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

I want my Ruby program to do different things on a Mac than on Windows. How can I find out on which system my program is running?

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

Try the Launchy gem (gem install launchy):

require 'launchy'
Launchy::Application.new.host_os_family # => :windows, :darwin, :nix, or :cygwin 

require 'rbconfig'
include Config

case CONFIG['host_os']
  when /mswin|windows/i
    # Windows
  when /linux|arch/i
    # Linux
  when /sunos|solaris/i
    # Solaris
  when /darwin/i
    #MAC OS X
  else
    # whatever
end

Update! Addition! Rubygems nowadays ships with Gem.win_platform?.

Example usages in the Rubygems repo, and this one, for clarity:

def self.ant_script
  Gem.win_platform? ? 'ant.bat' : 'ant'
end

For something readily accessible in most Ruby installations that is already somewhat processed for you, I recommend these:

1. Gem::Platform.local.os #=> eg. "mingw32", "java", "linux", "cygwin", "aix", "dalvik" (code)
2. Gem.win_platform? #=> eg. true, false (code)

Both these and every other platform checking script I know is based on interpreting these underlying variables:

1. RbConfig::CONFIG["host_os"] #=> eg. "linux-gnu" (code 1, 2)
2. RbConfig::CONFIG["arch"] #=> eg. "i686-linux", "i386-linux-gnu" (passed as parameter when the Ruby interpreter is compiled)
3. RUBY_PLATFORM #=> eg. "i386-linux-gnu", "darwin" - Note that this returns "java" in JRuby! (code)

• These are all Windows variants: /cygwin|mswin|mingw|bccwin|wince|emx/

4. RUBY_ENGINE #=> eg. "ruby", "jruby"

Libraries are available if you don't mind the dependency and want something a little more user-friendly. Specifically, OS offers methods like OS.mac? or OS.posix?. Platform can distinguish well between a variety of Unix platforms. Platform::IMPL will return, eg. :linux, :freebsd, :netbsd, :hpux. sys-uname and sysinfo are similar. utilinfo is extremely basic, and will fail on any systems beyond Windows, Mac, and Linux.

If you want more advanced libraries with specific system details, like different Linux distributions, see my answer for Detecting Linux distribution in Ruby.

We have been doing pretty good so far with the following code

  def self.windows?
    return File.exist? "c:/WINDOWS" if RUBY_PLATFORM == 'java'
    RUBY_PLATFORM =~ /mingw32/ || RUBY_PLATFORM =~ /mswin32/
  end

  def self.linux?
    return File.exist? "/usr" if RUBY_PLATFORM == 'java'
    RUBY_PLATFORM =~ /linux/
  end

  def self.os
    return :linux if self.linux?
    return :windows if self.windows?
    nil
  end

Using the os gem, when loading different binaries for IMGKit

# frozen_string_literal: true
IMGKit.configure do |config|
  if OS.linux? && OS.host_cpu == "x86_64"
    config.wkhtmltoimage =
      Rails.root.join("bin", "wkhtmltoimage-linux-amd64").to_s
  elsif OS.mac? && OS.host_cpu == "x86_64"
    config.wkhtmltoimage =
      Rails.root.join("bin", "wkhtmltoimage-macos-amd64").to_s
  else
    puts OS.report
    abort "You need to add a binary for wkhtmltoimage for your OS and CPU"
  end
end

When I just need to know if it is a Windows or Unix-like OS it is often enough to

is_unix = is_win = false
File::SEPARATOR == '/' ? is_unix = true : is_win = true