ENV['false'] is true

Thu, 09 Jul 2026 10:00:00 +0200

Here is a small Ruby ENV variable gotcha I encountered and how I fixed it.

I had an environment variable controlling a feature flag:

if ENV['DISABLE_THING']
  # skip the thing
end

Set DISABLE_THING=false, reloaded the app, and… the thing was still disabled. Which made no sense. I checked the value, I double-checked the spelling, I restarted the server. Same behavior.

The problem is obvious in retrospect. Environment variables are strings. ENV['DISABLE_THING'] returns "false". And in Ruby, "false" is truthy. It’s a non-empty string, so if ENV['DISABLE_THING'] always passes. The actual value doesn’t matter as long as it’s not empty or nil.

Ruby on Saroj Maharjan

ENV['false'] is true