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copied!<p>I am assuming you are using optparse here, although the same technique will work for other option parsing libraries. </p> <p>The simplest method is probably to parse the parameters using your chosen option parsing library and then raise an OptionParser::MissingArgument Exception if the value of host is nil.</p> <p>The following code illustrates</p> <pre><code>#!/usr/bin/env ruby require 'optparse' options = {} optparse = OptionParser.new do |opts| opts.on('-h', '--host HOSTNAME', "Mandatory Host Name") do |f| options[:host] = f end end optparse.parse! #Now raise an exception if we have not found a host option raise OptionParser::MissingArgument if options[:host].nil? puts "Host = #{options[:host]}" </code></pre> <p>Running this example with a command line of </p> <pre><code>./program -h somehost </code></pre> <p>simple displays "Host = somehost"</p> <p>Whilst running with a missing -h and no file name produces the following output</p> <pre><code>./program:15: missing argument: (OptionParser::MissingArgument) </code></pre> <p>And running with a command line of ./program -h produces</p> <pre><code>/usr/lib/ruby/1.8/optparse.rb:451:in `parse': missing argument: -h (OptionParser::MissingArgument) from /usr/lib/ruby/1.8/optparse.rb:1288:in `parse_in_order' from /usr/lib/ruby/1.8/optparse.rb:1247:in `catch' from /usr/lib/ruby/1.8/optparse.rb:1247:in `parse_in_order' from /usr/lib/ruby/1.8/optparse.rb:1241:in `order!' from /usr/lib/ruby/1.8/optparse.rb:1332:in `permute!' from /usr/lib/ruby/1.8/optparse.rb:1353:in `parse!' from ./program:13 </code></pre>

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