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PID files have a "fun" gotcha in daemons started as part of system boot — two instances of the same daemon may well get the same PID!

This does not prevent using a PID file, after all, you know that this instance of the program has not written the PID file yet, so if you read it and find that it matches $$, you know the PID has been recycled across reboots.

Perhaps the best option would be to place the PID file on a RAM filesystem that starts out empty every time the system boots. This way, there is no possibility of a stale PID file from the previous session and you avoid needless flash writes as a bonus.

Even if the daemon is run from cron, this issue could bite our questioner — the daemon reboots the system if the time is not set within some grace period, so a network failure could cause the system to start, fail to set the clock, reboot, start the daemon with the same PID again, fail to set the clock, ...

In reply to Re^2: Preventing multiple instances by jcb
in thread Preventing multiple instances by Bod

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