102.4 Lesson 1

Certificate:

LPIC-1

Version:

5.0

Topic:

102 Linux Installation and Package Management

Objective:

102.4 Use Debian package management

Lesson:

1 of 1

Introduction

A long time ago, when Linux was still in its infancy, the most common way to distribute software was a compressed file (usually a .tar.gz archive) with source code, which you would unpack and compile yourself.

However, as the amount and complexity of software grew, the need for a way to distribute pre-compiled software became clear. After all, not everyone had the resources, both in time and computing power, to compile large projects like the Linux kernel or an X Server.

Soon, efforts to standardize a way to distribute these software “packages” grew, and the first package managers were born. These tools made it much easier to install, configure or remove software from a system.

One of those was the Debian package format (.deb) and its package tool (dpkg). Today, they are widely used not only on Debian itself, but also on its derivatives, like Ubuntu and those derived from it.

Another package management tool that is popular on Debian-based systems is the Advanced Package Tool (apt), which can streamline many of the aspects of the installation, maintenance and removal of packages, making it much easier.

In this lesson, we will learn how to use both dpkg and apt to obtain, install, maintain and remove software on a Debian-based Linux system.

The Debian Package Tool (dpkg)

The Debian Package (dpkg) tool is the essential utility to install, configure, maintain and remove software packages on Debian-based systems. The most basic operation is to install a .deb package, which can be done with:

$ dkpg -i PACKAGENAME

Where PACKAGENAME is the name of the .deb file you want to install.

Package upgrades are handled the same way. Before installing a package, dpkg will check if a previous version already exists in the system. If so, the package will be upgraded to the new version. If not, a fresh copy will be installed.

Dealing with Dependencies

More often than not, a package may depend on others to work as intended. For example, an image editor may need libraries to open JPEG files, or another utility may need a widget toolkit like Qt or GTK for its user interface.

dpkg will check if those dependencies are installed on your system, and will fail to install the package if they are not. In this case, dpkg will list which packages are missing. However it cannot solve dependencies by itself. It is up to the user to find the .deb packages with the corresponding dependencies and install them.

In the example below, the user tries to install the OpenShot video editor package, but some dependencies were missing:

$ dpkg -i openshot-qt_2.4.3+dfsg1-1_all.deb
(Reading database ... 269630 files and directories currently installed.)
Preparing to unpack openshot-qt_2.4.3+dfsg1-1_all.deb ...
Unpacking openshot-qt (2.4.3+dfsg1-1) over (2.4.3+dfsg1-1) ...
dpkg: dependency problems prevent configuration of openshot-qt:
 openshot-qt depends on fonts-cantarell; however:
  Package fonts-cantarell is not installed.
 openshot-qt depends on python3-openshot; however:
  Package python3-openshot is not installed.
 openshot-qt depends on python3-pyqt5; however:
  Package python3-pyqt5 is not installed.
 openshot-qt depends on python3-pyqt5.qtsvg; however:
  Package python3-pyqt5.qtsvg is not installed.
 openshot-qt depends on python3-pyqt5.qtwebkit; however:
  Package python3-pyqt5.qtwebkit is not installed.
 openshot-qt depends on python3-zmq; however:
  Package python3-zmq is not installed.

dpkg: error processing package openshot-qt (--install):
 dependency problems - leaving unconfigured
Processing triggers for mime-support (3.60ubuntu1) ...
Processing triggers for gnome-menus (3.32.0-1ubuntu1) ...
Processing triggers for desktop-file-utils (0.23-4ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for man-db (2.8.5-2) ...
Errors were encountered while processing:
 openshot-qt

As shown above, OpenShot depends on the fonts-cantarell, python3-openshot, python3-pyqt5, python3-pyqt5.qtsvg, python3-pyqt5.qtwebkit and python3-zmq packages. All of those need to be installed before the installation of OpenShot can succeed.

Removing Packages

To remove a package, pass the -r parameter to dpkg, followed by the package name. For example, the following command will remove the unrar package from the system:

$ dpkg -r unrar
(Reading database ... 269630 files and directories currently installed.)
Removing unrar (1:5.6.6-2) ...
Processing triggers for man-db (2.8.5-2) ...

The removal operation also runs a dependency check, and a package cannot be removed unless every other package that depends on it is also removed. If you try to do so, you will get an error message like the one below:

$ dpkg -r p7zip
dpkg: dependency problems prevent removal of p7zip:
 winetricks depends on p7zip; however:
  Package p7zip is to be removed.
 p7zip-full depends on p7zip (= 16.02+dfsg-6).

dpkg: error processing package p7zip (--remove):
 dependency problems - not removing
Errors were encountered while processing:
 p7zip

You can pass multiple package names to dpkg -r, so they will all be removed at once.

When a package is removed, the corresponding configuration files are left on the system. If you want to remove everything associated with the package, use the -P (purge) option instead of -r.

Note

You can force dpkg to install or remove a package, even if dependencies are not met, by adding the --force parameter like in dpkg -i --force PACKAGENAME. However, doing so will most likely leave the installed package, or even your system, in a broken state. Do not use --force unless you are absolutely sure of what you are doing.

Getting Package Information

To get information about a .deb package, such as its version, architecture, maintainer, dependencies and more, use the dpkg command with the -I parameter, followed by the filename of the package you want to inspect:

$ dpkg -I google-chrome-stable_current_amd64.deb
 new Debian package, version 2.0.
 size 59477810 bytes: control archive=10394 bytes.
    1222 bytes,    13 lines      control
   16906 bytes,   457 lines   *  postinst             #!/bin/sh
   12983 bytes,   344 lines   *  postrm               #!/bin/sh
    1385 bytes,    42 lines   *  prerm                #!/bin/sh
 Package: google-chrome-stable
 Version: 76.0.3809.100-1
 Architecture: amd64
 Maintainer: Chrome Linux Team 
 Installed-Size: 205436
 Pre-Depends: dpkg (>= 1.14.0)
 Depends: ca-certificates, fonts-liberation, libappindicator3-1, libasound2 (>= 1.0.16), libatk-bridge2.0-0 (>= 2.5.3), libatk1.0-0 (>= 2.2.0), libatspi2.0-0 (>= 2.9.90), libc6 (>= 2.16), libcairo2 (>= 1.6.0), libcups2 (>= 1.4.0), libdbus-1-3 (>= 1.5.12), libexpat1 (>= 2.0.1), libgcc1 (>= 1:3.0), libgdk-pixbuf2.0-0 (>= 2.22.0), libglib2.0-0 (>= 2.31.8), libgtk-3-0 (>= 3.9.10), libnspr4 (>= 2:4.9-2~), libnss3 (>= 2:3.22), libpango-1.0-0 (>= 1.14.0), libpangocairo-1.0-0 (>= 1.14.0), libuuid1 (>= 2.16), libx11-6 (>= 2:1.4.99.1), libx11-xcb1, libxcb1 (>= 1.6), libxcomposite1 (>= 1:0.3-1), libxcursor1 (>> 1.1.2), libxdamage1 (>= 1:1.1), libxext6, libxfixes3, libxi6 (>= 2:1.2.99.4), libxrandr2 (>= 2:1.2.99.3), libxrender1, libxss1, libxtst6, lsb-release, wget, xdg-utils (>= 1.0.2)
 Recommends: libu2f-udev
 Provides: www-browser
 Section: web
 Priority: optional
 Description: The web browser from Google
  Google Chrome is a browser that combines a minimal design with sophisticated technology to make the web faster, safer, and easier.

Listing Installed Packages and Package Contents

To get a list of every package installed on your system, use the --get-selections option, as in dpkg --get-selections. You can also get a list of every file installed by a specific package by passing the -L PACKAGENAME parameter to dpkg, like below:

$ dpkg -L unrar
/.
/usr
/usr/bin
/usr/bin/unrar-nonfree
/usr/share
/usr/share/doc
/usr/share/doc/unrar
/usr/share/doc/unrar/changelog.Debian.gz
/usr/share/doc/unrar/copyright
/usr/share/man
/usr/share/man/man1
/usr/share/man/man1/unrar-nonfree.1.gz

Finding Out Which Package Owns a Specific File

Sometimes you may need to find out which package owns a specific file in your system. You can do so by using the dpkg-query utility, followed by the -S parameter and the path to the file in question:

$ dpkg-query -S /usr/bin/unrar-nonfree
unrar: /usr/bin/unrar-nonfree

Reconfiguring Installed Packages

When a package is installed there is a configuration step called post-install where a script runs to set-up everything needed for the software to run such as permissions, placement of configuration files, etc. This may also ask some questions of the user to set preferences on how the software will run.

Sometimes, due to a corrupt or malformed configuration file, you may wish to restore a package’s settings to its “fresh” state. Or you may wish to change the answers you gave to the initial configuration questions. To do this run the dpkg-reconfigure utility, followed by the package name.

This program will back-up the old configuration files, unpack the new ones in the correct directories and run the post-install script provided by the package, as if the package had been installed for the first time.

Advanced Package Tool (apt)

The Advanced Package Tool (APT) is a package management system, including a set of tools, that greatly simplifies package installation, upgrade, removal and management. APT provides features like advanced search capabilities and automatic dependency resolution.

APT is not a “substitute” for dpkg. You may think of it as a “front end”, streamlining operations and filling gaps in dpkg functionality, like dependency resolution.

APT works in concert with software repositories which contain the packages available to install. Such repositories may be a local or remote server, or (less common) even a CD-ROM disc.

Linux distributions, such as Debian and Ubuntu, maintain their own repositories, and other repositories may be maintained by developers or user groups to provide software not available from the main distribution repositories.

There are many utilities that interact with APT, the main ones being:

apt-get: used to download, install, upgrade or remove packages from the system.
apt-cache: used to perform operations, like searches, in the package index.
apt-file: used for searching for files inside packages.

There is also a “friendlier” utility named simply apt, combining the most used options of apt-get and apt-cache in one utility. Many of the commands for apt are the same as the ones for apt-get, so they are in many cases interchangeable. However, since apt may not be installed on a system, it is recommended to learn how to use apt-get and apt-cache.

Note	`apt` and `apt-get` may require a network connection, because packages and package indexes may need to be downloaded from a remote server.

Updating the Package Index

Before installing or upgrading software with APT, it is recommended to update the package index first in order to retrieve information about new and updated packages. This is done with the apt-get command, followed by the update parameter:

$ apt-get update
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:2 https://repo.skype.com/deb stable InRelease
Hit:3 http://us.archive.ubuntu.com/ubuntu disco InRelease
Hit:4 http://repository.spotify.com stable InRelease
Hit:5 http://dl.google.com/linux/chrome/deb stable Release
Hit:6 http://apt.pop-os.org/proprietary disco InRelease
Hit:7 http://ppa.launchpad.net/system76/pop/ubuntu disco InRelease
Hit:8 http://us.archive.ubuntu.com/ubuntu disco-security InRelease
Hit:9 http://us.archive.ubuntu.com/ubuntu disco-updates InRelease
Hit:10 http://us.archive.ubuntu.com/ubuntu disco-backports InRelease
Reading package lists... Done

Tip	Instead of `apt-get update`, you can also use `apt update`.

Installing and Removing Packages

With the package index updated you may now install a package. This is done with apt-get install, followed by the name of the package you wish to install:

$ apt-get install xournal
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.

Similarly, to remove a package use apt-get remove, followed by the package name:

$ apt-get remove xournal
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  xournal
0 upgraded, 0 newly installed, 1 to remove and 75 not upgraded.
After this operation, 1041 kB disk space will be freed.
Do you want to continue? [Y/n]

Be aware that when installing or removing packages, APT will do automatic dependency resolution. This means that any additional packages needed by the package you are installing will also be installed, and that packages that depend on the package you are removing will also be removed. APT will always show what will be installed or removed before asking if you want to continue:

$ apt-get remove p7zip
Reading package lists... Done
Building dependency tree
The following packages will be REMOVED:
  android-libbacktrace android-libunwind android-libutils
  android-libziparchive android-sdk-platform-tools fastboot p7zip p7zip-full
0 upgraded, 0 newly installed, 8 to remove and 75 not upgraded.
After this operation, 6545 kB disk space will be freed.
Do you want to continue? [Y/n]

Note that when a package is removed the corresponding configuration files are left on the system. To remove the package and any configuration files, use the purge parameter instead of remove or the remove parameter with the --purge option:

$ apt-get purge p7zip

$ apt-get remove --purge p7zip

Tip	You can also use `apt install` and `apt remove`.

Fixing Broken Dependencies

It is possible to have “broken dependencies” on a system. This means that one or more of the installed packages depend on other packages that have not been installed, or are not present anymore. This may happen due to an APT error, or because of a manually installed package.

To solve this, use the apt-get install -f command. This will try to “fix” the broken packages by installing the missing dependencies, ensuring that all packages are consistent again.

Tip	You can also use `apt install -f`.

Upgrading Packages

APT can be used to automatically upgrade any installed packages to the latest versions available from the repositories. This is done with the apt-get upgrade command. Before running it, first update the package index with apt-get update:

$ apt-get update
Hit:1 http://us.archive.ubuntu.com/ubuntu disco InRelease
Hit:2 http://us.archive.ubuntu.com/ubuntu disco-security InRelease
Hit:3 http://us.archive.ubuntu.com/ubuntu disco-updates InRelease
Hit:4 http://us.archive.ubuntu.com/ubuntu disco-backports InRelease
Reading package lists... Done

$ apt-get upgrade
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
  cups-ipp-utils cups-ppdc cups-server-common firefox-locale-ar (...)

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]

The summary at the bottom of the output shows how many packages will be upgraded, how many will be installed, removed or kept back, the total download size and how much extra disk space will be needed to complete the operation. To complete the upgrade, just answer Y and wait for apt-get to finish the task.

To upgrade a single package, just run apt-get upgrade followed by the package name. As in dpkg, apt-get will first check if a previous version of a package is installed. If so, the package will be upgraded to the newest version available in the repository. If not, a fresh copy will be installed.

Tip	You can also use `apt upgrade` and `apt update`.

The Local Cache

When you install or update a package, the corresponding .deb file is downloaded to a local cache directory before the package is installed. By default, this directory is /var/cache/apt/archives. Partially downloaded files are copied to /var/cache/apt/archives/partial/.

As you install and upgrade packages, the cache directory can get quite large. To reclaim space, you can empty the cache by using the apt-get clean command. This will remove the contents of the /var/cache/apt/archives and /var/cache/apt/archives/partial/ directories.

Tip	You can also use `apt clean`.

Searching for Packages

The apt-cache utility can be used to perform operations on the package index, such as searching for a specific package or listing which packages contain a specific file.

To conduct a search, use apt-cache search followed by a search pattern. The output will be a list of every package that contains the pattern, either in its package name, description or files provided.

$ apt-cache search p7zip
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

In the example above, the entry liblzma5 - XZ-format compression library does not seem to match the pattern. However, if we show the full information, including description, for the package using the show parameter, we will find the pattern there:

$ apt-cache show liblzma5
Package: liblzma5
Architecture: amd64
Version: 5.2.4-1
Multi-Arch: same
Priority: required
Section: libs
Source: xz-utils
Origin: Ubuntu
Maintainer: Ubuntu Developers 
Original-Maintainer: Jonathan Nieder 
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 259
Depends: libc6 (>= 2.17)
Breaks: liblzma2 (<< 5.1.1alpha+20110809-3~)
Filename: pool/main/x/xz-utils/liblzma5_5.2.4-1_amd64.deb
Size: 92352
MD5sum: 223533a347dc76a8cc9445cfc6146ec3
SHA1: 8ed14092fb1caecfebc556fda0745e1e74ba5a67
SHA256: 01020b5a0515dbc9a7c00b464a65450f788b0258c3fbb733ecad0438f5124800
Homepage: https://tukaani.org/xz/
Description-en: XZ-format compression library
 XZ is the successor to the Lempel-Ziv/Markov-chain Algorithm
 compression format, which provides memory-hungry but powerful
 compression (often better than bzip2) and fast, easy decompression.
 .
 The native format of liblzma is XZ; it also supports raw (headerless)
 streams and the older LZMA format used by lzma. (For 7-Zip's related
 format, use the p7zip package instead.)

You can use regular expressions with the search pattern, allowing for very complex (and precise) searches. However, this topic is out of scope for this lesson.

Tip	You can also use `apt search` instead of `apt-cache search` and `apt show` instead of `apt-cache show`.

The Sources List

APT uses a list of sources to know where to get packages from. This list is stored in the file sources.list, located inside the /etc/apt directory. This file can be edited directly with a text editor, like vi, pico or nano, or with graphical utilities like aptitude or synaptic.

A typical line inside sources.list looks like this:

deb http://us.archive.ubuntu.com/ubuntu/ disco main restricted universe multiverse

The syntax is archive type, URL, distribution and one or more components, where:

Archive type

A repository may contain packages with ready-to-run software (binary packages, type deb) or with the source code to this software (source packages, type deb-src). The example above provides binary packages.

URL

The URL for the repository.

Distribution

The name (or codename) for the distribution for which packages are provided. One repository may host packages for multiple distributions. In the example above, disco is the codename for Ubuntu 19.04 Disco Dingo.

Components

Each component represents a set of packages. These components may be different on different Linux distributions. For example, on Ubuntu and derivatives, they are:

main: contains officially supported, open-source packages.
restricted: contains officially supported, closed-source software, like device drivers for graphic cards, for example.
universe: contains community maintained open-source software.
multiverse: contains unsupported, closed-source or patent-encumbered software.

On Debian, the main components are:
main: consists of packages compliant with the Debian Free Software Guidelines (DFSG), which do not rely on software outside this area to operate. Packages included here are considered to be part of the Debian distribution.
contrib: contains DFSG-compliant packages, but which depend on other packages that are not in main.
non-free: contains packages that are not compliant with the DFSG.
security: contains security updates.
backports: contains more recent versions of packages that are in main. The development cycle of the stable versions of Debian is quite long (around two years), and this ensures that users can get the most up-to-date packages without having to modify the main core repository.

Note	You can learn more about the Debian Free Software Guidelines at: https://www.debian.org/social_contract#guidelines

To add a new repository to get packages from, you can simply add the corresponding line (usually provided by the repository maintainer) to the end of sources.list, save the file and reload the package index with apt-get update. After that, the packages in the new repository will be available for installation using apt-get install.

Keep in mind that lines starting with the # character are considered comments, and are ignored.

The `/etc/apt/sources.list.d` Directory

Inside the /etc/apt/sources.list.d directory you can add files with additional repositories to be used by APT, without the need to modify the main /etc/apt/sources.list file. These are simple text files, with the same syntax described above and the .list file extension.

Below you see the contents of a file called /etc/apt/sources.list.d/buster-backports.list:

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

Listing Package Contents and Finding Files

A utility called apt-file can be used to perform more operations in the package index, like listing the contents of a package or finding a package that contains a specific file. This utility may not be installed by default in your system. In this case, you can usually install it using apt-get:

$ apt-get install apt-file

After installation, you will need to update the package cache used for apt-file:

$ apt-file update

This usually takes only a few seconds. After that, you are ready to use apt-file.

To list the contents of a package, use the list parameter followed by the package name:

$ apt-file list unrar
unrar: /usr/bin/unrar-nonfree
unrar: /usr/share/doc/unrar/changelog.Debian.gz
unrar: /usr/share/doc/unrar/copyright
unrar: /usr/share/man/man1/unrar-nonfree.1.gz

Tip	You can also use `apt list` instead of `apt-file list`.

You can search all packages for a file using the search parameter, followed by the file name. For example, if you wish to know which package provides a file called libSDL2.so, you can use:

$ apt-file search libSDL2.so
libsdl2-dev: /usr/lib/x86_64-linux-gnu/libSDL2.so

The answer is the package libsdl2-dev, which provides the file /usr/lib/x86_64-linux-gnu/libSDL2.so.

The difference between apt-file search and dpkg-query is that apt-file search will also search uninstalled packages, while dpkg-query can only show files that belong to an installed package.

Guided Exercises

What is the command to install a package named package.deb using dpkg?
Using dpkg-query, find which package contains a file named 7zr.1.gz.
Can you remove a package called unzip from the system using dpkg -r unzip if the package file-roller depends on it? If not, what would be the correct way to do it?
Using apt-file, how can you find out which package contains the file unrar?
Using apt-cache, what is the command to show information for the package gimp?

Explorational Exercises

Consider a repository with Debian source packages for the xenial distribution, hosted at http://us.archive.ubuntu.com/ubuntu/ and with packages for the universe component. What would be the corresponding line to be added to /etc/apt/sources.list?
While compiling a program, you come across an error message complaining that the header file zzip-io.h is not present in your system. How can you find out with package provides that file?
How can you ignore a dependency warning and remove a package using dpkg, even if there are packages that depend on it in the system?
How can you get more information about a package called midori using apt?
Before installing or updating packages with apt, which command should be used to ensure that the package index is up-to-date?

Summary

In this lesson, you learned:

How to use dpkg to install and remove packages.
How to list installed packages and package contents.
How to reconfigure an installed package.
What is apt, and how to install, upgrade and remove packages using it.
How to use apt-cache to search for packages.
How the /etc/apt/sources.list file works.
How to use apt-file to show the contents of a package, or how to find which package contains a specific file.

The following commands were discussed:

dpkg -i: Installs a single package, or a space-separated list of packages.
dpkg -r: Removes a package, or a space-separated list of packages.
dpkg -I: Inspects a package, providing details on the software it installs and any needed dependencies.
dpkg --get-selections: Lists out every package that dpkg has installed on the system.
dpkg -L: Prints out a list of every file that a particular package installs.
dpkg-query: With a specified file name, this command will print out the package that installed the file.
dpkg-reconfigure: This command will re-run a packages post-install script so that an administrator can make configuration adjustments to the package’s installation.
apt-get update: This command will update the local package index to match what is available within the configured repositories under the /etc/apt/ directory.
apt-get install: This command will download a package from a remote repository and install it along with its dependencies, can also be used to install a Debian package that has already been downloaded.
apt-get remove: This command will uninstall the specified package(s) from the system.
apt-cache show: Just like the dpkg -I command, this command can be used to show details on a specific package.
apt-cache search: This command will search your local APT cached database for a particular package.
apt-file update: This command will update the package cache so that the apt-file command can query its contents.
apt-file search: This command can search for the name of a package that has installed a particular file, just like the dpkg-query command.
apt-file list: This command is used to list the contents of a package, just like the dpkg -L command.

Answers to Guided Exercises

What is the command to install a package named package.deb using dpkg?

Pass the -i parameter to dpkg:
```
$ dpkg -i package.deb
```
Using dpkg-query, find which package contains a file named 7zr.1.gz.

Add the -S parameter to dpkg-query:
```
$ dpkg-query -S 7zr.1.gz
```
Can you remove a package called unzip from the system using dpkg -r unzip if the package file-roller depends on it? If not, what would be the correct way to do it?

No. dpkg will not resolve dependencies, and will not let you remove a package if another installed package depends on it. In this example, you could first remove file-roller (assuming nothing depends on it) and then remove unzip, or remove both at the same time with:
```
$ dpkg -r unzip file-roller
```
How can you find out which package contains the file /usr/bin/unrar using the apt-file utility?

Use the search parameter followed by the path (or filename):
```
$ apt-file search /usr/bin/unrar
```
Using apt-cache, what is the command to show information for the package gimp?

Use the show parameter followed by the package name:
```
$ apt-cache show gimp
```

Answers to Explorational Exercises

Consider a repository with Debian source packages for the xenial distribution, hosted at http://us.archive.ubuntu.com/ubuntu/ and with packages for the universe component. What would be the corresponding line to be added to /etc/apt/sources.list?

Source packages are of the deb-src type, so the line should be:
```
deb-src http://us.archive.ubuntu.com/ubuntu/ xenial universe
```
This line could also be added inside a .list file in /etc/apt/sources.list.d/. The name is up to you but should be descriptive, something like xenial_sources.list.
While compiling a program, you come across an error message complaining that the header file zzip-io.h is not present on your system. How can you find out with package provides that file?

Use apt-file search to find which package contains a file not present in the system:
```
$ apt-file search zzip-io.h
```
How can you ignore a dependency warning and remove a package using dpkg, even if there are packages that depend on it in the system?

The parameter --force can be used, but this should never be done unless you know exactly what you are doing, since there is a great risk that your system will be left in an inconsistent or “broken” state.
How can you get more information about a package called midori using apt-cache?

Use apt-cache show followed by the package name:
```
$ apt-cache show midori
```
Before installing or updating packages with apt-get, which command should be used to ensure that the package index is up-to-date?

apt-get update should be used. This will download the latest package indexes from the repositories described in the /etc/apt/sources.list file or in the /etc/apt/sources.list.d/ directory.