this is the way i've always done it in my scripts, when i didn't want to change the environment variable outside of the program:

{
local %ENV{NAME_OF_VARIABLE}="new-value";
#do whatever you want with the new environment in here
}
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you should make sure that you don't stomp on global environment variables by always changing the variable with local. if you're really sure that you want to do that, check out the node above. but i think that normally want to think twice about changing environment variables when the user doesn't expect it. -- cat