# The APT package manager

## Table of contents <!-- omit in toc -->

1. [TL;DR](#tldr)
1. [Automate security upgrades](#automate-security-upgrades)
1. [Configuration](#configuration)
1. [Version pinning](#version-pinning)
1. [Troubleshooting](#troubleshooting)
   1. [Fix a "Problem with MergeList" or "status file could not be parsed" error](#fix-a-problem-with-mergelist-or-status-file-could-not-be-parsed-error)
1. [Further readings](#further-readings)
1. [Sources](#sources)

## TL;DR

```sh
# Check for broken dependencies.
sudo apt-get check

# Update the packages lists.
sudo apt update

# Upgrade the system.
sudo apt upgrade
sudo apt dist-upgrade

# Look for packages.
apt search 'ansible'
apt search --names-only 'python'

# Show details of packages.
apt show 'vlc'

# List a package's dependencies.
apt depends 'ufw'

# Install packages.
sudo apt install 'nano' 'python3-zstd/stable'

# Remove packages.
sudo apt remove 'dhclient' 'sudo'
sudo apt remove --purge 'bluez'
sudo apt purge 'crda'

# Remove orphaned packages.
sudo apt autoremove --purge

# List packages.
sudo apt list
sudo apt list --upgradable
sudo apt list --installed

# List explicitly installed packages only.
sudo apt-mark showmanual

# Mark packages as explicitly installed.
sudo apt-mark manual 'vim' 'unattended-upgrades'

# List non-explicitly installed packages only.
sudo apt-mark showauto

# Mark packages as non-explicitly installed.
sudo apt-mark auto 'zsh' 'bash-completion'
sudo apt-mark auto $(sudo apt-mark showmanual)

# Reconfigure packages.
sudo dpkg-reconfigure 'mariadb-server'
sudo dpkg-reconfigure -p 'low' 'unattended-upgrades'
```

## Automate security upgrades

Leverage `unattended-upgrades` for this.

```sh
# Configure the packages to keep up to date.
sudo dpkg-reconfigure -p 'low' 'unattended-upgrades'

# Check what packages would be installed.
sudo unattended-upgrade -d --dry-run

# Run manually.
sudo unattended-upgrade
```

## Configuration

See [Apt configuration] for more information.

```txt
# /etc/apt/apt.conf.d/90default-release
APT::Default-Release "stable";
```

```txt
# /etc/apt/apt.conf.d/99parallel-fetch
APT::Acquire::Queue-Mode "access";
APT::Acquire::Retries 3;
```

## Version pinning

See [`apt_preferences`'s man page][apt_preferences man page] for more information.

```txt
# /etc/apt/preferences.d/99perl
Package: perl
Pin: version 5.20*
Pin-Priority: 1001
```

```txt
# /etc/apt/preferences.d/90debian-release
Package: *
Pin: release a=testing
Pin-Priority: 990
Package: *
Pin: release a=stable
Pin-Priority: 500
Package: *
Pin: release a=unstable
Pin-Priority: -1
```

```txt
# /etc/apt/preferences.d/boinc
Package: boinc boinc-client boinc-manager libboinc7
Pin: release a=unstable
Pin-Priority: 995
```

## Troubleshooting

### Fix a "Problem with MergeList" or "status file could not be parsed" error

> E: Encountered a section with no Package: header
> E: Problem with MergeList /var/lib/apt/lists/deb.debian.org_debian_dists_bullseye_main_i18n_Translation-en
> E: The package lists or status file could not be parsed or opened.

```sh
sudo rm -vrf '/var/lib/apt/lists/'*
sudo apt update
```

## Further readings

- [Apt configuration]
- [Configuring Apt sources]
- [Unattended Upgrades]
- [`apt_preferences`'s man page][apt_preferences man page]
- [`dpkg`][dpkg]
- [`apt-file`][apt-file] to look for files in packages
- [`netselect-apt`][netselect-apt] to select the fastest APT mirror

## Sources

All the references in the [further readings] section, plus the following:

- [cheat.sh]
- [Fix a "Problem with MergeList" or "status file could not be parsed" error]

<!-- project's references -->
[apt configuration]: https://wiki.debian.org/AptConfiguration
[apt_preferences man page]: https://manpages.debian.org/testing/apt/apt_preferences.5.en.html
[configuring apt sources]: https://wiki.debian.org/SourcesList
[unattended upgrades]: https://wiki.debian.org/UnattendedUpgrades

<!-- internal references -->
[apt-file]: apt-file.md
[dpkg]: dpkg.md
[netselect-apt]: netselect-apt.md

<!-- external references -->
[cheat.sh]: https://cheat.sh/apt
[fix a "problem with mergelist" or "status file could not be parsed" error]: https://askubuntu.com/questions/30072/how-do-i-fix-a-problem-with-mergelist-or-status-file-could-not-be-parsed-err#30199