There is a tendency in programming to consider any spiffy new generalfeature, such as a good infinite undo system, as the way to deal withmany of your program's problems. This is often a mistake, and undo makesa good illustration of why.

(It's easy to see how this happens; you have this powerful system thatyou've invested a lot of effort into, so it's natural to apply it on asmuch as possible. And programmers really like general solutions, because

If you are for some reason absolutely forced to have a system that willsend email to user-entered addresses (given the principles of modernemail this is not a good idea ,but let's imagine that your management forces you), one of the thingsthat you should absolutely do is make your system so that it won't sendmail to certain user names. Spamming people is one thing; spamming abuse , postmaster , noc , security , and any number of otheradministrative user names is just carelessness.

(This is the story as I heard it and learned it in the past. It may ormay not be actually accurate, but then a lot of Unix history is passedaround as folklore, and in some ways the folklore is more important andmore influential than the truth.)

In the beginning, in V7, the Bourne shell didn't actually havecomments. Instead, all it had was : , which is not quite a comment (you can see this for yourself in the V7 shmanpage

I don't know how people learn the Bourne shell these days, but when Ilearned it, I first encountered ' : ' as a vaguely peculiar second wayof writing comments, one used by some old shell scripts that hung aroundour systems. This is true as far as it goes, which is not necessarilyvery far.

The difference between # and : is that # starts a real commentthat runs to the end of the line, while : is a real command thatjust does nothing (this is