Configure APT Repositories on Ubuntu 20.04: Add/Remove Repos, PPAs, and GPG Keys

Software installation on Linux systems is performed from repositories, which by default contain a large number of packages. But sometimes there are situations when the necessary software is not included, or the version is too old. In this case, you can add the necessary repository and install from it.

How to configure repositories on Ubuntu 20.04:

• How to view all repositories in Ubuntu
• How to add repositories in Ubuntu
• How to remove repositories in Ubuntu
• Dealing with PPA repositories in Ubuntu

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In this tutorial, we will configure repositories on Ubuntu server 20.04.

List of repositories in Ubuntu

Adding repositories in Ubuntu

Removing repositories

PPA repository in Ubuntu

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Conclusion

In this guide, we covered the core ways to manage software repositories on Ubuntu 20.04: where APT repository entries are stored, how to review and disable them, how to add and remove third-party repositories, and how PPAs work. After any repository change, always run apt update to refresh package indexes and confirm the source is valid. Since third-party repositories can introduce unexpected package versions (including core system components), it’s best to add only trusted sources, keep repository entries clean and well-documented, and remove unused repos to reduce update errors and security risks.

FAQ: Ubuntu 20.04 Repositories

• Q1: Where does Ubuntu 20.04 store the list of repositories?
  A: Repository entries are stored in /etc/apt/sources.list and, more commonly for third-party sources, as separate files inside /etc/apt/sources.list.d/.
• Q2: How do I quickly disable a repository without deleting it?
  A: Comment out the repository line by adding # at the beginning of the line in the .list file (or in sources.list). This keeps the entry for later but prevents APT from using it.
• Q3: What’s the safest way to add a third-party repository?
  A: Use the repository line provided by the vendor and ensure it’s signed with a trusted key. Prefer repository-specific signing (keyring) rather than trusting a key globally, and only use repositories from reputable sources.
• Q4: What is a PPA and when should I use it?
  A: A PPA (Personal Package Archive) is a Launchpad-hosted repository commonly used to distribute newer versions of software for Ubuntu. Use PPAs when you trust the maintainer and need a version that isn’t available in official Ubuntu repositories.
• Q5: How do I remove a repository or PPA correctly?
  A: Remove it using add-apt-repository --remove ... (for repos or PPAs) or delete the corresponding .list file from /etc/apt/sources.list.d/. After removal, run apt update to refresh package indexes.
• Q6: After adding a repository, why does apt update fail with a key error (NO_PUBKEY)?
  A: That means APT can’t verify the repository signature because the signing key is missing or incorrect. Install the correct vendor key and ensure the repository line matches the key being used.
• Q7: What does “Release file not found” mean during apt update?
  A: Usually the repository URL is wrong, the Ubuntu codename (like focal) doesn’t match what the repository supports, or the repository structure has changed. Double-check the repo line and vendor documentation.
• Q8: Do I need to run anything after adding or removing repositories?
  A: Yes — run apt update to refresh the package list. If you’re installing from the new repository right away, follow it with apt install for the package you need.
• Q9: Can adding third-party repositories affect system stability?
  A: Yes. Some repositories include newer or experimental builds that may upgrade core libraries or dependencies. It’s best to add only what you need, avoid mixing many third-party sources, and review what will be upgraded before confirming updates.