Easy way to check for valid command line parameters?

Question

I'm looking for an easy way to check for a correct number of command line parameters, displaying a usage message if an error occurs and then immediately exit.

I thought of something like

if (@ARGV < 3) {
  print STDERR "Usage: $0 PATTERN [FILE...]\n";
  exit 1;
}

Is this a valid pattern?

Answer 1

Also, I would STRONGLY suggest using the idiomatic way of processing command line arguments in Perl, Getopt::Long module (and start using named parameters and not position-based ones).

You don't really CARE if you have <3 parameters. You usually care if you have parameters a, b and C present.

As far as command line interface design, 3 parameters is about where the cut-off is between positional parameters (cmd <arg1> <arg2>) vs. named parameters in any order (cmd -arg1 <arg1> -arg2 <arg2>).

So you are better off doing:

use Getopt::Long;
my %args;
GetOptions(\%args,
           "arg1=s",
           "arg2=s",
           "arg3=s",
) or die "Invalid arguments!";
die "Missing -arg1!" unless $args{arg1};
die "Missing -arg2!" unless $args{arg2};
die "Missing -arg3!" unless $args{arg3};

Answer 2

Another common way to do that is to use die

die "Usage: $0 PATTERN [FILE...]\n" if @ARGV < 3;

You can get more help on the @ARGV special variable at your command line:

perldoc -v @ARGV