This is the sort of thing that's second nature to a command-line geek, but I thought it was cool enough (in its little way) to share.
The problem: I have a process that runs on a remote server for quite a while. I wish to be alerted when it completes. As long as it's running, it creates a lockfile in /var/lock. So when the lockfile goes away, I want to be alerted.
Part 1: In my .bashrc file, I've aliased the command "alert" to play a sound: (This works only on Linux. I'm sure you could do something similar on OS X.)
alias alert='aplay /usr/share/sounds/k3b_success1.wav'
But of course, I can only play that on my local machine -- does me no good to sound an alarm on a computer in the server room, even if it had speakers.
Part 2: I can create a "while" one-liner that waits until the file is deleted, then executes the next line:
while [ -e ~/lockfile ]; do sleep 10; done; alert
However, this can only test against files on my local system. This brings me to...
Part 3: You can use ssh to run a command on a remote system, instead of just logging into the remote system:
ssh email@example.com "while [ -e /var/lock/program.lock ]; do sleep 10; done" ; alert
It runs the while loop on the remote system, not giving back control to the local system until the while loop exits (which happens when the lock file no longer exists). Once the while loop exits, ssh exits, and we run the "alert" command, which plays a sound alerting me to the change.