Parsing arguments in shell script

I write a good amount of shell scripts, but I tend to use arguments pretty
sparingly. Because of this, my implementation to handle said arguments tends to be pretty weak. Some good examples are the arguments needing to be passed in
a specific order and a lack of using long or shorthand arguments.

While this works out well enough, especially if you only have a single argument,
things can always be better.

With a new script needing to be written last week, I set out to learn how to
improve parsing of command-line arguments in shell scripts. In doing so, I ended
up learning some new stuff, like the build-in shift to help iterate through
the argument vector.

I’ve also embraced starting my script by writing up a help section that tells
you a bit about the script as well as demonstrates the usage.

Let’s say we wanted to write a script that accepted a required argument to
specify the environment we’re using, and an optional argument that tells the
script to perform an action, like deleting data, we can start by writing a help
function like this:

function usage {
  echo ""
  echo "Usage: $0 -e|--env <dev|qa|prod> [-d|--delete]"
  echo ""
  echo "List out stuff in environment, optionally delete it."
  echo ""
  echo "Options:"
  echo "  -d, --delete     Delete the stuff."
  exit 1
}

Not much to it, but it not only helps a new user to the script understand what
it’s about, but also sets the stage for the code we’re going to write to parse
the referenced arguments.

This is where shift comes in. Instead of just interrogating $1, $2, and
such, we’re going to loop through the argument vector. Depending on if the
argument will accept a value or not, we’ll use shift to remove the first item
from the array. This also moves the rest of the items up to the previous index,
so $3 becomes $2, $2 becomes $1, and so on.

For arguments that accept values, we’ll want to use shift twice and for
everything else, we will use shift once. If the --help argument is
specified, or if the argument isn’t recognized at all, we’ll present the user
with our help information:

while [[ $# -gt 0 ]] do
  case $1 in
    -e|--env)
      ENV="$2"
      shift
      shift
      
    -d|--delete)
      DELETE=true
      shift
      
    -h|--help)
      usage
      shift
      
    *)
      echo "Invalid option: $1"
      usage
      
  esac
done

In parsing through the arguments, we created a couple of variables that
represents the information. You can change those as you see fit, or even expand
the case statement to handle even more options.

Even though we’re done parsing the arguments, we still have a bit of work to do.
One of the arguments, --env is not only required, but it also only accepts a
fixed set of values.

We can simply check that the variable we set above, $ENV, is in fact
set. If so, we can do an additional check to ensure that it matches of
the expected set of values:

if [ -z "$ENV" ] then
  echo "Missing required option: -e|--env"
  usage
elif [[ ! "$ENV" =~ ^(dev|qa|prod)$ ]] then
  echo "Invalid environment: $ENV"
  usage
fi

At this point, we’ve successfully parsed the command-line arguments passed to
our script, and ensured that the necessary arguments are there and properly set.

The arguments can also be passed in any order, since we’re looping through the
argument vector and not expecting anything in a certain position.

If we were to put everything together, it would look like this:

#!/usr/bin/env bash

# Function to print usage instructions
function usage {
  echo ""
  echo "Usage: $0 -e|--env <dev|qa|prod> [-d|--delete]"
  echo ""
  echo "List out stuff in environment, optionally delete it."
  echo ""
  echo "Options:"
  echo "  -d, --delete     Delete the stuff."
  exit 1
}

# Parse command line arguments
while [[ $# -gt 0 ]] do
  case $1 in
    -e|--env)
      ENV="$2"
      shift
      shift
      
    -d|--delete)
      DELETE=true
      shift
      
    -h|--help)
      usage
      shift
      
    *)
      echo "Invalid option: $1"
      usage
      
  esac
done

# Check if our environment is set and valid
if [ -z "$ENV" ] then
  echo "Missing required option: -e|--env"
  usage
elif [[ ! "$ENV" =~ ^(dev|qa|prod)$ ]] then
  echo "Invalid environment: $ENV"
  usage
fi

# List out stuff in environment

# Delete stuff in environment if requested
if [ "$DELETE" = true ] then
  echo "Deleting stuff in ${ENV} environment... (not really)"
fi