trollop output file with stdout as default

Question

What's the best way to optionally specify an output file but use stdout by default?

Right now I'm trying:

opts = Trollop::options do
  opt :output, "Output File", :default => $stdout
  opt :input, "Input File", :default => $stdin
end

But when I try using it, I get:

$ ./test.rb -o temp.txt
Error: file or url for option '-o' cannot be opened: No such file or directory - temp.txt.
Try --help for help.

Obviously I don't want to require the output file to exist prior to running my script.

(Also, is the way I'm specifying the input okay?)

Answer 1

Looking at the code in trollop (specifically parse_io_parameter) I believe that currently, trollop (version 1.16.2) assumes any IO type argument (as in the question) is assumed to be for input.

The workaround is as follows.

Use a String to specify the output file in trollop:

opts = Trollop::options do
  opt :input, "Input File", :default => $stdin
  opt :output, "Output File", :default => "<stdout>"
end

Create an output object (out) based on the parsed argument:

out = if opts[:output] =~ /^<?stdout>?$/i
  $stdout
else
  fd = IO.sysopen(opts[:output], "w")
  a = IO.new(fd, "w")
end

Then you can use it like this:

out.puts("this text gets written to output")