Fahrenheit / Celsius temperature converter - follow-up

Question

This is an updated version of the script Fahrenheit / Celsius temperature converter. I have changed the script to be non-interactive, using positional parameters instead of asking for user input once the script is running. In my testing of the script, everything seems to work as it should. But I am quite new to using positional parameters and I'm wondering if this is the best way of using them or if there might be a more streamlined approach.

help shift says:

shift: shift [n]
    Shift positional parameters.

    Rename the positional parameters $N+1,$N+2 ... to $1,$2 ...  If N is
    not given, it is assumed to be 1.

    Exit Status:
    Returns success unless N is negative or greater than $#.

Which leaves me still uncertain as to what that means exactly or how to implement it correctly. The script seems to work but I'd like to know if I could / should do anything differently.

#!/bin/bash

usage() {
    echo -e "usage: temp [-f][-c]|[number]|[-h]\nExample:\n\ttemp -f 54\n\ttemp -c 24.5\n"
}
re='^-?[0-9]+([.][0-9]+)?$'
if [ "$1" = "" ]; then
    usage
    exit 1
elif [[ ! "$2" =~ $re ]]; then
    usage
    exit 1
elif [[ "$3" != "" ]]; then
    usage
    exit 1
fi
while [ "$1" != "" ]; do
    case $1 in
        -f )    shift
                RES="$(echo "scale=2; (${1}-32)*5/9" | bc -q -l; printf "\b˚C")"
                echo "${1}˚F = "${RES}
                exit
                ;;

        -c )    shift
                RES="$(echo "scale=2; ${1}*9/5+32" | bc -q -l; printf "\b˚F")" 
                echo "${1}˚C = "${RES}
                exit
                ;;

        -h )    usage
                exit
                ;;

        * )     usage
                exit 1
    esac
done

Answer 1

shift shifts the command line argument list by n, or by 1 if n is unspecified. For example, given -c 25, the value of $1 is -c. After you call shift without parameters, it shifts the parameters by 1, so $1 becomes 25. You used it correctly.

Use here-documents

This is hard to read:

echo -e "usage: temp [-f][-c]|[number]|[-h]\nExample:\n\ttemp -f 54\n\ttemp -c 24.5\n"

Using a here-document, it becomes much easier to read:

usage() {
    cat << EOF
usage: temp [-f][-c]|[number]|[-h]
Example:
    temp -f 54
    temp -c 24.5
EOF
}

Simplify conditions

This can be written slightly simpler:

if [ "$1" = "" ]; then

Like this:

if [ ! "$1" ]; then

This can be written much simpler:

elif [[ "$3" != "" ]]; then
    usage
    exit 1
fi
while [ "$1" != "" ]; do

Like this:

elif [[ "$3" ]]; then
    usage
    exit 1
fi
while [ "$1" ]; do

Simplify the bc calls

These statements are unnecessarily complicated:

-f )    shift
        RES="$(echo "scale=2; (${1}-32)*5/9" | bc -q -l; printf "\b˚C")"
        echo "${1}˚F = "${RES}
        exit
        ;;

-c )    shift
        RES="$(echo "scale=2; ${1}*9/5+32" | bc -q -l; printf "\b˚F")" 
        echo "${1}˚C = "${RES}
        exit
        ;;

That is:

• Doing part of the output formatting when you create RES and the other part when you echo it is confusing, it would be better to separate
• Printing a backspace character becomes unnecessary when you separate the calculation and display formatting of RES
• The quoting around $(...) is unnecessary
• Instead of echo-ing the command for bc, a here-string would be better

Like this:

RES=$(bc -q <<< "scale=2; ${1}*9/5+32")
echo "${1}˚C = ${RES}˚F"

Notice how the assignment of RES has only calculation logic, no display logic. All the display logic is on the next line. The result is cleaner and shorter.