Hi all,
I'm learning bash scripting and need clarification on the double parentheses in bash. I've read that double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash. Is that all its used for?

Can someone explain in laymans terms.

No--a layman will not relate to the differences between BASH and the traditional Bourne shell......

Really, really good idea to have this: http://tldp.org/LDP/abs/abs-guide.pdf on you computer, and use it for searches. In this case, go to section 8.3.

From that section:

arithmetic expressions

Short form, it evaluates (integer-based) arithmetic expressions inside them and returns an exit code based on the result. This makes it possible to use it in conditional constructs like for, while, and if. The c-style-for loop is one common use of it. You should generally use them any time you want to test for a numerical condition, or apply an expression without having to deal with printed output (e.g. incrementing variables).

The related, posix-compliant structure is $((..)). It also evaluates arithmetic, except that it only prints the resulting values to stdout, and doesn't produce an exit code.

Also see here for the other commonly-used test constructions. In general you should use [[..]] for string and file conditions.

http://wiki.bash-hackers.org/syntax/...nal_expression
http://wiki.bash-hackers.org/commands/classictest