Question

Is it possible to add an option to an existing Bash command?

For example I would like to run a shell script when I pass -foo to a specific command (cp, mkdir, rm…).

Answer 1

You can make an alias for e.g. cp which calls a special script that checks for your special arguments, and in turn call the special script:

$ alias cp="my-command-script cp $*"

And the script can look like

#!/bin/sh

# Get the actual command to be called
command="$1"
shift

# To save the real arguments
arguments=""

# Check for "-foo"
for arg in $*
do
    case $arg in
    -foo)
        # TODO: Call your "foo" script"
        ;;
    *)
        arguments="$arguments $arg"
        ;;
esac
done

# Now call the actual command
$command $arguments