Let Bash do the math: Doing calculations using that bash

Sunday, 26th Aug 2007 at 18:52 (linux)
Tags: arithmetics, bash, calc, calculator, cml, command line, linux, math, scripting

Have you ever wanted to “just do some math” without much fuss? I noticed a while ago (but haven’t really used it since) that bash (born again shell) has, amongst many other evaluations, an arithmetic one. Using the syntax $((expression)) you can evaluate the expression to arithmetically. Operations allowed are quite a few. From man bash:

ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circumstances (see the let and declare builtin commands and Arithmetic Expansion). Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, asso ciativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

       id++ id--   variable post-increment and post-decrement
       ++id --id   variable pre-increment and pre-decrement
       - +             unary minus and plus
       ! ~             logical and bitwise negation
       **              exponentiation
       * / %         multiplication, division, remainder
       + -             addition, subtraction
       <>       left and right bitwise shifts
       =
                        comparison
       == !=       equality and inequality
       &               bitwise AND
       ^               bitwise exclusive OR
       |                bitwise OR
       &&             logical AND
       ||              logical OR
       expr?expr:expr
                        conditional operator
       = *= /= %= += -= <>= &= ^= |=
                        assignment
       expr1 , expr2
                        comma

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is ref erenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Other wise, numbers take the form [base#]n, where base is a decimal number between 2 and 64 representing the arith metic base, and n is a number in that base. If base# is omitted, then base 10 is used. The digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.