(octave.info)Controlling Subprocesses
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 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".
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);
or
more on;
printf ("%s\n", nthargout (2, "system", "cmd"));
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: Note: unix, Note: 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: Note: dos, Note: system, Note:
isunix, Note: ismac, Note: 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: Note: unix, Note: system, Note:
isunix, Note: ismac, Note: 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.
Recognized file types are
‘.m’
Open file in the editor.
‘.mat’
Load the file in the base workspace.
‘.exe’
Execute the program (on Windows systems only).
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: Note: system, Note: 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: Note: system, Note: 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: Note: 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: Note: fclose, Note: 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: Note: popen, Note: 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: Note: IMAGE_PATH, *note OCTAVE_HOME:
XREFOCTAVE_HOME, Note: 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: Note: 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: Note: fopen, Note: fclose, Note:
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.
See also: Note: WCONTINUE, *note WCOREDUMP:
XREFWCOREDUMP, Note: WEXITSTATUS, Note:
WIFCONTINUED, Note: WIFSIGNALED,
Note: WIFSTOPPED, Note: WNOHANG, Note:
WSTOPSIG, Note: WTERMSIG, Note:
WUNTRACED.
-- : 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: Note: waitpid, Note: WNOHANG,
Note: 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).
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, Note:
WIFSIGNALED, Note: WTERMSIG, Note:
WIFSTOPPED, Note: WSTOPSIG, Note:
WIFCONTINUED.
-- : 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.
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WIFSIGNALED, *note WTERMSIG:
XREFWTERMSIG, Note: WCOREDUMP, *note WIFSTOPPED:
XREFWIFSTOPPED, Note: WSTOPSIG, *note WIFCONTINUED:
XREFWIFCONTINUED.
-- : WIFCONTINUED (STATUS)
Given STATUS from a call to ‘waitpid’, return true if the child
process was resumed by delivery of ‘SIGCONT’.
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, Note:
WIFSIGNALED, Note: WTERMSIG, Note:
WCOREDUMP, Note: WIFSTOPPED, Note:
WSTOPSIG.
-- : WIFSIGNALED (STATUS)
Given STATUS from a call to ‘waitpid’, return true if the child
process was terminated by a signal.
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, *note WTERMSIG:
XREFWTERMSIG, Note: WCOREDUMP, *note WIFSTOPPED:
XREFWIFSTOPPED, Note: WSTOPSIG, *note WIFCONTINUED:
XREFWIFCONTINUED.
-- : 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)).
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, Note:
WIFSIGNALED, Note: WTERMSIG, Note:
WCOREDUMP, Note: WSTOPSIG, Note:
WIFCONTINUED.
-- : WIFEXITED (STATUS)
Given STATUS from a call to ‘waitpid’, return true if the child
terminated normally.
See also: Note: waitpid, *note WEXITSTATUS:
XREFWEXITSTATUS, Note: WIFSIGNALED, Note:
WTERMSIG, Note: WCOREDUMP, Note:
WIFSTOPPED, Note: WSTOPSIG, Note:
WIFCONTINUED.
-- : 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: Note: waitpid, *note WUNTRACED:
XREFWUNTRACED, Note: 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.
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, Note:
WIFSIGNALED, Note: WTERMSIG, Note:
WCOREDUMP, Note: WIFSTOPPED, Note:
WIFCONTINUED.
-- : 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.
See also: Note: waitpid, *note WIFEXITED:
XREFWIFEXITED, Note: WEXITSTATUS, Note:
WIFSIGNALED, Note: WCOREDUMP, Note:
WIFSTOPPED, Note: WSTOPSIG, Note:
WIFCONTINUED.
-- : 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: Note: waitpid, Note: WNOHANG,
Note: WCONTINUE.
-- : [ERR, 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, ERR is 0 and MSG is an empty string. Otherwise, ERR
is nonzero and MSG contains a system-dependent error message.
See also: Note: fopen, Note: dup2.
-- : [ERR, 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.
Return 0 if successful, otherwise return -1.
-- : SIG ()
Return a structure containing Unix signal names and their defined
values.
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