. It can also be nil, which says to discard the output unless a filter function handles it. (See Filter Functions, and Read and Print.) Normally, you should avoid having multiple processes send output to the same buffer because their output would be intermixed randomly. For synchronous processes, you can send the output to a file instead of a buffer.

All three of the subprocess-creating functions have a &rest argument, args. The args must all be strings, and they are supplied to program as separate command line arguments. Wildcard characters and other shell constructs have no special meanings in these strings, since the strings are passed directly to the specified program.

The subprocess inherits its environment from Emacs, but you can specify overrides for it with process-environment. See System Environment. The subprocess gets its current directory from the value of default-directory.

— Variable: exec-directory

The value of this variable is a string, the name of a directory that contains programs that come with GNU Emacs and are intended for Emacs to invoke. The program movemail is an example of such a program; Rmail uses it to fetch new mail from an inbox.

— User Option: exec-path

The value of this variable is a list of directories to search for programs to run in subprocesses. Each element is either the name of a directory (i.e., a string), or nil, which stands for the default directory (which is the value of default-directory). The value of exec-path is used by call-process and start-process when the program argument is not an absolute file name.

Generally, you should not modify exec-path directly. Instead, ensure that your PATH environment variable is set appropriately before starting Emacs. Trying to modify exec-path independently of PATH can lead to confusing results.