We can see them all using pp, the pretty printer of Ruby.

examples/ruby/env.rb

require 'pp'

pp ENV

We can also access the value directly. For example: puts ENV['PATH'], we can add new environment variables or change existing ones with one big caveat. Once our Ruby program ends these changes will be gone.

If we start a new process from our Ruby program after we have made modifications ENV, all those modifications will be seen by the other process. However the changes cannot propagate to the process that launched our Ruby program:

For example if run this program:

examples/ruby/change_path_and_system.rb

system("echo $PATH")
ENV['PATH'] = '/nothing/here'
system("echo $PATH")

The output will look something like this:

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
/nothing/here

The first time we called system the new shell saw the original content of the PATH environment variable. Then we changed it and set it to something horribly bad. When we called system the second time the new shell saw the new value.

After tunning the above script execute the following in the Unix/Linux shell

echo $PATH

It will print the same path as it did with the first call to system.

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

This means our changes in the Ruby code have not changed the environment variable for the parent process.

This is a feature of most or all of the Operating Systems.