Missing argument handling

When a required argument is silently missing, the command either crashes mid-execution with a raw TypeError: Cannot read properties of undefined — dumping a stack trace onto the user — or it runs with undefined as a value and produces silent corruption downstream (a deploy target of undefined, a file named undefined.json, a database query with a null key). CWE-233 (Improper Handling of Parameters) applies: the input is invalid, but no validation gate catches it before execution begins. ISO 25010:2011 usability.learnability penalizes CLIs that make users guess what went wrong — the error message must name the missing argument and show the correct usage.

Use the framework's native required-argument syntax so validation happens before your handler runs. In commander, angle brackets mark required arguments:

program
  .command('deploy <target>')
  .description('Deploy to the specified target')
  .action((target) => {
    // commander errors automatically: "error: missing required argument 'target'"
    deploy(target)
  })

In click, arguments are required by default:

@cli.command()
@click.argument('target')  # required — click handles: "Error: Missing argument 'TARGET'."
def deploy(target):
    """Deploy to the specified TARGET."""
    pass

Never access argv[n] or opts.target without first confirming the value is defined — opt into your framework's built-in validation rather than writing manual if (!target) guards that are easy to miss.