Question

I am new to bash-scripting & trying to understand how things work. It’s all a bit strange..

The following can be put into a script or entered into the shell:

declare -a A=("foo" "bar")
B=1
[ ${A[B]} == ${A[$B]} ] && echo "wTF" || echo ";)"

This gives me “wTF” on my debian squeeze & also on cygwin 1.7.11-1

So. Why does ${A[B]} work?

Answer 1

From the Bash Reference Manual, §6.7 “Arrays”:

Indexed arrays are referenced using integers (including arithmetic expressions […]) and are zero-based; […] ¶ The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero.

So in effect, ${A[B]} means ${A[$((B))]}. This is convenient when you want something like ${A[B-1]}.

Arithmetic expressions are explained in §6.5 “Shell Arithmetic”, which says in part:

Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax.

So, $((B)) means $(($B)) (except that the former is smarter about some things, e.g. using zero instead of blank as a default for uninitialized variables).