OS Converter Exercise (With program parameters)

#Author -- MR
#Date modified -- 2/11/14

fah_to_cel()
{
    echo
    echo -n "Enter fahrenheit: "
    read fah
    echo
    a=32
    b=0.56
    echo -n "$fah FAHRENHEIT in CELSIUS is: "
    echo -n "$fah $a $b" | awk '{ print ($1-$2) * $3 }'
    echo
}

cel_to_fah()
{
    echo
    echo -n "Enter celsius: "
    read cel
    echo
    c=1.8
    d=32
    echo -n "$cel CELSIUS in FAHRENHEIT is: "
    echo -n "$cel $c $d" | awk '{ print ($1*$2) + $3 }'
    echo
}

help()
{
    echo
    echo "PROGRAM DESCRIPTION"
    echo
    echo "This program converts temperature from C to F, vice versa"
    echo
}

menu()
{
while [ "$choice" != "3" ]
do
    echo
    echo "~~~~What do you want to do?~~~~"
    echo "================================"
    echo "1. Convert Fahrenheit to Celsius"
    echo "2. Convert Celsius to Fahrenheit"
    echo "3. Exit"
    echo "================================"
    echo
    echo -n "Enter choice: "
    read choice
    echo

    if [ "$choice" == "1" ]
    then
        fah_to_cel
    elif [ "$choice" == "2" ]
    then
        cel_to_fah
    elif [ "$choice" == "3" ]
    then
        echo "Goodbye"
        echo
    else
        echo "Invalid choice"
    fi
done
}


if [ "$#" -eq 0 ]
then
    menu
else
    while getopts hcfi j
        do
            case $j in
                h) help;;
                c) cel_to_fah;;
                f) fah_to_cel;;
                i) menu;;
            esac
        done
fi