PHP exec in background using & is not working

Question

I am using this code on Ubuntu 13.04,

$cmd = "sleep 20 &> /dev/null &";
exec($cmd, $output);

Although it actually sits there for 20 seconds and waits :/ usually it works fine when using & to send a process to the background, but on this machine php just won't do it :/
What could be causing this??

Answer 1

Try

<?PHP
$cmd = '/bin/sleep';
$args = array('20');

$pid=pcntl_fork();
if($pid==0)
{
  posix_setsid();
  pcntl_exec($cmd,$args,$_ENV);
  // child becomes the standalone detached process
}

echo "DONE\n";

I tested it for it works. Here you first fork the php process and then exceute your task.

Or if the pcntl module is not availabil use:

<?PHP

$cmd = "sleep 20 &> /dev/null &";
exec('/bin/bash -c "' . addslashes($cmd) . '"');

Answer 2

The REASON this doesn't work is that exec() executes the string you're passing into it. Since & is interpreted by the shell as "execute in the background", but you don't execute a shell in your exec call, the & is just passed along with 20 to the /bin/sleep executable - which probably just ignores that.

The same applies to the redirection of output, since that is also parsed by the shell, not in exec.

So, you either need to find a way to fork your process (as described above), or a way to run the subprocess as a shell.