How can I check a file exists and execute a command if not?

I have a daemon I have written using Python. When it is running, it has a PID file located at /tmp/filename.pid. If the daemon isn't running then PID file doesn't exist.

On Linux, how can I check to ensure that the PID file exists and if not, execute a command to restart it?

The command would be

python daemon.py restart

which has to be executed from a specific directory.

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[ -f /tmp/filename.pid ] || python daemon.py restart

-f checks if the given path exists and is a regular file (just -e checks if the path exists)

the [] perform the test and returns 0 on success, 1 otherwise

the || is a C-like or , so if the command on the left fails, execute the command on the right.

So the final statement says, if /tmp/filename.pid does NOT exist then start the daemon.

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If it is bash scripting you are wondering about, something like this would work:

if [ ! -f "$FILENAME" ]; then
   python daemon.py restart
fi

A better option may be to look into lockfile

---

The other answers are fine for detecting the existence of the file. However for a complete solution you probably should check that the PID in the pidfile is still running, and that it's your program.

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