Waits for one of a number of files to be changed, then exits. Simple as that. Mac OS X only (probably works for pretty old versions, even).
monitor <file> …
To try it out, this will do:
$ touch monitor-this.txt
$ (monitor monitor-this.txt; echo "Monitor exited.") & # Run in background.
$ # Wait a while
$ touch monitor-this.txt
Monitor exited.
$
It's useful particularly as the condition in a while loop around a frequent command. See below.
Give depend a command to run, and touch away; the command will be repeated every time a file changes.
I recommend depend 'rake spec' spec/* for ad-hoc spec creation.
I hope to expand this to include directory monitoring. I may have to break into Obj-C for it. Some day it will happen, lest a kind soul lend a helping fork before then.