Environment Variable

An environment variable is a name-value pair held in a process's environment and inherited by every process it spawns. It is the lowest-common-denominator configuration mechanism: available in every language, on every platform, and requiring no file format.

Its defining property is inheritance, which is also the source of most of its confusions. A variable set in one shell is invisible to another. A variable set at the session-manager level is inherited by everything, including things launched years later, which is how a stale credential set once can override a properly configured one indefinitely with no file anywhere to explain it. When a tool reports using the wrong identity, the first question is which source it actually read: an environment variable almost always takes precedence over a stored credential.

Two further points. The absence of a required variable should be fatal, not defaulted; see Fail Fast and Defensive Default. And making a variable required has a wide blast radius — every entry point that lacks it will now fail, including the ones nobody thinks about: the test configuration, the container image, the provisioning script, and the code-generation step in continuous integration.

Directory-scoped environment loaders make per-project variables ergonomic, which is how one machine can hold several isolated configurations for the same tool.