= Waiting On Another Process =

== Problem ==

I want to run a process after another process completes.

== Solution ==

Calling {{{kill -0 PID}}} will exit with either 0 for success (the process exists '''and''' this user may send a signal to it) or 1 otherwise. On STDERR an error string will indicate the cause.

'''Note:''' this does not work if you do not have permission to send a signal to the process (ie it's owned by another user). To do this for a process owned by a different user root access is needed.

So use a simple loop - I'm waiting for a process with process-id {{{PID}}} and then want to run {{{/path/to/my/command}}}:
{{{
notroot@host:~$ while kill -0 PID ; do echo -n . ; sleep 5 ; done ; /path/to/my/command
}}}

== Examples ==

=== waiting for my process ===
{{{
notroot@host:~$ sleep 30 &
[1] 10567
notroot@host:~$ while kill -0 10567 ; do echo -n . ; sleep 2; done; echo "done at `date`"
...........[1]+  Done                    sleep 30
bash: kill: (10567) - No such process
done at Wed Feb 19 15:36:10 EST 2014
}}}

=== waiting for a process owned by another user ===
This requires sudo access (or to be running the loop as root) - the change in the example is to run sleep as another user and to call kill with sudo.

{{{
otheruser@host:~$ sleep 30 &
[1] 11483

notroot@host:~$ while sudo kill -0 11483 ; do echo -n . ; sleep 2; done; echo "done at `date`"
.........kill: No such process
done at Wed Feb 19 15:43:30 EST 2014
}}}

=== kill -0 examples ===
A process which exists, but this user does not have access to it:
{{{
notroot@host:~$ kill -0 1
bash: kill: (1) - Operation not permitted
notroot@host:~$ echo $?
1
}}}

A process which does not exist:
{{{
notroot@host:~$ kill -0 234123
bash: kill: (234123) - No such process
notroot@host:~$ echo $?
1
}}}

An invalid argument:
{{{
notroot@host:~$ kill -0 sdfasd
bash: kill: sdfasd: arguments must be process or job IDs
notroot@host:~$ echo $?
1
}}}

But here, we have access (now root) and the process exists:
{{{
notroot@host:~$ /usr/bin/sudo kill -0 1
notroot@host:~$ echo $?
0
}}}

And here we have access to the process because it's owned by the user:
{{{
notroot@host:~$ sleep 10 &
[1] 8966
notroot@host:~$ kill -0 8966
notroot@host:~$ echo $?
0

notroot@host:~$ ... <wait for the sleep to go away> ...
[1]+  Done                    sleep 10

notroot@host:~$ kill -0 8966
bash: kill: (8966) - No such process
notroot@host:~$ echo $?
1
}}}