Thanks for the advice but I still couldn't get it to quite do what I want it to do. I have added in a loop after some advice from a computer scientist. The updated version of the problematic section of my code:

Code:

list="inches hands feet yards furlongs miles leagues au"
for item in $list
do      
        if [[ "$2" = "$item" ]]
        then
                if [[ "$4" != "$item" ]]
                then
                        echo "unrecognized unit: $4"
                fi
        else
                echo "unrecognized unit: $2"
        fi
done

This of course produces 8 error messages for any input where either $2 or $4 is not part of $list. So to put it in english, this is what I want to do: "go through the whole list and if ANY match with $2, check $4, if none match, produce 1 error message (unrecognized unit: $4). Also if nothing in the list matches with $2, produce 1 error message (unrecognized unit: $4)". What do I need to do/change to get my code to do this.