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What are pid and lock files for?

I often see that programs specify pid and lock files. And I'm not quite sure what they do.

For example, when compiling nginx:

--pid-path=/var/run/nginx.pid \
--lock-path=/var/lock/nginx.lock \

Can somebody shed some light on this one?

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  • 1
    +1 Explaining the use of having both the lock file and the pid file. Commented Oct 6, 2015 at 2:00
  • 2
    @Caleb - Please explain why both a PID file and a lock file would be used. It seems a PID file would be sufficient. If the PID file exists, the PID could be checked to see if the process is running, just takes less steps than check for a lockfile, checking for a PID file, and then verifying the existence of the process. Commented Jan 25, 2017 at 16:57
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    @MVaughan To avoid race conditions if nothing else. Some apps have uses for times when the still need the PID but can relinquish the lock. But at a more fundamental level if you overload one file for both operations you open the door to failures such as a crash leaving an inconsistent state on the system. Commented Jan 25, 2017 at 21:06