Installing and Removing Software on Ubuntu

The process of installing and removing software is one of the greatest strengths of Linux. The majority of software you will need as an administrator is able to be installed from a trusted repository using a package manager. This process automates most of what can be challenging about installing and removing software and makes removal of troublesome software complete and easy.

Commands

• dpkg

• apt

Configuration

• /etc/apt/sources.list

• /etc/apt/trusted.gpg

Introduction

Installing and removing software on Linux means making yourself familiar with the package manager on your system. There are several package managers on Linux and people argue over which one is the best. They all have strengths and weaknesses. Your choice of Linux distribution comes with the distribution’s preferred package manager. This lecture will focus on Debian’s package manager called apt, but the concepts transfer directly to other distributions. Below is a table of Linux distributions and their package managers:

Package Manager	Distributions
Advanced Package Manager (apt)	Debian and derivatives including Ubuntu, Raspbian (Raspberry Pi), Knoppix, Linux Mint, Security Onion and Kali Linux
Redhat Package Manager (rpm)	Fedora, RedHat Enterprise Linux and CentOS, SuSE Linux
Itsy Package Manager (ipkg)	OpenWRT (some versions), Openmoko, webOS, QNAP Linux NAS appliances
Open Package Manager (opkg)	OpenWRT (some versions), Open Embedded, Yocto, Angstrom
Package Manager (pacman)	Arch Linux

The list isn’t complete and all package managers aren’t as full-featured as apt is. What’s common to them all are some basic operations:

• Adding and removing package repositories

• Installing and removing packages

• Keeping packages updated

• Finding what packages are installed

• Resolving package dependencies

• Finding installed files

• Fixing broken packages

The Package Index

The apt command needs to know what packages are available to your system and what they depend on. That information comes from the repositories that your VM is configured to use (more on that later). When your VM is freshly created with Vagrant package information is missing. The apt update command retreives package lists.

$ apt update 

The update command does not update packages, that’s the job of apt upgrade.

Understanding Dependencies

When a program in one package uses libraries or commands from another it’s said to depend on the other package. For example, in order to use Firefox you must have a graphical desktop (Firefox doesn’t have a command line interface). Therefore the Firefox package depends on Xwindows and you cannot install Firefox if X-windows is not installed. Also, you cannot remove X-windows without first removing Firefox. The package manager understands these dependencies and does two important things for you: It automatically installs dependencies when you ask for a package and it prevents you from breaking a package by uninstalling something it depends on.

To see a package’s dependencies do this:

apt-cache depends <package>

For example:

$ apt depends firefox 
firefox
  Depends: lsb-release
  Depends: libatk1.0-0 (>= 1.12.4)
  Depends: libc6 (>= 2.18)
  Depends: libcairo-gobject2 (>= 1.10.0)
  Depends: libcairo2 (>= 1.10.0)
  Depends: libdbus-1-3 (>= 1.9.14)
  Depends: libdbus-glib-1-2 (>= 0.78)
  Depends: libfontconfig1 (>= 2.11.94)
  Depends: libfreetype6 (>= 2.3.5)
  Depends: libgcc1 (>= 1:4.2)
  Depends: libgdk-pixbuf2.0-0 (>= 2.22.0)
  Depends: libglib2.0-0 (>= 2.31.8)
  Depends: libgtk-3-0 (>= 3.4)
  Depends: libpango-1.0-0 (>= 1.22.0)
  Depends: libpangocairo-1.0-0 (>= 1.14.0)
  Depends: libstartup-notification0 (>= 0.8)
  Depends: libstdc++6 (>= 4.6)
  Depends: libx11-6
  Depends: libx11-xcb1
  Depends: libxcb-shm0
  Depends: libxcb1
  Depends: libxcomposite1 (>= 1:0.3-1)
  Depends: libxdamage1 (>= 1:1.1)
  Depends: libxext6
  Depends: libxfixes3
  Depends: libxrender1
  Depends: libxt6
  Recommends: xul-ext-ubufox
  Recommends: libcanberra0
  Recommends: libdbusmenu-glib4
  Recommends: libdbusmenu-gtk3-4
  Suggests: fonts-lyx
  Replaces: <kubuntu-firefox-installer>

You can also see what packages list a package as a dependency (a reverse-dependency):

$ apt rdepends <package>

Packages may also depend on a particular versionof another package. The software on Linux is always changing and being updated. Most of the time new versions of software are backwards compatible with older versions. When a version is not backwards compatible that creates a problem for packages that depend on it: Use the old version or use the new version. When this happens Ubuntu usually tries to have both versions installed. When that’s impossible it’s possible that your packages can get stuck. You can’t install something without first removing something else.

Managing Repositories

A repository is an online source of packages. By default Ubuntu comes installed with repositories from Canonical Ltd (the company behind Ubuntu). The default repositories are ever changing with new packages and updated versions of existing packages. A key role of an administrator is keeping software up to date. The apt commands from the previous section (as its name implies) uses the cache stored on your local disk. The cache is built from a list of all packages available online. The following command updates the list of packages available and rebuilds the cache:

$ sudo apt update

The update command only updates the cache, not the packages. I’ll talk about updating packages later. It’s always safe to run the apt update command and it’s a good thing to do before you run any other apt commands. Running apt update shows you what repositories you have enabled. Those repositories are listed in your /etc/apt/sources.list file. Here’s a few lines from the default sources.list on Xenial:

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu xenial main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu xenial-updates main restricted

Each ‘deb’ line tells apt where to find binary packages and ‘deb-src’ lines tells apt where to find the source code for those packages. Most open source software requires people who distribute binary versions of software also make the source code available. This is a key part of what it means to be open source. It’s common to have third party repositories installed. Software vendors that support Linux are increasingly realizing that using the package manager is a great way to distribute your software and keep it up to date. Installing Google Chrome on Linux works by installing this file:

$ cat /etc/apt/sources.list.d/google-chrome.list
### THIS FILE IS AUTOMATICALLY CONFIGURED ###
# You may comment out this entry, but any other modifications may be lost.
deb http://dl.google.com/linux/chrome/deb/ stable main

After adding this deb line the next apt-get update will “see” Google Chrome and related packages as able to be installed. Also, updates will be done automatically with the rest of system.

Repositories and Trust

One of the great strengths of the repository system is that it’s resistant to the malware problems that make installing software on Windows a nightmare. Software from repositories is signed to protect it from being altered by someone who can intercept your network traffic. Before you can use a repository you must import its public key into your list of trusted keys. Be careful when you do this, once you do you will automatically trust all software from the repository. To see what repositories you trust do this:

$ apt-key list
/etc/apt/trusted.gpg
--------------------
pub   1024D/437D05B5 2004-09-12
uid                  Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
sub   2048g/79164387 2004-09-12

pub   4096R/C0B21F32 2012-05-11
uid                  Ubuntu Archive Automatic Signing Key (2012) <ftpmaster@ubuntu.com>

pub   4096R/EFE21092 2012-05-11
uid                  Ubuntu CD Image Automatic Signing Key (2012) <cdimage@ubuntu.com>

pub   1024D/FBB75451 2004-12-30
uid                  Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>

See the manual page for apt-key to see how to add and remove keys.

Installing Software

Installing and removing software is easy! To install a package simply run:

apt install <package-name>

Knowing what package you want is a bit more tricky. When you execute a command that doesn’t exist but could if you installed a package Ubuntu very helpfully tells you what package you might need:

$ ntop
The program 'ntop' is currently not installed. You can install it by typing:
sudo apt-get install ntop

This is one of the things I love most about Ubuntu. This is not just a nicety for n00bs it’s something that this seasoned admin finds useful all the time. Sometimes, however, the help isn’t enough to locate the package you need. When that happens you can search the apt cache for a package that matches your input:

$ apt search ntop

Notice that related packages are also returned. These packages may have the word “ntop” somewhere in their description. The apt-cache command will also show you detailed information about a package:

$ apt show ntop 
Package: ntop
Version: 3:5.0.1+dfsg1-2.2ubuntu1
Priority: optional
Section: universe/net
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Ludovico Cavedon <cavedon@debian.org>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 1,613 kB
Depends: libc6 (>= 2.15), libgdbm3 (>= 1.8.3), libgeoip1, libpcap0.8 (>= 0.9.8), libpython2.7 (>= 2.7), librrd4 (>= 1.3.0), zlib1g (>= 1:1.1.4), debconf (>= 0.5) | debconf-2.0, ntop-data (= 3:5.0.1+dfsg1-2.2ubuntu1), python-mako, net-tools, adduser, passwd
Suggests: graphviz, gsfonts
Homepage: http://www.ntop.org/
Download-Size: 590 kB
APT-Sources: http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
Description: display network usage in web browser
 ntop is a network traffic probe that shows the network usage, similar to what
 the popular top Unix command does. ntop is based on libpcap and it has been
 written in a portable way in order to virtually run on every Unix platform and
 on Win32 as well.
 .
 ntop users can use a web browser to navigate through ntop (that acts as a
 web server) traffic information and get a dump of the network status. In the
 latter case, ntop can be seen as a simple RMON-like agent with an embedded web
 interface.
 The use of:
  * a web interface
  * limited configuration and administration via the web interface
  * reduced CPU and memory usage (they vary according to network size
  and traffic)
 make ntop easy to use and suitable for monitoring various kind of networks.
 .
 This package contains the ntop daemon.

Installing ntop installs it’s dependencies automatically (though if it’s a lot of packages apt-get will ask you to confirm that you still want to install):

$ sudo apt install ntop 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  fontconfig fontconfig-config fonts-dejavu-core libcairo2 libdatrie1 libdbi1 libfontconfig1 libgraphite2-3 libharfbuzz0b libpango-1.0-0 libpangocairo-1.0-0 libpangoft2-1.0-0
  libpixman-1-0 libpython-stdlib libpython2.7 libpython2.7-minimal libpython2.7-stdlib librrd4 libthai-data libthai0 libxcb-render0 libxcb-shm0 libxrender1 ntop-data python
  python-mako python-markupsafe python-minimal python2.7 python2.7-minimal
Suggested packages:
  graphviz gsfonts geoip-database-contrib libjs-jquery libjs-jquery-ui python-doc python-tk python-beaker python-mako-doc python2.7-doc binutils binfmt-support
The following NEW packages will be installed:
  fontconfig fontconfig-config fonts-dejavu-core libcairo2 libdatrie1 libdbi1 libfontconfig1 libgraphite2-3 libharfbuzz0b libpango-1.0-0 libpangocairo-1.0-0 libpangoft2-1.0-0
  libpixman-1-0 libpython-stdlib libpython2.7 libpython2.7-minimal libpython2.7-stdlib librrd4 libthai-data libthai0 libxcb-render0 libxcb-shm0 libxrender1 ntop ntop-data
  python python-mako python-markupsafe python-minimal python2.7 python2.7-minimal
0 upgraded, 31 newly installed, 0 to remove and 77 not upgraded.
Need to get 9,767 kB of archives.
After this operation, 37.6 MB of additional disk space will be used.
Do you want to continue? [Y/n] 

Removing Software

So what if we get tired of ntop? We can remove it just as easily as we installed it. There are three apt commands that are used to remove software, and they are subtly different.

Command	Use
apt remove <package>...	Removes the package from the system, leaves any configuration for the package intact so a re-install will be configured the same way.
apt purge <package>...	Removes the package and its configuration.
apt autoremove [<package>...]	Removes the packages (if listed) and any packages that were installed to satisfy dependencies and are no longer required.

Let’s purge ntop:

$ sudo apt purge ntop 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  fontconfig fontconfig-config fonts-dejavu-core libcairo2 libdatrie1 libdbi1 libfontconfig1 libgraphite2-3 libharfbuzz0b libpango-1.0-0 libpangocairo-1.0-0 libpangoft2-1.0-0
  libpixman-1-0 libpython2.7 librrd4 libthai-data libthai0 libxcb-render0 libxcb-shm0 libxrender1 ntop-data python-mako python-markupsafe
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
  ntop*
0 upgraded, 0 newly installed, 1 to remove and 77 not upgraded.
After this operation, 1,613 kB disk space will be freed.
Do you want to continue? [Y/n] 

What about the dependencies that were installed? They were not removed automatically. If you want to get rid of them run the command:

$ sudo apt autoremove
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  fontconfig fontconfig-config fonts-dejavu-core libcairo2 libdatrie1 libdbi1 libfontconfig1 libgraphite2-3 libharfbuzz0b libpango-1.0-0 libpangocairo-1.0-0 libpangoft2-1.0-0
  libpixman-1-0 libpython2.7 librrd4 libthai-data libthai0 libxcb-render0 libxcb-shm0 libxrender1 ntop-data python-mako python-markupsafe
0 upgraded, 0 newly installed, 23 to remove and 77 not upgraded.
After this operation, 19.4 MB disk space will be freed.
Do you want to continue? [Y/n] 

The autoremove command removes all packages that were installed to meet dependencies but are no longer necessary. It’s a nice way to tidy up after removing something.

Metapackages

There is a special kind of package called a metapackage. A metapackage is a package that doesn’t install any software but it depends on a bunch of other packages. Metapackages are useful if you want to install a bundle of software related to a particular task or function. If you want to convert your Ubuntu Server VM to an Ubuntu Desktop VM all you have to do is install the ubuntu-desktop metapackage.

$ sudo apt install ubuntu-desktop

This will install the almost 2G of packages needed to convert Ubuntu Server to Ubuntu Desktop. You can convert it back:

$ sudo apt autoremove ubuntu-desktop

Updating to the Latest Versions

Desktop Ubuntu uses a graphical updater to help you keep your software up to date. On Ubuntu Server you must do this process by hand. Luckily it’s easy. The first step is to make sure you have the latest packages available:

$ sudo apt update

Next you ask apt to upgrade any packages that have newer versions in the repository:

$ sudo apt upgrade

The upgrade command will not install some packages if those packages require that another package be removed. To upgrade all packages run:

$ sudo apt full-upgrade

Pro Tips

Here are a few commands that every Ubuntu admin should know. To list all packages, installed and available:

dpkg -l

If you want to filter just on installed packages use grep:

$ dpkg -l | grep -e '^ii'

Packages come in *.deb files. You can install a package that’s not in a repository directly using dpkg:

dpkg -i <packagefile.deb>

This won’t automatically install dependencies and will fail if dependencies are missing. If dpkg fails manually install dependencies using apt-get. To find out what package a file belongs to do this:

dpkg -S <path-to-file>

For example:

$ dpkg -S /bin/cat
coreutils: /bin/cat

To see what files a package installed:

dpkg-query -L <package-name>

For example:

$ dpkg-query -L bash
/.
/etc
/etc/bash.bashrc
/etc/skel
/etc/skel/.profile
/etc/skel/.bash_logout
/etc/skel/.bashrc
/bin
/bin/bash
/usr
[...output snipped...]

There are many other useful things to know about apt. If you use Ubuntu Desktop I suggest installing the synaptic program which gives you a GUI that you can use to explore and install packages. It’s the first thing I install after a fresh install of Ubuntu.