Cron job to prevent simultaneous script

Shell Programming and Scripting
1

I'm using a shared server on Hostgator (Linux CentOS). I'm trying to set a cron job using the Control Panel that will check if its already running before starting a new one.

I've tried the following...

* * * * * [ -z "$(ps ax|grep /pathtoscript/parse_script.php|grep -v grep)" ] && [ php /pathtoscript/parse_script.php ]

but I get this error emailed to me...

/bin/sh: line 0: [: php: unary operator expected

Any ideas how to make this work?

2

Hi.

Why are you testing this (as it looks like you're trying to):

&& [ php /pathtoscript/parse_script.php ]

and not just running it?

&& php /pathtoscript/parse_script.php

You may also need to provide the full path to the php binary.

3

Traditional way is to have the script create a file somewhere containing the process' PID, and check if the file's present and the PID's valid to see if it's still running. The start-stop-daemon utility can help simplify this.

Your way may work if you're only ever running one instance at a time, ever. You've got some extra [ ] there. Try this:

[ -z "$(ps ax|grep /pathtoscript/parse_script.php|grep -v grep)" ] && php /pathtoscript/parse_script.php
4

I removed the extra braces. I set it to run every 2 minutes like this.

*/2 * * * * [ -z "$(ps ax|grep /pathtoscript/parse_script.php|grep -v grep)" ] && php /pathtoscript/parse_script.php

Then after a couple minutes, there are 2 jobs running. Here's what I saw in the Control Panel...

PID    Process

20010    /bin/sh -c [ -z "$(ps ax|grep /pathtoscript/parse_script.php|grep -v grep)" ] && php /pathtoscript/parse_script.php    

22072    /bin/sh -c [ -z "$(ps ax|grep /pathtoscript/parse_script.php|grep -v grep)" ] && php /pathtoscript/parse_script.php

I checked my email and saw that the command failed...

/bin/sh: fork: Resource temporarily unavailable
/bin/sh: fork: Resource temporarily unavailable
grep: write error: Broken pipe

Most of the time this cron command works, but every so often, it fails. How could I change the command so that when the command gets an error, it simply does NOT execute the php script?

5

Wouldn't it be simpler, and more robust, to put the check (if the script is already running) in the script itself?

1 Like
6

I tried it in my script but wasn't getting results from exec statement, but now I have it working...

exec("ps  ax", $results)

Thanks for pointing me in the right direction.