What does `kill -0 $pid` in a shell script do?

by

sending the signal 0 to a given PID just checks if any process with the given PID is running and you have the permission to send a signal to it.

For more information see the following manpages:

kill(1)

$ man 1 kill
...
If sig is 0, then no signal is sent, but error checking is still performed.
...

kill(2)

$ man 2 kill
...
If sig is 0, then no signal is sent, but error checking is still performed; this 
can be used to check for the existence of a process ID or process group ID.
...

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