A post by Ronald reminded me I wanted to write about watch. The Linux watch command is a fab tool, it allows you to run a program at a set interval, such as every 5 seconds. Handy for keeping an eye on stuff!
Update 2009-06-30: yesterday I wrote “… watch does not re-load and run the program, but goes in and resets the program counter.” (and was un-recommending it for use on production systems for that reason). I was blindly quoting an otherwise trusted source, and it was wholly wrong. Numerous people had a peek at the source code (all the way back to 1997) and found the popen(). So, I stand corrected and my apologies for not doing extra homework in the first place before posting! (I’m leaving the comments as they are)