Environment Variable

An environment variable is a named string value supplied to a process when it starts. Programs use these values for configuration such as a database host or a locale. PATH is a list of directories used when locating external commands; its value can explain why the same command name selects different executables in different jobs.

A newly assigned shell variable is not automatically part of the environment passed to external commands. In Bash, export marks a variable for inclusion in subsequently launched commands. A command-specific assignment can provide a value for one invocation. Changing a child’s environment does not rewrite its parent’s environment or the configuration of a process that is already running.

For example, an interactive shell may export a database host before launching a Python job. A scheduled invocation can lack that value because it did not start from the same shell. Check the job’s configured environment and fail clearly when a required value is missing. Treat an unset variable and an explicitly empty string according to the application’s contract, rather than assuming they are interchangeable.

An environment is a delivery mechanism for configuration, not a vault. A secret injected there can reach child processes or appear in debugging and diagnostic output. Inspect only the non-secret values needed for the investigation. Keep credentials out of source control and obtain them through the approved runtime mechanism; rotating a credential and changing the environment of a future job are separate operations.

Reference: GNU Bash: Environment. Practice these ideas in Linux, the Shell, and Git for Data Work.