<p>I am trying to set my DOS environment variable in Ruby, and have it persist after the script exits. For example, if I want a ruby script <code>set_abc_env.rb</code> to set environment variable 'ABC' to 'blah', I expect to run the following:</p>

<pre><code>C:&gt; echo %ABC%
C:&gt; set_abc_env.rb
C:&gt; echo %ABC% blah
</code></pre>

<p>How do I do this?</p>

You can access environment variables via Ruby ENV object:
```
```
i = ENV['ABC']; # nil
ENV['ABC'] = '123';
i = ENV['ABC']; # '123'
```
```
Bad news is, as MSDN
says
, a process can never directly change the environment variables of another process that is not a child of that process. So when script exits, you lose all changes it did.
Good news is what Microsoft Windows stores environment variables in the registry and it's possible to
propagate
environment variables to the system. This is a way to modify user environment variables:
```
```
require 'win32/registry.rb'

Win32::Registry::HKEY_CURRENT_USER.open('Environment', Win32::Registry::KEY_WRITE) do |reg|
  reg['ABC'] = '123'
end
```
```
The documentation also says you should log off and log back on or broadcast a WM_SETTINGCHANGE message to make changes seen to applications. This is how broadcasting can be done in Ruby:
```
```
require 'Win32API'  

SendMessageTimeout = Win32API.new('user32', 'SendMessageTimeout', 'LLLPLLP', 'L') 
HWND_BROADCAST = 0xffff
WM_SETTINGCHANGE = 0x001A
SMTO_ABORTIFHUNG = 2
result = 0
SendMessageTimeout.call(HWND_BROADCAST, WM_SETTINGCHANGE, 0, 'Environment', SMTO_ABORTIFHUNG, 5000, result)
```
```