How to add a progress bar to a shell script?

Question

When scripting in bash or any other shell in *NIX, while running a command that will take more than a few seconds, a progress bar is needed.

For example, copying a b

Answer 1

use the linux command pv:

http://linux.die.net/man/1/pv

it doesn't know the size if it's in the middle of the stream, but it gives a speed and total and from there you can figure out how long it should take and get feedback so you know it hasn't hung.