What does apt-get update do?

1. Fetches package lists

apt-get update fetches the package lists from /etc/apt/sources.list and files from the /etc/apt/sources.list.d/ directory. If you inspect a file from these locations, you’ll see something like:

deb http://ppa.launchpad.net/ondrej/php/ubuntu focal main

Let’s break down the components:

• deb: This specifies the archive type. deb is used for pre-compiled, ready-to-use Debian packages that are ready to be installed without any compilation steps. You can also have deb-src which contains the package source and compilation instructions – useful for when you want to inspect or edit the source.
• http://ppa.launchpad.net/ondrej/php/ubuntu
  • http://ppa.launchpad.net – a popular platform for hosting open-source pages. You can think of it like GitHub for Ubuntu packages.
  • ondrej/php/ubuntu is the specific Personal Package Archive (PPA). You can think of it as a GitHub repository.
• focal: Specifies the distribution codename that the repository targets. See here for full list of distribution codenames.
• main: The component of the repository. A PPA repo can contain multiple components. The main component usually contains free and open-source software that’s official supported.

2. Updates local package index

Once it’s fetched the package list, it’ll your system’s local page index – a local copy of all the available packages and versions available from the repositories your system knows about.

3. Prepares system for upgrade

apt-get update only updates your system’s package index. It doesn’t actually upgrade anything. To upgrade, you’ll want to run apt-get upgrade.