After a synchronous process is created, Emacs waits for the
process to terminate before continuing. Starting Dired on GNU or
Unix18 is an example of this: it
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
call-process-region is encoded using a coding
system, much like text written into a file. See Coding Systems.
call-processprogram &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:
Insert the output in that buffer, before point. This includes both the standard output stream and the standard error stream of the process.
Insert the output in a buffer with that name, before point.
Insert the output in the current buffer, before point.
Discard the output.
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.
Send the output to the file name specified, overwriting it if it already exists.
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 when the subprocess finishes.
If display is non-
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. Each string is passed to program as a separate argument.
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. If you
call-process not to wait, it returns
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
(call-process insert-directory-program nil t nil switches (if full-directory-p (concat (file-name-as-directory file) ".") file))
process-fileprogram &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
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
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
This variable indicates whether a call of
By default, this variable is always set to t, meaning that a
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
call-process-regionstart 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
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
nil if you told it to return without
waiting; otherwise, a number or string which indicates how the
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: 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
call-process-shell-commandcommand &optional infile destination display
This function executes the shell command command synchronously.
The other arguments are handled as in
call-process. An old
calling convention allowed passing any number of additional arguments
after display, which were concatenated to command; this is
still supported, but strongly discouraged.
process-file-shell-commandcommand &optional infile destination display
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. An old calling
convention allowed passing any number of additional arguments after
display, which were concatenated to command; this is still
supported, but strongly discouraged.
This function executes command (a string) as a shell command, then returns the command’s output as a string.
process-linesprogram &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