Test dir var (eval shell)

 Test if the given variable contains a valid directory. If not, exit. (This uses the eval function of the shell)
testdirvar () {
    tmp=$1
    if [ -d "${!tmp}" ] ; then
        echo "${tmp}=${!tmp} (variable ${tmp}: directory exists)"
    else
        echo "!!! ${tmp}=${!tmp} is not a directory (variable ${tmp}) !!!"
        exit
    fi
}