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HELP RUN_UNIX_PROGRAM                            John Williams, Oct 1994

The procedure run_unix_program (defined in LIB * RUN_UNIX_PROGRAM)
enables you to run Unix programs from within Poplog; send input to such
a program, and receive any output it produces.

It takes six (!) arguments, and returns five results, and should be
called as:

    run_unix_program(name, arglist, input, output, errs, wait)
                        -> (indev, outdev, errdev, status, pid);

The possible argument and result values are described below.

1  Argument Values

The first argument, name, is the name (as a string) of the Unix program
to be run. If name starts with a '/', e.g. '/bin/csh', it is assumed to
be the full name of the Unix program file. Otherwise, run_unix_program
uses * sys_search_unix_program  to locate the command in the Unix
environment variable $PATH. An error is signalled if the command cannot
be found.

arglist is a list of argument strings to pass to the program named by
name. (For example, if name were 'csh', arglist might be ['-f']).
arglist may of course be the empty list [].

The arguments input and output specify the standard input and standard
output of the Unix program. These arguments can take one of four values:
a filename, a device record, the boolean true, or the boolean false.

If a filename (or device), then standard input and/or standard output
will be bound to that file (or device). For example, the call

    run_unix_program('wc', ['-w'], 'foo', 'baz', false, false)

will count the number of words in the file foo and output the count to
the file baz.

If the argument input is true, then run_unix_program will return, as the
value of indev, a device record which when written to sends input to the
running Unix program. Similarly, if output is true, outdev will be a
device which when read returns the output of the program. See below for
an example.

The fifth argument, errs, controls where error messages produced by the
program should be sent. Like the input and output arguments, its value
may be either a filename, device, or boolean. In the case of a filename
or device, error messages are written to that file or device. If errs is
true, the result errdev will be a device from which the error output of
the program can be read.

In addition to these four values, errs can also take the value 1,
signifying that error output should be sent to the same place as normal
output. In particular, if output is true and errs is 1, the values of
the results outdev and errdev are the same device, and so you can read
the standard output and error output of the Unix program together. This
is probably the most useful value for the errs parameter.

Supplying false as the value for input, output, or errs indicates that
the standard input and/or standard output and/or error output for the
Unix program will be inherited from the parent Poplog process.

Finally, the wait argument indicates whether run_unix_program should
wait for the Unix program named by name to complete execution. Note that
if either input, output, or errs is true, run_unix_program will not
wait, regardless of the actual value of the wait argument.

2  Result Values

The values of the results indev, outdev, and errdev depend on the values
of the corresponding arguments input, output, and errs. If the argument
value is true, then the result value will be a device for sending input
to, or receiving output from, the running Unix process. Otherwise, the
result value will be false. (See below for an example).

The result status will be an integer representing the Unix program's
exit status if the wait argument is true, and false otherwise. The exit
code 0 normally indicates succesful execution of the program.

Finally, the result pid will be the Unix process identification number
of the Unix process created by this call to run_unix_program. This
result is only useful if the wait argument is false, since otherwise the
Unix process will have terminated by the time the pid result is returned
from the call to run_unix_program.

3  Examples

The following example uses run_unix_program to create a Unix C shell
process from Poplog, send commands to it, and then receive the output.

First we create the csh process:

    vars indev, outdev, errdev, status, pid;

    run_unix_program('csh', ['-f'], true, true, 1, false)
        -> (indev, outdev, errdev, status, pid);

We define two procedures to simplify communication with the running csh

    define send(input, dev);
        lvars input, dev;
        syswrite(dev, 1, input, length(input));

    define rec(dev) -> output;
        lvars dev, output, n;
        lconstant buff = inits(127);
        '' -> output;
        while sys_input_waiting(dev) do
            sysread(dev, 1, buff, 127) -> n;
            output <> substring(1, n, buff) -> output

We can now send the shell command "date" to the csh process, via the
device indev:

    send('date\n', indev);

And we can retrieve the output from the "date" command via the device


Also, any errors produced will be sent to outdev (because the value 1
was given for the errs parameter in the original call to
run_unix_program). For example:

    send('foo\n', indev);

Note that if you call rec to retrieve the output too soon after sending
the command, no output may appear. This is because the command has not
finished executing.

When you have finished using the C shell process it is a good idea to
terminate it using * syskill:

    syskill(pid) =>

4  Related documentation

run_unix_program uses the lower level procedure sys_fork to actually
fork the child Unix process. This is described in REF * SYSUTIL, as is
the procedure sysobey which is much simpler to use if you just want to
execute a single C-shell command.

--- C.unix/help/run_unix_program
--- Copyright University of Sussex 1994. All rights reserved.