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In this guide, we’ll dive deeper into Debian package management using APT (Advanced Package Tool). APT is a powerful front-end to dpkg that automates dependency resolution, offers advanced search capabilities, and simplifies package installation, upgrades, and removals. It works with software repositories—which can be remote servers, local mirrors, or even CD-ROMs. Official repositories are maintained by distributions like Debian and Ubuntu, and you can add third-party or custom repos as needed.
The image is a slide describing the Advanced Package Tool (APT), highlighting its features such as advanced search, dependency resolution, and its role in working with software repositories.

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APT Command-Line Utilities

CommandPurpose
sudo apt-get [options]Install, upgrade, or remove packages
sudo apt-cache [options]Search and display package information
sudo apt-file [options]Search for files within packages (inst./uninst.)
sudo apt [options]Unified interface combining apt-get & apt-cache
The apt command is more user-friendly, but it may not be installed on older systems. Always know how to use both apt-get and apt-cache.

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1. Updating the Package Index

Before installing or upgrading any software, refresh your local package index: 
sudo apt-get update
This fetches the latest package lists from all configured repositories.

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2. Installing and Upgrading Packages

To install a new package—or upgrade it if already present—use: 
sudo apt-get install xournal
Sample output: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  xournal
0 upgraded, 1 newly installed, 0 to remove and 75 not upgraded.
Need to get 285 kB of archives.
After this operation, 1041 kB of additional disk space will be used.
You can install multiple packages at once, for example:
sudo apt-get install git curl vim

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3. Removing and Purging Packages

  • Remove (keep config files): 
    sudo apt-get remove xournal
  • Purge (remove config files too): 
    sudo apt-get purge p7zip
# or equivalently:
sudo apt-get remove --purge p7zip
APT will list affected packages and ask for confirmation before proceeding.

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4. Fixing Broken Dependencies

When a manual .deb install triggers unmet dependencies: 
sudo dpkg -i --force openshot-qt_2.4.3+dfsg1-1_all.deb
# ... dpkg: dependency problems prevent configuration of openshot-qt: ...
Restore consistency by installing missing dependencies: 
sudo apt-get install -f
This will automatically fetch and install the required packages.

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5. Upgrading All Packages

  1. Refresh your index: 
    sudo apt-get update
  2. Upgrade upgradable packages: 
    sudo apt-get upgrade
Example summary: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  gnome-control-center
The following packages will be upgraded:
  cups cups-bsd cups-client cups-common cups-core-drivers cups-daemon ...
74 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Need to get 243 MB of archives.
After this operation, 30.7 kB of additional disk space will be used.
Do you want to continue? [Y/n]
To upgrade one package without affecting others, use:
sudo apt-get install --only-upgrade unrar

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6. Cleaning the Package Cache

APT caches downloaded .deb files in /var/cache/apt/archives. Free up disk space by running: 
sudo apt-get clean

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7. Searching Packages with apt-cache

  • Search by keyword: 
    sudo apt-cache search p7zip
    Sample output: 
    liblzma-dev     - XZ-format compression library - development files
liblzma5        - XZ-format compression library
forensics-extra - Forensics Environment - extra console components (metapackage)
p7zip           - 7zr file archiver with high compression ratio
p7zip-full      - 7z and 7za file archivers with high compression ratio
p7zip-rar       - non-free rar module for p7zip
  • Show package details: 
    sudo apt-cache show liblzma5
    Key fields: 
    Package: liblzma5
Version: 5.2.4-1
Depends: libc6 (>= 2.17)
Description-en: XZ-formatted compression library
 LZMA is the successor to the Lempel-Ziv-Markov-chain Algorithm...

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8. Configuring Software Repositories

Repository entries reside in /etc/apt/sources.list or under /etc/apt/sources.list.d/. Each line follows: 
<type> <URL> <distribution> <components>
Example (Ubuntu “disco”): 
deb http://us.archive.ubuntu.com/ubuntu/ disco main restricted universe multiverse
ComponentDescription
mainOfficially supported open-source packages
restrictedSupported closed-source software (e.g., proprietary drivers)
universeCommunity-maintained open-source packages
multiverseUnsupported closed-source or patented software
contribDFSG-compliant packages depending on non-main components
non-freePackages not compliant with the Debian Free Software Guidelines
securitySecurity updates
backportsNewer versions backported from testing or unstable branches
After editing or adding a .list file, always run: 
sudo apt-get update

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Example: Adding Debian Buster Backports

sudo vi /etc/apt/sources.list.d/buster-backports.list
Append: 
deb     http://deb.debian.org/debian buster-backports main contrib non-free
deb-src http://deb.debian.org/debian buster-backports main contrib non-free

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9. Using apt-file

The apt-file tool lets you search for individual files within packages, even if they aren’t installed.
  1. Install and initialize: 
    sudo apt-get install apt-file
sudo apt-file update
  2. List package contents: 
    sudo apt-file list unrar
# unrar: /usr/bin/unrar-nonfree
# unrar: /usr/share/doc/unrar/changelog.Debian.gz
  3. Search for a specific file: 
    sudo apt-file search libsdl2.so
Unlike dpkg-query, apt-file can search across all available (but not yet installed) packages.
Test your knowledge with the included quiz!

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