make sure file exists before writing to it

gistfile1.rb

if options.has_key? :file then
  file = options[:file]
  dir = File.dirname(file)
  if File.directory?(dir) && File.writable?(dir) then
    if File.exists?(file) && ! File.writable?(file) then
      abort "Can't write to file. No permission?"
    else
      $stdout = File.new file, 'w'
    end
  else
    abort "Can't write to directory. No permission?"
  end
end

This would capture all the potential error cases and provide the default error message for each case.

if options.has_key? :file then
    path = options[:file]
    begin
        $stdout.reopen(path, 'a')
    rescue => e
        abort e.message
    end
end

It won't give a different error for an unwriteable file or unwriteable directory, but you could provide this by handling Errno::EACCES explicitly; e.g.

if options.has_key? :file then
    path = options[:file]
    begin
        $stdout.reopen(path, 'a')
    rescue Errono::EACCES=> e
        dir = File.dirname(path)
        abort File.writable?(dir) ? "Can't write to directory.  No permission." : "Can't write to file. No Permission."
    rescue => e
        abort e.message
    end
end

Err, flip the error messages around in that ternary. :)

Before refreshing the page and seeing your solution, I took your advice from twitter and did the following:

if options.has_key? :file then
  begin
    $stdout.reopen(options[:file], 'w')
  rescue Errno::ENOENT => e
    abort "Can't write into that directory. Does it exist?"
  rescue Errno::EACCES => e
    abort "Can't write to file or directory. Do you have permission?"
  rescue Errno::EISDIR => e
    abort "Please pass in a file name, not a directory name."
  rescue => e
    abort e.message
  end
end

But I like your solution even better. Thanks!