8.1. Operators (Revision #5)

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 program 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.

Example 8-1. Greatest common divisor

#!/bin/bash
# gcd.sh: greatest common divisor
#         Uses Euclid's algorithm

#  The "greatest common divisor" (gcd) of two integers
#+ is the largest integer that will divide both, leaving no remainder.

#  Euclid's algorithm uses successive division.
#    In each pass,
#+      dividend <---  divisor
#+      divisor  <---  remainder
#+   until remainder = 0.
#    The gcd = dividend, on the final pass.
#
#  For an excellent discussion of Euclid's algorithm, see
#+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm.


# ------------------------------------------------------
# Argument check
ARGS=2
E_BADARGS=85

if [ $# -ne "$ARGS" ]
then
  echo "Usage: `basename $0` first-number second-number"
  exit $E_BADARGS
fi
# ------------------------------------------------------


gcd ()
{

  dividend=$1             #  Arbitrary assignment.
  divisor=$2              #! It doesn't matter which of the two is larger.
                          #  Why not?

  remainder=1             #  If an uninitialized variable is used inside
                          #+ test brackets, an error message results.

  until [ "$remainder" -eq 0 ]
  do    #  ^^^^^^^^^^  Must be previously initialized!
    let "remainder = $dividend % $divisor"
    dividend=$divisor     # Now repeat with 2 smallest numbers.
    divisor=$remainder
  done                    # Euclid's algorithm

}                         # Last $dividend is the gcd.


gcd $1 $2

echo; echo "GCD of $1 and $2 = $dividend"; echo


# Exercises :
# ---------
# 1) Check command-line arguments to make sure they are integers,
#+   and exit the script with an appropriate error message if not.
# 2) Rewrite the gcd () function to use local variables.

exit 0

+=

plus-equal (increment variable by a constant) ¹

let "var += 5" results in var being incremented by 5.

-=

minus-equal (decrement variable by a constant)

*=

times-equal (multiply variable by a constant)

let "var *= 4" results in var being multiplied by 4.

  /=  

  slash-equal  (divide variable by a constant)

 %=  

mod-equal (remainder of dividing variable by a constant)

Arithmetic operators often occur in an expr or let expression.