How do I handle a missing mandatory argument in Ruby OptionParser?

Question

In OptionParser I can make an option mandatory, but if I leave out that value it will take the name of any following option as the value, screwing up the rest of the command line parsing. Here is a test case that echoes the values of the options:

$ ./test_case.rb --input foo --output bar
output  bar
input  foo

Now leave out the value for the first option:

$ ./test_case.rb --input  --output bar
input  --output

Is there some way to prevent it taking another option name as a value? Thanks!

Here is the test case code:

#!/usr/bin/env ruby
require 'optparse'
files = Hash.new

option_parser = OptionParser.new do |opts|
  opts.on('-i', '--input FILENAME', 'Input filename - required') do |filename|
    files[:input] = filename
  end
  opts.on('-o', '--output FILENAME', 'Output filename - required') do |filename|
    files[:output] = filename
  end
end

begin
  option_parser.parse!(ARGV)
rescue OptionParser::ParseError
  $stderr.print "Error: " + $! + "\n"
  exit
end

files.keys.each do |key|
  print "#{key}  #{files[key]}\n"
end

Answer 1

What you want to do is not a good idea. What if you really have a file named "--output"? This is a perfectly valid filename on Unix. Every Unix program's option parsing works the way the ruby one is doing, so you shouldn't change it, because then your program will be arbitrarily different from everything else, which is confusing and violates the "principle of least surprise."

The real question is: why are you having this problem in the first place? Perhaps you're running your program from another program, and the parent program is providing a blank filename as the parameter to --input, which makes it see --output as the parameter to --input. You can work around this by always quoting the filenames you pass on the command line:

./test_case.rb --input "" --output "bar"

Then --input will be blank, and that's easy to detect.

Also note that if --input is set to --output (and --output is not a real file) you can just try to open the --input file. If it fails, print a message like:

can't open input file: --output: file not found

And that should make it clear to the user what they did wrong.