The *expr*
command does arithmetic.
It's not just for incrementing a variable in a Bourne shell loop.
It's handy for quick integer math on the command line.
For instance, if *wc*
tells me a file has 2545 lines and I want to
know
how many 66-line pages
that is:

$ **expr 2545 / 66**

38

It's 38 pages - actually, 39, because *expr*
rounds down.

Article
45.28
has a complete list of *expr*
math operators.