Perform arithmetic as specified by one or more expressions. expressions consist of numbers, operators, and shell variables (which don’t need a preceding $). Expressions must be quoted if they contain spaces or other special characters. The (( )) form does the quoting for you. For more information and examples, see Arithmetic Expressions in Arithmetic Expressions. See also expr in Chapter 3.
Each of these examples adds 1 to variable i:
i=`expr $i + 1` let i=i+1 let "i = i + 1" (( i = i + 1 )) (( i += 1 )) (( i++ ))