[Entry] General: Environment Variables

[Dusch4593]

Reason/inspiration (optional)

Please write a new concept entry for environment variables under the General topic.

• Since this is a General entry, try to avoid language-specific information.
• But do include info about its relationship with Unix and Unix-like variants (Linux).
  • Go ahead and link out to the Docs entries for Linux and Unix, as well.
• Include a list of the most common environment variables. (PATH, HOME, etc.)

Please refer to the entry templates, content standards, and markdown style guide when working on the PR for this issue.

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New Concept Entry for Existing Topic

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