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37.3 Creating a Synchronous Process

After a synchronous process is created, Emacs waits for the process to terminate before continuing. Starting Dired on GNU or Unix1 is an example of this: it runs ls in a synchronous process, then modifies the output slightly. Because the process is synchronous, the entire directory listing arrives in the buffer before Emacs tries to do anything with it.

While Emacs waits for the synchronous subprocess to terminate, the user can quit by typing C-g. The first C-g tries to kill the subprocess with a SIGINT signal; but it waits until the subprocess actually terminates before quitting. If during that time the user types another C-g, that kills the subprocess instantly with SIGKILL and quits immediately (except on MS-DOS, where killing other processes doesn't work). See Quitting.

The synchronous subprocess functions return an indication of how the process terminated.

The output from a synchronous subprocess is generally decoded using a coding system, much like text read from a file. The input sent to a subprocess by call-process-region is encoded using a coding system, much like text written into a file. See Coding Systems.

— Function: call-process program &optional infile destination display &rest args

This function calls program and waits for it to finish.

The current working directory of the subprocess is default-directory.

The standard input for the new process comes from file infile if infile is not nil, and from the null device otherwise. The argument destination says where to put the process output. Here are the possibilities:

a buffer
Insert the output in that buffer, before point. This includes both the standard output stream and the standard error stream of the process.
a string
Insert the output in a buffer with that name, before point.
t
Insert the output in the current buffer, before point.
nil
Discard the output.
0
Discard the output, and return nil immediately without waiting for the subprocess to finish.

In this case, the process is not truly synchronous, since it can run in parallel with Emacs; but you can think of it as synchronous in that Emacs is essentially finished with the subprocess as soon as this function returns.

MS-DOS doesn't support asynchronous subprocesses, so this option doesn't work there.

(:file file-name)
Send the output to the file name specified, overwriting it if it already exists.
(real-destination error-destination)
Keep the standard output stream separate from the standard error stream; deal with the ordinary output as specified by real-destination, and dispose of the error output according to error-destination. If error-destination is nil, that means to discard the error output, t means mix it with the ordinary output, and a string specifies a file name to redirect error output into.

You can't directly specify a buffer to put the error output in; that is too difficult to implement. But you can achieve this result by sending the error output to a temporary file and then inserting the file into a buffer.

If display is non-nil, then call-process redisplays the buffer as output is inserted. (However, if the coding system chosen for decoding output is undecided, meaning deduce the encoding from the actual data, then redisplay sometimes cannot continue once non-ASCII characters are encountered. There are fundamental reasons why it is hard to fix this; see Output from Processes.)

Otherwise the function call-process does no redisplay, and the results become visible on the screen only when Emacs redisplays that buffer in the normal course of events.

The remaining arguments, args, are strings that specify command line arguments for the program.

The value returned by call-process (unless you told it not to wait) indicates the reason for process termination. A number gives the exit status of the subprocess; 0 means success, and any other value means failure. If the process terminated with a signal, call-process returns a string describing the signal.

In the examples below, the buffer ‘foo’ is current.

          (call-process "pwd" nil t)
               ⇒ 0
          
          ---------- Buffer: foo ----------
          /home/lewis/manual
          ---------- Buffer: foo ----------
          
          (call-process "grep" nil "bar" nil "lewis" "/etc/passwd")
               ⇒ 0
          
          ---------- Buffer: bar ----------
          lewis:x:1001:1001:Bil Lewis,,,,:/home/lewis:/bin/bash
          
          ---------- Buffer: bar ----------

Here is an example of the use of call-process, as used to be found in the definition of the insert-directory function:

          (call-process insert-directory-program nil t nil switches
                        (if full-directory-p
                            (concat (file-name-as-directory file) ".")
                          file))
— Function: process-file program &optional infile buffer display &rest args

This function processes files synchronously in a separate process. It is similar to call-process, but may invoke a file handler based on the value of the variable default-directory, which specifies the current working directory of the subprocess.

The arguments are handled in almost the same way as for call-process, with the following differences:

Some file handlers may not support all combinations and forms of the arguments infile, buffer, and display. For example, some file handlers might behave as if display were nil, regardless of the value actually passed. As another example, some file handlers might not support separating standard output and error output by way of the buffer argument.

If a file handler is invoked, it determines the program to run based on the first argument program. For instance, suppose that a handler for remote files is invoked. Then the path that is used for searching for the program might be different from exec-path.

The second argument infile may invoke a file handler. The file handler could be different from the handler chosen for the process-file function itself. (For example, default-directory could be on one remote host, and infile on a different remote host. Or default-directory could be non-special, whereas infile is on a remote host.)

If buffer is a list of the form (real-destination error-destination), and error-destination names a file, then the same remarks as for infile apply.

The remaining arguments (args) will be passed to the process verbatim. Emacs is not involved in processing file names that are present in args. To avoid confusion, it may be best to avoid absolute file names in args, but rather to specify all file names as relative to default-directory. The function file-relative-name is useful for constructing such relative file names.

— Variable: process-file-side-effects

This variable indicates whether a call of process-file changes remote files.

By default, this variable is always set to t, meaning that a call of process-file could potentially change any file on a remote host. When set to nil, a file handler could optimize its behavior with respect to remote file attribute caching.

You should only ever change this variable with a let-binding; never with setq.

— Function: call-process-region start end program &optional delete destination display &rest args

This function sends the text from start to end as standard input to a process running program. It deletes the text sent if delete is non-nil; this is useful when destination is t, to insert the output in the current buffer in place of the input.

The arguments destination and display control what to do with the output from the subprocess, and whether to update the display as it comes in. For details, see the description of call-process, above. If destination is the integer 0, call-process-region discards the output and returns nil immediately, without waiting for the subprocess to finish (this only works if asynchronous subprocesses are supported; i.e., not on MS-DOS).

The remaining arguments, args, are strings that specify command line arguments for the program.

The return value of call-process-region is just like that of call-process: nil if you told it to return without waiting; otherwise, a number or string which indicates how the subprocess terminated.

In the following example, we use call-process-region to run the cat utility, with standard input being the first five characters in buffer ‘foo’ (the word ‘input’). cat copies its standard input into its standard output. Since the argument destination is t, this output is inserted in the current buffer.

          ---------- Buffer: foo ----------
          input-!-
          ---------- Buffer: foo ----------
          
          (call-process-region 1 6 "cat" nil t)
               ⇒ 0
          
          ---------- Buffer: foo ----------
          inputinput-!-
          ---------- Buffer: foo ----------

For example, the shell-command-on-region command uses call-process-region in a manner similar to this:

          (call-process-region
           start end
           shell-file-name      ; name of program
           nil                  ; do not delete region
           buffer               ; send output to buffer
           nil                  ; no redisplay during output
           "-c" command)        ; arguments for the shell
— Function: call-process-shell-command command &optional infile destination display &rest args

This function executes the shell command command synchronously. The final arguments args are additional arguments to add at the end of command. The other arguments are handled as in call-process.

— Function: process-file-shell-command command &optional infile destination display &rest args

This function is like call-process-shell-command, but uses process-file internally. Depending on default-directory, command can be executed also on remote hosts.

— Function: shell-command-to-string command

This function executes command (a string) as a shell command, then returns the command's output as a string.

— Function: process-lines program &rest args

This function runs program, waits for it to finish, and returns its output as a list of strings. Each string in the list holds a single line of text output by the program; the end-of-line characters are stripped from each line. The arguments beyond program, args, are strings that specify command-line arguments with which to run the program.

If program exits with a non-zero exit status, this function signals an error.

This function works by calling call-process, so program output is decoded in the same way as for call-process.


Footnotes

[1] On other systems, Emacs uses a Lisp emulation of ls; see Contents of Directories.