Question

In my script, how can I distinguish when the asterisk wildcard character was used instead of strongly typed parameters?

This

# myscript *

from this

# myscript p1 p2 p3 ... (where parameters are unknown number)

Answer 1

The shell expands the wildcard. By the time a script is run, the wildcard has been expanded, and there is no way a script can tell whether the arguments were a wildcard or an explicit list.

Which means that your script will need help from something else which is not a script. Specifically, something which is run before command-line processing. That something is an alias. This is your alias

alias myscript='set -f; globstopper /usr/bin/myscript'

What this does is set up an alias called ‘myscript’, so when someone types ‘myscript’, this is what gets run. The alias does two things: firstly, it turns off wildcard expansion with set -f, then it runs a function called globstopper, passing in the path to your script, and the rest of the command-line arguments.

So what’s the globstopper function? This:

globstopper() {
  if [[ "$2" == "*" ]]
    then echo "You cannot use a wildcard"
    return
  fi
  set +f
  "$@";
}

This function does three things. Firstly, it checks to see if the argument to the script is a wildcard (caveat: it only checks the first argument, and it only checks to see if it’s a simple star; extending this to cover more cases is left as an exercise to the reader). Secondly, it switches wildcard expansion back on. Lastly, it runs the original command.

For this to work, you do need to be able to set up the alias and the shell function in the user’s shell, and require your users to use the alias, not the script. But if you can do that, it ought to work.

I should add that i am leaning heavily on the resplendent Simon Tatham’s essay ‘Magic Aliases: A Layering Loophole in the Bourne Shell‘ here.