Controlling Subprocesses (GNU Octave (version 7.3.0))

36.5 Controlling Subprocesses

Octave includes some high-level commands like system and popen for starting subprocesses. If you want to run another program to perform some task and then look at its output, you will probably want to use these functions.

Octave also provides several very low-level Unix-like functions which can also be used for starting subprocesses, but you should probably only use them if you can’t find any way to do what you need with the higher-level functions.

: system ("string") ¶

: system ("string", return_output) ¶

: system ("string", return_output, type) ¶

: [status, output] = system (…) ¶

Execute a shell command specified by string.

If system is called with one or more output arguments, or if the optional argument return_output is true and the subprocess is started synchronously, then the output from the command is returned as a variable. Otherwise, if the subprocess is executed synchronously, its output is sent to the standard output. To send the output of a command executed with system through the pager, use a command like

[~, text] = system ("cmd");
more on;
disp (text);

more on;
printf ("%s\n", nthargout (2, "system", "cmd"));

If the optional argument type is "async", the process is started in the background and the process ID of the child process is returned immediately. Otherwise, the child process is started and Octave waits until it exits. If the type argument is omitted, it defaults to the value "sync".

The system function can return two values. The first is the exit status of the command and the second is any output from the command that was written to the standard output stream. For example,

[status, output] = system ("echo foo & exit 2");

will set the variable output to the string ‘foo’, and the variable status to the integer ‘2’.

For commands run asynchronously, status is the process id of the command shell that is started to run the command.

The shell used for executing commands varies with operating system and is typically /bin/sh for UNIX systems and cmd.exe for Windows systems.

See also: unix, dos.

: unix ("command") ¶

: status = unix ("command") ¶

: [status, text] = unix ("command") ¶

: […] = unix ("command", "-echo") ¶

Execute a system command if running under a Unix-like operating system, otherwise do nothing.

Octave waits for the external command to finish before returning the exit status of the program in status and any output in text.

When called with no output argument, or the "-echo" argument is given, then text is also sent to standard output.

See also: dos, system, isunix, ismac, ispc.

: dos ("command") ¶

: status = dos ("command") ¶

: [status, text] = dos ("command") ¶

: […] = dos ("command", "-echo") ¶

Execute a system command if running under a Windows-like operating system, otherwise do nothing.

Octave waits for the external command to finish before returning the exit status of the program in status and any output in text.

When called with no output argument, or the "-echo" argument is given, then text is also sent to standard output.

See also: unix, system, isunix, ismac, ispc.

: open file ¶

: output = open (file) ¶

Open the file file in Octave or in an external application based on the file type as determined by the filename extension.

By default, recognized file types are

.m: Open file in the editor. No output value is returned.
.mat
octave-workspace: Open the data file with load. If no return value output is requested, variables are loaded in the base workspace. Otherwise output will be a structure containing loaded data. See load function.
.ofig: Open the figure with hgload. See hgload function.
.fig, .ofig: Load the figure
.exe: Execute the program (on Windows systems only). No output value is returned.

Custom file extensions may also be handled if a function openxxx, where xxx is the extension, is found in the load path. The function must accept the file name as input. For example, in order to load ".dat" data files in the base workspace, as is done by default for ".mat" files, one may define "opendat.m" with the following contents:

function retval = opendat (fname)
  evalin ("base", sprintf ("load ('%s');", fname));
endfunction

Other file types are opened in the appropriate external application.

: output = perl (scriptfile) ¶

: output = perl (scriptfile, argument1, argument2, …) ¶

: [output, status] = perl (…) ¶

Invoke Perl script scriptfile, possibly with a list of command line arguments.

Return output in output and optional status in status. If scriptfile is not an absolute filename it is searched for in the current directory and then in the Octave loadpath.

See also: system, python.

: output = python (scriptfile) ¶

: output = python (scriptfile, argument1, argument2, …) ¶

: [output, status] = python (…) ¶

Invoke Python script scriptfile, possibly with a list of command line arguments.

Return output in output and optional status in status. If scriptfile is not an absolute filename it is searched for in the current directory and then in the Octave loadpath.

See also: system, perl.

: fid = popen (command, mode) ¶

Start a process and create a pipe.

The name of the command to run is given by command. The argument mode may be

"r": The pipe will be connected to the standard output of the process, and open for reading.
"w": The pipe will be connected to the standard input of the process, and open for writing.

The file identifier corresponding to the input or output stream of the process is returned in fid.

For example:

fid = popen ("ls -ltr / | tail -3", "r");
while (ischar (s = fgets (fid)))
  fputs (stdout, s);
endwhile

   -| drwxr-xr-x  33 root  root  3072 Feb 15 13:28 etc
   -| drwxr-xr-x   3 root  root  1024 Feb 15 13:28 lib
   -| drwxrwxrwt  15 root  root  2048 Feb 17 14:53 tmp

See also: popen2.

: pclose (fid) ¶

Close a file identifier that was opened by popen.

The function fclose may also be used for the same purpose.

See also: fclose, popen.

: [in, out, pid] = popen2 (command, args) ¶

Start a subprocess with two-way communication.

The name of the process is given by command, and args is an array or cell array of strings containing options for the command.

The file identifiers for the input and output streams of the subprocess are returned in in and out. If execution of the command is successful, pid contains the process ID of the subprocess. Otherwise, pid is -1.

For example:

[in, out, pid] = popen2 ("sort", "-r");
fputs (in, "these\nare\nsome\nstrings\n");
fclose (in);
EAGAIN = errno ("EAGAIN");
done = false;
do
  s = fgets (out);
  if (ischar (s))
    fputs (stdout, s);
  elseif (errno () == EAGAIN)
    pause (0.1);
    fclear (out);
  else
    done = true;
  endif
until (done)
fclose (out);
waitpid (pid);

   -| these
   -| strings
   -| some
   -| are

Note that popen2, unlike popen, will not "reap" the child process. If you don’t use waitpid to check the child’s exit status, it will linger until Octave exits.

See also: popen, waitpid.

: val = EXEC_PATH () ¶

: old_val = EXEC_PATH (new_val) ¶

: EXEC_PATH (new_val, "local") ¶

Query or set the internal variable that specifies a colon separated list of directories to append to the shell PATH when executing external programs.

The initial value of is taken from the environment variable OCTAVE_EXEC_PATH, but that value can be overridden by the command line argument --exec-path PATH.

When called from inside a function with the "local" option, the variable is changed locally for the function and any subroutines it calls. The original variable value is restored when exiting the function.

See also: IMAGE_PATH, OCTAVE_HOME, OCTAVE_EXEC_HOME.

In most cases, the following functions simply decode their arguments and make the corresponding Unix system calls. For a complete example of how they can be used, look at the definition of the function popen2.

: [pid, msg] = fork () ¶

Create a copy of the current process.

Fork can return one of the following values:

> 0: You are in the parent process. The value returned from fork is the process id of the child process. You should probably arrange to wait for any child processes to exit.
0: You are in the child process. You can call exec to start another process. If that fails, you should probably call exit.
< 0: The call to fork failed for some reason. You must take evasive action. A system dependent error message will be waiting in msg.

: [err, msg] = exec (file, args) ¶

Replace current process with a new process.

Calling exec without first calling fork will terminate your current Octave process and replace it with the program named by file. For example,

exec ("ls", "-l")

will run ls and return you to your shell prompt.

If successful, exec does not return. If exec does return, err will be nonzero, and msg will contain a system-dependent error message.

: [read_fd, write_fd, err, msg] = pipe () ¶

Create a pipe and return the reading and writing ends of the pipe into read_fd and write_fd respectively.

If successful, err is 0 and msg is an empty string. Otherwise, err is nonzero and msg contains a system-dependent error message.

See also: mkfifo.

: [fid, msg] = dup2 (old, new) ¶

Duplicate a file descriptor.

If successful, fid is greater than zero and contains the new file ID. Otherwise, fid is negative and msg contains a system-dependent error message.

See also: fopen, fclose, fcntl.

: [pid, status, msg] = waitpid (pid, options) ¶

Wait for process pid to terminate.

The pid argument can be:

-1: Wait for any child process.
0: Wait for any child process whose process group ID is equal to that of the Octave interpreter process.
> 0: Wait for termination of the child process with ID pid.

The options argument can be a bitwise OR of zero or more of the following constants:

0: Wait until signal is received or a child process exits (this is the default if the options argument is missing).
WNOHANG: Do not hang if status is not immediately available.
WUNTRACED: Report the status of any child processes that are stopped, and whose status has not yet been reported since they stopped.
WCONTINUE: Return if a stopped child has been resumed by delivery of SIGCONT. This value may not be meaningful on all systems.

If the returned value of pid is greater than 0, it is the process ID of the child process that exited. If an error occurs, pid will be less than zero and msg will contain a system-dependent error message. The value of status contains additional system-dependent information about the subprocess that exited.

: WCONTINUE () ¶

Return the numerical value of the WCONTINUE macro.

WCONTINUE is the option argument that may be passed to waitpid to indicate that it should also return if a stopped child has been resumed by delivery of a SIGCONT signal.

See also: waitpid, WNOHANG, WUNTRACED.

: WCOREDUMP (status) ¶

Given status from a call to waitpid, return true if the child produced a core dump.

This function should only be employed if WIFSIGNALED returned true. The macro used to implement this function is not specified in POSIX.1-2001 and is not available on some Unix implementations (e.g., AIX, SunOS).

: WEXITSTATUS (status) ¶

Given status from a call to waitpid, return the exit status of the child.

This function should only be employed if WIFEXITED returned true.

: WIFCONTINUED (status) ¶

Given status from a call to waitpid, return true if the child process was resumed by delivery of SIGCONT.

: WIFSIGNALED (status) ¶

Given status from a call to waitpid, return true if the child process was terminated by a signal.

: WIFSTOPPED (status) ¶

Given status from a call to waitpid, return true if the child process was stopped by delivery of a signal.

This is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).

: WIFEXITED (status) ¶

Given status from a call to waitpid, return true if the child terminated normally.

: WNOHANG () ¶

Return the numerical value of the WNOHANG macro.

WNOHANG is the option argument that may be passed to waitpid to indicate that it should return its status immediately instead of waiting for a process to exit.

See also: waitpid, WUNTRACED, WCONTINUE.

: WSTOPSIG (status) ¶

Given status from a call to waitpid, return the number of the signal which caused the child to stop.

This function should only be employed if WIFSTOPPED returned true.

: WTERMSIG (status) ¶

Given status from a call to waitpid, return the number of the signal that caused the child process to terminate.

This function should only be employed if WIFSIGNALED returned true.

: WUNTRACED () ¶

Return the numerical value of the WUNTRACED macro.

WUNTRACED is the option argument that may be passed to waitpid to indicate that it should also return if the child process has stopped but is not traced via the ptrace system call

See also: waitpid, WNOHANG, WCONTINUE.

: fcntl (fid, request, arg) ¶

: [status, msg] = fcntl (fid, request, arg) ¶

Change the properties of the open file fid.

The following values may be passed as request:

F_DUPFD ¶

Return a duplicate file descriptor.

F_GETFD ¶

Return the file descriptor flags for fid.

F_SETFD ¶

Set the file descriptor flags for fid.

F_GETFL ¶

Return the file status flags for fid. The following codes may be returned (some of the flags may be undefined on some systems).

O_RDONLY ¶: Open for reading only.
O_WRONLY ¶: Open for writing only.
O_RDWR ¶: Open for reading and writing.
O_APPEND ¶: Append on each write.
O_CREAT ¶: Create the file if it does not exist.
O_NONBLOCK ¶: Non-blocking mode.
O_SYNC ¶: Wait for writes to complete.
O_ASYNC ¶: Asynchronous I/O.

F_SETFL ¶

Set the file status flags for fid to the value specified by arg. The only flags that can be changed are O_APPEND and O_NONBLOCK.

If successful, status is 0 and msg is an empty string. Otherwise, status is -1 and msg contains a system-dependent error message.

See also: fopen, dup2.

: kill (pid, sig) ¶

: [status, msg] = kill (pid, sig) ¶

Send signal sig to process pid.

If pid is positive, then signal sig is sent to pid.

If pid is 0, then signal sig is sent to every process in the process group of the current process.

If pid is -1, then signal sig is sent to every process except process 1.

If pid is less than -1, then signal sig is sent to every process in the process group -pid.

If sig is 0, then no signal is sent, but error checking is still performed.

If successful, status is 0 and msg is an empty string. Otherwise, status is -1 and msg contains a system-dependent error message.

: SIG () ¶: Return a structure containing Unix signal names and their defined values.