Environment variables are values that control the behavior of operating systems, individual utilities like Git, shell scripts, user applications such as the Google Chrome browser, or deployed applications like a Python web app. They can be set using commands like `export` and accessed using `$`. Shell variables have local scope, while environment variables have global scope. Environment variables are designed to be accessible to scripts or child processes, whereas shell variables are only accessible in the current shell. To change an environment variable, use `export`, and to delete one, use `unset`. Variable names should follow a specific syntax and convention. Environment variables can be used to capture output from commands, avoid spaces around equals signs, and quote values to prevent word-splitting issues. They are also useful for supplying config and secrets to deployed applications and configuring CI/CD jobs and environments. To execute a script in the context of the current shell, prefix it with a leading period (`.`). When using `sudo`, use the `--preserve-env` flag to pass through environment variables from the current shell. Additionally, there are resources available for further learning on shell programming, such as Bite Size Bash and Shellcheck static analysis tool.