Returning STDERR value with Shell_Exec 1

[Guggly]

Hi! How can I get the value of STDERR for a program that I’ve launched using Shell_Exec? I can output STDERR to a file using >FILENAME but I’d like to be able to have the STDERR value sent back directly to a variable. Thanks!

 

[sleipnir214]

If your version of PHP is at least 4.3.0, you might try proc_open().

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[Guggly]

Thanks, but how can I get it to return the value of stderr without writing it to a file? If I change the script so that $descriptorspec is as follows:

$descriptorspec = array(
   0 => array("pipe", "r"),
   1 => array("pipe", "w"),
   2 => array("pipe", "a")
);

When I call "echo($pipes[2]);" I just get a resource ID as the return. What am I doing wrong here?

Also, do I need to explicitly close pipes 0 and 1 if I never read or write to them? All I'm interested is in stderr. Thanks!

 

[sleipnir214]

That's because $pipes[0], $pipes[1], and $pipes[2] are all resource handles that you treat as file handles.

Look at the example code for what it does with $pipes[1].

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[Guggly]

I've tried using the same syntax for $pipes[2] but nothing gets returned. When I use the original "file" setting, sderr value is being written to the file. Am I missing something?

while (!feof($pipes[2])) {
       echo fgets($pipes[2], 1024);
   }

 

[sleipnir214]

Are you sure that the app you're invoking using proc_open() is sending anything to stderr?

Here's my test PHP script, based on the code from the online manual page:

<?php
$descriptorspec = array
(
   1 => array("pipe", "w"),
   2 => array("pipe", "w")
);

$process = proc_open("./foo", $descriptorspec, $pipes);

if (is_resource($process))
{
   while (!feof($pipes[1]))
   {
       $a .= fgets($pipes[1], 1024);
   }
   fclose($pipes[1]);
   
   while (!feof($pipes[2]))
   {
       $b .= fgets($pipes[2], 1024);
   }
   fclose($pipes[2]);
   
   $return_value = proc_close($process);

   print "command returned $return_value\n";
   print 'stdout from program: ' . $a ."\n";
   print 'stderr from program: ' . $b ."\n";
}
?>

The app "foo" that script invokes is a c-language program, the source code of which consists of:

#include <stdio.h>

int main()
{
	fprintf (stdout, "To STDOUT:  test test test\n");
	fprintf (stderr, "To STDERR:  tset tset tset\n");

	return 19;
}

When I invoke my PHP script, the output I get is:

command returned 19
stdout from program: To STDOUT:  test test test

stderr from program: To STDERR:  tset tset tset

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[Guggly]

Okay, I found the problem, the stderr pipe needs to use "w" (like you had it) in it's array instead of the original "a":

I had it like this:
$descriptorspec = array
(
2 => array("pipe", "a")
);

It should be like this:
$descriptorspec = array
(
2 => array("pipe", "w")
);

If you're sending stderr to a file it doesn't seem to make a difference what that second value is.

Thanks for your help!

 

[sleipnir214]

If you're outputting STDERR to a file, it will matter whether there's an "a" or "w". But only on successive invocations of the script.

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TANSTAAFL!!

 

[Guggly]

That's good to know. Thanks!