testmath.sh

#!/bin/bash
#
# This script do the following:
#
# 1. Each operation should have its own function.
# 2. Any of the three methods for bash arithmetic, ( $((..)), let , or expr) may be used.
# 3. The user should give 3 arguments when executing the script:
#    The first should be one of the letters a, s, m, or d to specify which math operation.
#    The second and third arguments should be the numbers that are being operated on.
# 4. The script should detect for bad or missing input values and display the results when done.
#
# You can test this script using: for n in a s m d x ; do bash testmath.sh $n 21 7 ; done

# Functions.

add(){
    answer1=$(($1 + $2))
    let answer2=($1 + $2)
    answer3=`expr $1 + $2`
}
sub(){
    answer1=$(($1 - $2))
    let answer2=($1 - $2)
    answer3=`expr $1 - $2`
}
mult(){
    answer1=$(($1 * $2))
    let answer2=($1 * $2)
    answer3=`expr $1 \* $2`
}
div(){
    answer1=$(($1 / $2))
    let answer2=($1 / $2)
    answer3=`expr $1 / $2`
}

# need 3 arguments, and parse to make sure they are valid types
op=$1; arg1=$2; arg2=$3

[[ $# -lt 3 ]] && \
    echo "Usage: Provide an operation (a,s,m,d) and two numbers" && exit 1

[[ $op != a ]] && [[ $op != s ]] && [[ $op != d ]] && [[ $op != m ]] && \
    echo operator must be a, s, m or d, not $op as supplied

if [[ $op == a ]] ; then add $arg1 $arg2
elif [[ $op == s ]] ; then sub $arg1 $arg2
elif [[ $op == m ]] ; then mult $arg1 $arg2
elif [[ $op == d ]] ; then div $arg1 $arg2
else
    echo $op is not a, s, m or d, aborting; exit 2
fi

# show the answers
echo $arg1 $op $arg2
echo 'Method 1, $((..)),' Answer is $answer1
echo 'Method 2, let,    ' Answer is $answer2
echo 'Method 3, expr,   ' Answer is $answer3