Testing whether or not a flag has an argument

Question

My typical setup for parsing command-line options is:

CONF=""
INPUT=""
while getopts ":c:i:" FLAG; do
   case $FLAG in
      i) INPUT=$OPTARG;;
      c) CONF=$OPTARG;;
      \?) echo -e "\nInvalid option: -$OPTARG"
         usage;;
      :) echo -e "Option -$OPTARG requires an argument."
         usage;;
   esac
done
if [ "$#" -eq 0 ]; then
    usage
fi

I'm looking for a way to catch when a valid flag is provided but no argument is - for example:

./Script.sh -c -i 

Returns usage. I was under the impression that this line:

:) echo -e "Option -$OPTARG requires an argument."

Handled this however when running the script as above using flags without arguments, the usage function is not firing nor is the echo.

What am I doing wrong?

Answer 1

If you invoke your script with either ./script.sh -c -i -c or ./script.sh -c, both will show the error message Option -c requires an argument.

However, when invoking ./script.sh -c -i, you are passing value "-i" for the -c argument, so that at the end of arguments parsing, you end up with CONF=-c and INPUT not set.