From: Robert Klemme <bob.news@...>
Date: 2004-06-08T16:23:43+09:00
Subject: Re: Newbie wondering about command line arguments


"Paul Emmons" <pemmons@voicenet.com> schrieb im Newsbeitrag
news:cln9c0tv0r26fgt25hq43htmlh5sk709id@4ax.com...
> I'm experimenting with writing a text filter, as a learning
> experience.
>
> Right now, I want the program to:
> * work as a pure filter if there are no command-line arguments
> * use the argument as an input file, send output to STOUT if there is
> one argument
> * If there are more than one argument, use the last argument as
> an output file name and the others as input file names.
>
> My question is not how to get my program to do this.   It is doing it
> exactly as I wanted, like a mind reader.  My question is, how is it so
> smart?  I expect I'll have problems later on if I don't really
> understand what is happening.
>
> I have read that unless somehow told otherwise, a Ruby program uses
> the command-line arguments as input file names and reads them one
> after the other as though they came from STDIN.  If I have used the
> last argument as an output file name, why doesn't it run into trouble
> when it comes to this argument and tries to use it as in input file?
>
> And how, in general, do I make sure that a Ruby script does not try to
> use an argument as an input file name if I want it to mean something
> else?
>
> Scratching my head (with delight for a change).
>
> Thank you for any help.
>
> (This is my program so far.  :)
>
> #!/usr/bin/env ruby
> # Reverses characters in every line of a file
>
> if STDIN.tty?
>     puts "No. of args = #{ARGV.size}"
>    ARGV.each { |a| puts "Arg = #{a}" }
>    if ARGV.size>1
>    $stdout=File.new(ARGV[ARGV.size-1], "w")
>   end
> end
>

#!/usr/bin/ruby

out = nil

case ARGV.size
  when 0, 1
    out = $stdout
  else
    out = File.open( ARGV.slice!(-1,1)[0], "w" )
end

begin
  while line = gets
    line.chomp!
    out.puts line.reverse
  end
ensure
  out.close unless out.equal? $stdout
end

    robert