php exec() background process issues

Question

I'm trying to process a file in the background with the following command, but it does nothing.

exec("php csv.php $file $user > /dev/null &", $output);

If I remove > /dev/null & then the file processes, but not in the background.

exec("php csv.php $file $user", $output);

Any ideas?

Did you try using passthru or popen? I have never used exec, but I'm sure it works with both of them. — Ben, Jan 28 '13 at 05:33
You shouldn't be expecting an output if you want it to run async. Also i would precede the exec with an "@" — Emery King, Jan 28 '13 at 05:35

Emery King · Accepted Answer · 2013-01-28T05:56:32.877

48

Note:

If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.

http://php.net/manual/en/function.exec.php

so:

exec("php csv.php $file $user > /dev/null &"); // no $output

edited Jan 28 '13 at 05:56

answered Jan 28 '13 at 05:38

Emery King

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He is redirecting the output to /dev/null – Ben Jan 28 '13 at 05:40
5

There shouldn't be a second parameter supplied then. – Emery King Jan 28 '13 at 05:41
So one can't run this in the background with additional parameters? – kylex Jan 28 '13 at 05:46
My bad +1. You may be able to use proc_open with descriptors. – Ben Jan 28 '13 at 05:49
1

Removed the second input and while the script immediately stops, the actual command doesn't run. – kylex Jan 28 '13 at 05:53
try running that exact command from the command line and see what the output is. – Emery King Jan 28 '13 at 06:00
There is no output, but the process that is supposed to happen doesn't. – kylex Jan 28 '13 at 06:04
are you handling the output within the csv.php script now? or are you still handling it on the exec script? because you need to handle it within that csv.php file now. That may be why you are not seeing results. – Emery King Jan 28 '13 at 06:06
It saves lots of time for me, Thanks. – Lakhan Nov 26 '19 at 09:58

score 1 · Answer 2 · answered Oct 11 '15 at 00:29

Have you considered using screen? You can start up a screen session that runs in a detached process. Output will go to the screen session, which you can reattach to in another terminal while it is still running.

exec("screen -d -m -S my_php_session csv.php $file $user", $output);

score 0 · Answer 3 · answered Jul 30 '21 at 04:23

In addition to Emery King's answer, you must use a single exec() call in order to kill a background process. At first, I was under the impression that if the process was put in the background, I could go on my merry way and kill it later as long as I had the process ID, but that is not the case.

For example:

// Runs the background process for 10 seconds
// and then kills it and allows php to continue
exec('sh -c \'echo $$ > pid; exec long_running_process\' > /dev/null 2>&1 & sleep 10 && kill $(cat pid)');

// Runs the background process but does not allow
// php to continue until the background process finishes.
exec('sh -c \'echo $$ > pid; exec long_running_process\' > /dev/null 2>&1 &');
exec(' sleep 10 && kill $(cat pid)'); // <- does not execute until background process is done (at which point pid is already dead)

echo $$ > pid writes the process ID of the long_running_process into a file named pid.
> /dev/null 2>&1 & redirects stdout and stderr to /dev/null and puts long_running_process into the background.
sleep 10 && kill $(cat pid) waits 10 seconds before killing the process who's ID is in the pid file.

php exec() background process issues

3 Answers3

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