8.1. Operators

#!/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=65

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


gcd ()
{

                                 #  Arbitrary assignment.
  dividend=$1                    #  It does not matter
  divisor=$2                     #+ which of the two is larger.
                                 #  Why?

  remainder=1                    #  If uninitialized variable used in loop,
                                 #+ it results in an error message
				 #+ on first pass through loop.

  until [ "$remainder" -eq 0 ]
  do
    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


# Exercise :
# --------
#  Check command-line arguments to make sure they are integers,
#+ and exit the script with an appropriate error message if not.

exit 0

#!/bin/bash
# Counting to 6 in 5 different ways.

n=1; echo -n "$n "

let "n = $n + 1"   # let "n = n + 1"   also works.
echo -n "$n "

: $((n = $n + 1))
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "

n=$(($n + 1))
echo -n "$n "

: $[ n = $n + 1 ]
#  ":" necessary because otherwise Bash attempts
#+ to interpret "$[ n = $n + 1 ]" as a command.
# Works even if "n" was initialized as a string.
echo -n "$n "

n=$[ $n + 1 ]
#  Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
echo -n "$n "; echo

# Thanks, Stephane Chazelas.

exit 0

#!/bin/bash

a=24
b=47

if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]
then
  echo "Test #1 succeeds."
else
  echo "Test #1 fails."
fi

# ERROR:   if [ "$a" -eq 24 && "$b" -eq 47 ]
#          attempts to execute  ' [ "$a" -eq 24 '
#          and fails to finding matching ']'.
#
#    if [[ $a -eq 24 && $b -eq 24 ]]   works
#    (The "&&" has a different meaning in line 17 than in line 6.)
#    Thanks, Stephane Chazelas.


if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
then
  echo "Test #2 succeeds."
else
  echo "Test #2 fails."
fi


#  The -a and -o options provide
#+ an alternative compound condition test.
#  Thanks to Patrick Callahan for pointing this out.


if [ "$a" -eq 24 -a "$b" -eq 47 ]
then
  echo "Test #3 succeeds."
else
  echo "Test #3 fails."
fi


if [ "$a" -eq 98 -o "$b" -eq 47 ]
then
  echo "Test #4 succeeds."
else
  echo "Test #4 fails."
fi


a=rhino
b=crocodile
if [ "$a" = rhino ] && [ "$b" = crocodile ]
then
  echo "Test #5 succeeds."
else
  echo "Test #5 fails."
fi

exit 0