Question

I’m writing a simple calculator in BASH. Its aim is to loop through the arguments given, check if they’re correct, do the power function and pass the rest to the expr to do the calculation. Everything except multiplication works.
When I do something like

my_script.sh 2 \* 2  

I get syntax error from expr. Checking with bash -x lets me know that

expr 2 '\*' 2  

The * is in apostrophes. I don’t know how to get rid of it so the expr can parse it properly.

if [ $# -le 0 ]
then
    usage
    exit 1
fi

ARGS=("${@}")

while [ $# -gt 0 ]
do
    if [ $OP -eq 0 ]
    then
        if [[ $1 =~ ^[-]{0,1}[0-9]+$ ]]
        then
            ELEMS[$J]=$1
            shift
            let OP=1
            let J=$J+1
        else
            echo $1' is not a number'
            usage
            exit 3
        fi
    else
        if [[ $1 =~ ^[-+/\^\*]{1}$ ]]
        then
            if [[ $1 =~ ^[\^]{1}$ ]]
            then
                if ! [[ $2 =~ ^[0-9]+$ ]]
                then
                    echo 'Bad power exponent'
                    usage
                    exit 3
                fi
                let BASE=${ELEMS[$J-1]}
                let EX=$2
                pow $BASE $EX
                let ELEMS[$J-1]=$RES
                shift 2
            else
                if [[ $1 =~ [\*]{1} ]]
                then
                    ELEMS[$J]=\\*
                else
                ELEMS[$J]=$1
                fi
                let J=$J+1
                shift
                let OP=0
            fi
        else
            echo $1' is not an operator'
            if [[ $1 =~ ^[0-9]+$ ]]
            then
                let TMP=${ELEMS[$J-1]}
                echo "Are you missing an operator beetwen $TMP and $1?"
            fi
            usage
            exit 3
        fi
    fi    
done

if [ $OP -eq 0 ]
then
    echo 'Missing argument after last operator'
    usage
    exit 3
fi
echo "Calculation: ${ARGS[*]}"
echo -n 'Result: '
expr ${ELEMS[*]}

Answer 1

Change ELEMS[$J]=\\* to ELEMS[$J]="*" (or ELEMS[$J]=\*) and use:

expr "${ELEMS[@]}"

The key is to use @ instead of * in the array dereference, which allows you to use double quotes. This is equivalent to expr "2" "*" "2", instead of expr "2 * 2" which you get when using expr "${ELEMS[*]}"