Arithmetic evaluation in the Korn shell or POSIX shell

Edit online

The Korn shell or POSIX shell regular built-in let command enables you to perform integer arithmetic.

Constants are of the form [Base]Number. The Base parameter is a decimal number between 2 and 36 inclusive, representing the arithmetic base. The Number parameter is a number in that base. If you omit the Base parameter, the shell uses a base of 10.

Arithmetic expressions use the same syntax, precedence, and associativity of expression as the C programming language. All of the integral operators, other than double plus (++), double hyphen (—), question mark-colon (?: ), and comma (,), are supported. The following table lists valid Korn shell or POSIX shell operators in decreasing order of precedence:

Operator Definition
- Unary minus
! Logical negation
~ Bitwise negation
* Multiplication
/ Division
% Remainder
+ Addition
- Subtraction
<<, >> Left shift, right shift
<=,>=, <>, ==, != Comparison
& Bitwise AND
^ Bitwise exclusive OR
| Bitwise OR
&& Logical AND
|| Logical OR
= *=, /=, &= +=, -=, <<=, > >=, &=, ^=, |= Assignment
Many arithmetic operators, such as *, &, <, and >, have special meaning to the Korn shell or POSIX shell. These characters must be quoted. For example, to multiply the current value of y by 5 and reassign the new value to y, use the expression: 
let "y = y * 5"

Enclosing the expression in quotation marks removes the special meaning of the * character.

You can group operations inside let command expressions to force grouping. For example, in the expression: 
let "z = q * (z - 10)"
the command multiplies q by the reduced value of z.
The Korn shell or POSIX shell includes an alternative form of the let command if only a single expression is to be evaluated. The shell treats commands enclosed in (( )) as quoted expressions. Therefore, the expression: 
((x = x / 3))
is equivalent to: 
let "x = x / 3"

Named parameters are referenced by name within an arithmetic expression without using the parameter substitution syntax. When a named parameter is referenced, its value is evaluated as an arithmetic expression.

Specify an internal integer representation of a named parameter with the -i flag of the typeset special built-in command. Using the -i flag, arithmetic evaluation is performed on the value of each assignment to a named parameter. If you do not specify an arithmetic base, the first assignment to the parameter determines the arithmetic base. This base is used when parameter substitution occurs.