Writing PID File on Linux

Question

I am currently working on a linux daemon that needs to be single instance (i.e restricted to 1 user 1 process). What would be the best way of doing so without having to use getpid() to manually write the pid out to /var/run/ and then lock it using flock()?

Solution 1

Just lock the executable file itself.

Solution 2

Wrap the start-up and shut-down with start-stop-daemon.

Solution 3

I use something like this in a couple of initd scripts I've written. Replace the COMMAND with whatever you need

PIDFILE=/var/run/service.pid
COMMAND="java -jar start.jar"
$COMMAND > /dev/null 2>&1 &
echo $! > $PIDFILE

Edited with @dogane 's suggestion, tested as well.

Solution 4

Just use libunique. It is the simplest way.

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Comments

[Flexo]

That looked really interesting, but it seems like it's heavily tied to GNOME unless I missed something?

[dogbane]

$! will give you the process id. You don't need to grep ps.

[E. Verda]

It depends on GTK, not Gnome. It is difficult to find Linux desktop without GTK. However, if you are going to use your daemon on a headless server, libunique isn't the best choice.

[William Pursell]

This fails, and is a classic race condition. There is no guarantee that only one instance of COMMAND will be running.

[Jeff Busby]

Admittedly this is only a partial answer to his question, it shows how to write the pid file. I use this as a start up script which is used along side a init script which actually checks if the process is already running. I'll post that code tonight when I have time if @Error1f1f hasn't already moved on.

[Matt Joiner]

I think you mean bind a socket.

[Pihhan]

Would not it restrict it to whole system as single instance? Ie. 2 different users cannot have their own instance on single system then?