Replaces your clipboard with an underscore delimited string. Throw in your ~/Library/Scripts/ directory and `chmod a+x`.

clipboard_manipulation.rb

#!/usr/bin/ruby

# Save file to a directory in your $PATH.
# Ensure it is executable `chmod a+x clipboard_manipulation.rb`.
#
# Usage `copy some text like this`. 
# Then from the commandline run `clipboard_manipulation.rb space underscore`.
# Your clipborad will now contain the text `copy_some_text_like_this`.
# 
# This is a simply and silly script that scratched an itch I was having.

def pbcopy(input)
  str = input.to_s
  IO.popen('pbcopy', 'w') { |f| f << str }
  str
end

def pbpaste
  `pbpaste`
end

types = {
  comma:      ', ',
  dash:       '-',
  decimal:    '.',
  period:     '. ',
  space:      ' ',
  underscore: '_'
}

from = ARGV[0].to_sym
to   = ARGV[1].to_sym

return 'Not valid `from`' unless types.key?(from)
return 'Not valid `to`'   unless types.key?(to)

pbcopy(pbpaste.gsub(types[from], types[to]))