Inside which type of parenthesis (), (()), {}, [[ ]], $( ), $(( )) should we use the dollar sign before the variable to expand it?

CodePudding user response：

The arithmetic expansion is the one that doesn't need dollar sings, which means both ((...)) and $((...)). But you don't need the dollar sign in other arithmetic contexts, either, e.g. in array indices.

arr=(a b c)
x=1
echo "${arr[x]}"  # b

See "Arithmetic Evaluation" in man bash.