Environment variables and the shell configuration

An environment variable is a named value available to every process running in a given shell session — configuration that lives outside any specific program's own code, readable by whatever process needs it:

PATH is a specific, critical environment variable: a list of directories the shell searches, in order, when you type a command name, looking for a matching executable file:

When you type python, the shell checks each directory in PATH, in order, until it finds an executable file named python — and runs the first one it finds. This is the direct, mechanical explanation behind "command not found" (no directory in PATH contains that executable) and behind "why is the wrong version of this tool running" (an earlier directory in PATH contains an older or different version than the one you actually intended).

Files like ~/.bashrc or ~/.zshrc (depending on which shell you use) run automatically every time you open a new terminal session, and are where persistent configuration lives — custom PATH additions, aliases, exported environment variables you want available every time, not just for one session:

Changes to these files only take effect in new terminal sessions, or after running source ~/.bashrc to reload the current one manually — editing the file alone doesn't retroactively affect a session that's already running.

A scheduled task (cron) or a CI pipeline typically runs in a minimal environment that does not automatically load your interactive shell's ~/.bashrc the way opening a normal terminal does — meaning any PATH additions or environment variables set only in that file simply aren't present there. This is the actual, mechanical explanation behind a script that runs perfectly when you test it manually but fails mysteriously when run by a scheduler or a CI job — not a different bug, the same environment- variable gap, showing up in a different, less obvious context.

Prefixing a single command with a variable assignment sets it just for that one invocation, without affecting the wider session at all:

This is a genuinely useful pattern for testing a one-off configuration change without permanently altering your shell's exported environment.