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.

Answer 1

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

Answer 2

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);

Answer 3

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.