How can I find which operating system my Ruby program is running on?

Question

Use the RUBY_PLATFORM constant, and optionally wrap it in a module to make it more friendly:

module OS
  def OS.windows?
    (/cygwin|mswin|mingw|bccwin|wince|emx/ =~ RUBY_PLATFORM) != nil
  end

  def OS.mac?
   (/darwin/ =~ RUBY_PLATFORM) != nil
  end

  def OS.unix?
    !OS.windows?
  end

  def OS.linux?
    OS.unix? and not OS.mac?
  end

  def OS.jruby?
    RUBY_ENGINE == 'jruby'
  end
end

It is not perfect, but works well for the platforms that I do development on, and it's easy enough to extend.

(Warning: read @Peter Wagenet's comment ) I like this, most people use rubygems, its reliable, is cross platform

irb(main):001:0> Gem::Platform.local
=> #<Gem::Platform:0x151ea14 @cpu="x86", @os="mingw32", @version=nil>
irb(main):002:0> Gem::Platform.local.os
=> "mingw32"

update use in conjunction with "Update! Addition! Rubygems nowadays..." to mitigate when Gem::Platform.local.os == 'java'

Either

irb(main):002:0> require 'rbconfig'
=> true
irb(main):003:0> Config::CONFIG["arch"]
=> "i686-linux"

or

irb(main):004:0> RUBY_PLATFORM
=> "i686-linux"

I have a second answer, to add more options to the fray. The os rubygem, and their github page has a related projects list.

require 'os'

>> OS.windows?
=> true   # or OS.doze?

>> OS.bits
=> 32

>> OS.java?
=> true # if you're running in jruby.  Also OS.jruby?

>> OS.ruby_bin
=> "c:\ruby18\bin\ruby.exe" # or "/usr/local/bin/ruby" or what not

>> OS.posix?
=> false # true for linux, os x, cygwin

>> OS.mac? # or OS.osx? or OS.x?
=> false