Work the Shell - Parsing Command-Line Options with getopt

Make your shell scripts more flexible and more command-line-friendly by accepting command-line arguments/flags.

The double semicolon is an oddity, but that's how you indicate the end of an individual case match, hence the notation shown above.

Grabbing the argument for the -k flag is easy too, because getopt has made sure that it's a separate argument, and since we're using shift as we go along to move things around, $2 will always be the argument itself.

Finally, also notice that as a stylistic approach, I have the double semicolon with a leading space. That's just so when I eyeball the script, I quickly can recognize if there are any cases that are missing the double semicolon.

The only piece missing is some error handling, because right now, if a bad flag is encountered, here's what happens:

$ scale -ax 100 *png
getopt: illegal option -- x

Nice, but the script doesn't catch the error condition or stop running—not so good.

To fix it, immediately after the call to getopt, simply test the return code:

if [ $? != 0 ] ; then ...

In the conditional, you probably would put a usage statement and an exit command. For my script, I actually also test to ensure that there are a minimum of two arguments on the command line as well, because the script is never valid without them:

if [ $? != 0 -o $# -lt 2 ] ; then
  echo ""
  echo "Usage: scale {args} factor [file or files]"
  echo ""

  ... stuff skipped ...

  exit 0
fi

At this point in our shell script writing journey, I certainly hope you can read that rather cryptic conditional statement and understand what it does.

Ultimately, it's a bit of work to parse command-line flags the right way, but it makes for a far more flexible and robust shell script.

Dave Taylor has been involved with UNIX since he first logged in to the on-line network in 1980. That means that, yes, he's coming up to the 30-year mark now. You can find him just about everywhere on-line, but start here: www.DaveTaylorOnline.com.

______________________

Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at www.DaveTaylorOnline.com.

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