8.1. Operators (Revision #4)

assignment

variable assignment

Initializing or changing the value of a variable

=

All-purpose assignment operator, which works for both arithmetic and string assignments.

var=27
category=minerals  # No spaces allowed after the "=".

    Do not confuse the "=" assignment operator with the = test operator.

#   =  as a test operator

if [ "$string1" = "$string2" ]
then
   command
fi

#  if [ "X$string1" = "X$string2" ] is safer,
#+ to prevent an error message should one of the variables be empty.
#  (The prepended "X" characters cancel out.)

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

**

exponentiation

# Bash, version 2.02, introduced the "**" exponentiation operator.

let "z=5**3"    # 5 * 5 * 5
echo "z = $z"   # z = 125

%

modulo, or mod (returns the remainder of an integer division operation)

bash$ expr 5 % 3
2

5/3 = 1, with remainder 2

This operator finds use in, among other things, generating numbers within a specific range (see    Example 9-11    and   Example 9-15  ) and formatting pro gram output (see  Example 27-16  and Example A-6)  . It can even be used to generate prime n  umbers, (see Example A-15). Modulo turns up surprisingly often in numerical recipes.