let

let expressions
     or 
((expressions))

Korn shell only. Perform arithmetic as specified by one or more integer 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" earlier in this section. See also expr in Section 2.

Examples

Each example below adds 1 to variable i.

i=`expr $i + 1`
let i=i+1
let "i = i + 1"
(( i = i + 1 ))