apt: E: Unable to locate package

What the message looks like

$ sudo apt install htop
Reading package lists... Done
Building dependency tree... Done
E: Unable to locate package htop

Common causes

• apt update was never run, for example on a fresh server or in a new container, so the package lists are empty.
• The package name is wrong, or the package has another name on your release.
• The repository component that carries the package is not enabled: universe on Ubuntu, contrib or non-free on Debian.
• The release is end of life and its repositories moved to an archive server, so apt update ends with 404 errors.
• A broken or typed-wrong entry in the sources makes apt update fail, so the lists stay incomplete.

Check

1. Does apt know the package at all?
   apt-cache policy htopCopy
2. Search for the real package name
   apt-cache search htop | head -n 10Copy
3. Which repositories and components are active?
   grep -rE '^(deb |Types:|URIs:|Suites:|Components:)' /etc/apt/sources.list /etc/apt/sources.list.d/ 2>/dev/nullCopy
4. Which release is this?
   cat /etc/os-releaseCopy

Fix

• Refresh the lists and read the output for errors: sudo apt update.
• Use the exact package name that apt-cache search shows for your release.
• Enable the missing component: on Ubuntu sudo add-apt-repository universe, on Debian add contrib or non-free to the Components of your sources, then run apt update again.
• If apt update shows 404 for your release, it is end of life. Upgrade to a supported release instead of pointing the sources to an archive for production.
• In a Dockerfile, run apt-get update and apt-get install in the same RUN step.

FAQ

Why does it work on my laptop but not in the container?

Container images ship without package lists to stay small. Run apt-get update first, in the same build step as the install.

Is it safe to add third-party repositories to fix this?

Only if you trust the source. First check whether the package exists under another name or in an official component of your release.