Starting a daemon from PHP

Question

For a website, I need to be able to start and stop a daemon process. What I am currently doing is

exec("sudo /etc/init.d/daemonToStart start");

The daemon process is started, but Apache/PHP hangs. Doing a ps aux revealed that sudo itself changed into a zombie process, effectively killing all further progress. Is this normal behavior when trying to start a daeomon from PHP?

And yes, Apache has the right to execute the /etc/init.d/daemonToStart command. I altered the /etc/sudoers file to allow it to do so. No, I have not allowed Apache to be able to execute any kind of command, just a limited few to allow the website to work.

Anyway, going back to my question, is there a way to allow PHP to start daemons in a way that no zombie process is created? I ask this because when I do the reverse, stopping an already started daemon, works just fine.

True, I forgot to add that to the question. In my code, I do have it. But, I'll edit the question for completeness. — ThaMe90, Dec 05 '11 at 15:31

DaveRandom · Accepted Answer · 2011-12-05T14:57:46.643

13

Try appending > /dev/null 2>&1 & to the command.

So this:

exec("sudo /etc/init.d/daemonToStart > /dev/null 2>&1 &");

Just in case you want to know what it does/why:

> /dev/null - redirect STDOUT to /dev/null (blackhole it, in other words)
2>&1 - redirect STDERR to STDOUT (blackhole it as well)
& detach process and run in the background

edited Dec 05 '11 at 14:57

answered Dec 05 '11 at 14:52

DaveRandom

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Thanks for the further explanation, but because the output is blackholed, any code that should have been executed after the daemon has started, is onmitted aswell. I have to check if the process has started, but that code is never fired... – ThaMe90 Dec 05 '11 at 15:04
1

When you start a process using this method, it should output the PID of the process that was created. You can take this, and then (for example) execute `ps | grep $pidOfProcessYouJustStarted` to make sure it is running. – DaveRandom Dec 05 '11 at 15:08
Actually, you could just execute `ps | grep /etc/init.d/daemonToStart` to do the same thing... – DaveRandom Dec 05 '11 at 15:10
I know, I am already doing that. ;) It seemed that that code was never executed though, as my browser didn't present me with an alert with the result value I presented. But my logs show that the code is actually being run. So thanks again. – ThaMe90 Dec 05 '11 at 15:11

score 2 · Answer 2 · answered Jun 12 '14 at 12:34

I had the same problem.

I agree with DaveRandom, you have to suppress every output (stdout and stderr). But no need to launch in another process with the ending '&': the exec() function can't check the return code anymore, and returns ok even if there is an error...

And I prefer to store outputs in a temporary file, instead of 'blackhole'it. Working solution:

$temp = tempnam(sys_get_temp_dir(), 'php');
exec('sudo /etc/init.d/daemonToStart >'.$temp.' 2>&1');

Just read file content after, and delete temporary file:

$output = explode("\n", file_get_contents($temp));
@unlink($temp);

score 0 · Answer 3 · answered Dec 05 '11 at 14:58

I have never tried starting a daemon from PHP, but I have tried running other shell commands, with much trouble. Here are a few things I have tried, in the past:

As per DaveRandom's answer, append /dev/null 2>&1 & to the end of your command. This will redirect errors to standard output. You can then use this output to debug.
Make sure your webserver's user's PATH contains all referenced binaries inside your daemon script. You can do this by calling exec('echo $PATH; whoami;). This will tell you the user PHP is running under, and it's current PATH variable.

Starting a daemon from PHP

3 Answers3

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