There are two different ways of parsing command line arguments while using getopt(3). There is an utility called getopt (man 1 getopt). This utility is available in all shells. Then in bash, there is another built-in tool for parsing arguments called getopts (it’s a built-in, so it doesn’t have it’s own man-page — try help getopts).

getopt(1)

Here’s an example script that demonstrates the usage of getopt:

#!/bin/bash

# Execute getopt
ARGS=`getopt -o "123:" -l "one,two,three:" \
      -n "getopt.sh" -- "$@"`

#Bad arguments
if [ $? -ne 0 ];
then
  exit 1
fi

# A little magic
eval set -- "$ARGS"

# Now go through all the options
while true;
do
  case "$1" in
    -1|--one)
      echo "Uno"
      shift;;

    -2|--two)
      echo "Dos"
      shift;;

    -3|--three)
      echo "Tres"

      # We need to take the option argument
      if [ -n "$2" ];
      then
        echo "Argument: $2"
      fi
      shift 2;;

    --)
      shift
      break;;
  esac
done

At first, the getopt utility is called with desired parameters (see man getopt for detailed description of all the options). If it returns anything else than 0, something was wrong and we’ll end the script. There is no error message necessary, because the getopt itself will inform user about what went wrong.

After that, there’s a little magic line with eval and set. It’s there to preserve whitespaces inside options arguments. Detailed description of this technique is here.

All options are evaluated and appropriate actions take place in the while loop at the end of the script.

getopts

Here’s an example script that demonstrates the usage of getopts:

#!/bin/bash

while getopts "123:" OPTION
do
  case $OPTION in
    1)  echo "Uno";;
    2)  echo "Dos";;
    3)  echo "Tres: $OPTARG";;

    # Unknown option. No need for an error, getopts informs
    # the user itself.
    \?) exit 1;;
  esac
done

As you can see, the bash built-in version is easier to use, but it can’t handle long options like --option.

Conclusion

Which one you should use? Well, it’s up to you, what you need. If you’re looking for compatibility of your script among more shells than just bash or want to have long options, you’ll need to use the getopt utility. If not, I’d go for the getopts built-in, which I personally consider more user-friendly.

Sources:

• http://aplawrence.com/Unix/getopts.html
• http://bashcurescancer.com/the-60-second-getopts-tutorial.html
• http://rsalveti.wordpress.com/2007/04/03/bash-parsing-arguments-with-getopts/
• http://wiki.bash-hackers.org/howto/getopts_tutorial

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