A repository in Debian is simply a server (or set of servers) that hosts packages your system can download and install.

A Debian repository contains .deb packages and metadata indexes: lists of available packages, versions, dependencies and signatures: cryptographic verification to ensure authenticity.

On Debian-based systems apt and dpkg are both package management tools, but they operate at different levels:

apt modifies system-wide software, so it requires administrative privileges.

dpkg (Debian Package Manager)

low-level tool that directly installs .deb files on your system.

Example usage:

Install a local package:

sudo dpkg -i package.deb

Remove a package:

sudo dpkg -r package_name

List installed packages:

dpkg -l

Show files installed by a package:

dpkg -L package_name

apt (Advanced Package Tool)

high-level tool. It is a frontend (wrapper) around dpkg + repository system.

apt is the modern, user-friendly interface designed for humans to use interactively in the terminal. apt-get is a low-level, stable interface primarily recommended for scripts and automation because its behavior and output remain consistent across versions.

Example usage:

Install software:

sudo apt install firefox

Remove software:

sudo apt remove firefox

apt update

This does NOT install or upgrade anything. It only refreshes your package list from repositories.

• Updates your local database (in /var/lib/apt/lists/)

sudo apt update

Now your system now knows newer versions exist. Nothing is installed yet.

apt upgrade

This actually installs newer versions of installed packages. It compares installed packages vs repository versions

• Does NOT remove packages
• Does NOT install new required dependencies if that would require removals

sudo apt upgrade

apt full-upgrade

Updates all packages like upgrade BUT also installs new dependencies if needed, removes conflicting or obsolete packages

Upgrade everything even if system structure must change. Neither command upgrades your Linux distribution version. They only upgrade packages within the same release.

sudo apt full-upgrade

or older name:

sudo apt dist-upgrade

Where repositories are configured

Main file:

/etc/apt/sources.list

Optional additional files:

/etc/apt/sources.list.d/*.list

Example:

deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware

• deb for binary packages (deb-src for source)
• URL for repository server
• bookworm Debian release (Debian 12)

Sections:

main

• Fully free and open-source software
• Meets Debian Free Software Guidelines
• Officially supported

contrib

• Free software, but depends on non-free components
• Example: an open-source tool that needs proprietary firmware

non-free

• Proprietary software or firmware
• Not fully open-source

non-free-firmware

• Introduced in newer Debian versions
• proprietary, low-level software required by hardware components, which does not provide its source code or allow modification

By default, Debian may only enable main due to philosophical and legal reasons