The dpkg command

Firstly, the dpkg utility is a low-level system tool to extract, analyze, unpack, install, and remove packages with a .deb extension. The scripts read by dpkg inside each .deb file are important as they give information to the program regarding the packages' installation, removal, and configuration. The dpkg utility resides at the base of the package management system in Debian-based distributions. The Debian package, dpkg, provides the dpkg utility, as well as several other programs necessary for runtime functioning of the packaging system; namely: dpkg-deb, dpkg-split, dpkg-query, dpkg-statoverride, dpkg-divert, and dpkg-trigger. We can take a glance at the /var/log/dpkg.log file. There are a lot of verbose details about triggers and packages going through varying stages of unpacking and configuration. 

Let's take a look at /var/log/dpkg.log:

philip@ubuntu:~$ cat /var/log/dpkg.log
2018-07-02 06:43:57 startup archives unpack
2018-07-02 06:44:01 install linux-image-4.4.0-130-generic:amd64 <none> 4.4.0-130.156
2018-07-02 06:44:01 status half-installed linux-image-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:09 status unpacked linux-image-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:09 status unpacked linux-image-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:09 install linux-image-extra-4.4.0-130-generic:amd64 <none> 4.4.0-130.156
2018-07-02 06:44:09 status half-installed linux-image-extra-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:20 status unpacked linux-image-extra-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:20 status unpacked linux-image-extra-4.4.0-130-generic:amd64 4.4.0-130.156
2018-07-02 06:44:21 upgrade linux-generic:amd64 4.4.0.128.134 4.4.0.130.136
2018-07-02 06:44:21 status half-configured linux-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status unpacked linux-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status half-installed linux-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status half-installed linux-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status unpacked linux-generic:amd64 4.4.0.130.136
2018-07-02 06:44:21 status unpacked linux-generic:amd64 4.4.0.130.136
2018-07-02 06:44:21 upgrade linux-image-generic:amd64 4.4.0.128.134 4.4.0.130.136
2018-07-02 06:44:21 status half-configured linux-image-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status unpacked linux-image-generic:amd64 4.4.0.128.134
2018-07-02 06:44:21 status half-installed linux-image-generic:amd64 4.4.0.128.134

From the preceding output, we have learned about the various packages that the dpkg utility is managing. If we want to see a list of packages on this system, we can use the l option:

philip@ubuntu:~$ dpkg -l
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                          Version             Architecture        Description
+++-=============================-===================-===================-================================================================
ii  a11y-profile-manager-indicato 0.1.10-0ubuntu3      amd64               Accessibility Profile Manager - Unity desktop indicator
ii  account-plugin-facebook       0.12+16.04.20160126 all                 GNOME Control Center account plugin for single signon - facebook
ii  account-plugin-flickr         0.12+16.04.20160126 all                 GNOME Control Center account plugin for single signon - flickr
ii  account-plugin-google         0.12+16.04.20160126 all                 GNOME Control Center account plugin for single signon
ii  accountsservice               0.6.40-2ubuntu11.3  amd64               query and manipulate user account information
ii  activity-log-manager          0.9.7-0ubuntu23.16. amd64               blacklist configuration user interface for Zeitgeist
ii  adduser                       3.113+nmu3ubuntu4   all                 add and remove users and groups
ii  adium-theme-ubuntu            0.3.4-0ubuntu1.1    all                 Adium message style for Ubuntu
ii  app-install-data              15.10               all                 Ubuntu applications (data files)
ii  app-install-data-partner      16.04               all                 Application Installer (data files for partner applications/repos
ii  apparmor                      2.10.95-0ubuntu2.9  amd64               user-space parser utility for AppArmor
ii  appmenu-qt:amd64              0.2.7+14.04.2014030 amd64               application menu for Qt
ii  appmenu-qt5                   0.3.0+16.04.2017021 amd64               application menu for Qt5
ii  apport                        2.20.1-0ubuntu2.18  all                 automatically generate crash reports for debugging
ii  apport-gtk                    2.20.1-0ubuntu2.18  all                 GTK+ frontend for the apport crash report system

In the preceding output, we read this from left to right. We should now focus our attention on the far right of the output. This is the description section; the packages are presented in a human-readable summary for each package that is currently installed onto the system.

We can also narrow down our output by filtering the dpkg command; let's look for the xterm program:

philip@ubuntu:~$ dpkg -l xterm
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                          Version             Architecture        Description
+++-=============================-===================-===================-================================================================
ii  xterm                         322-1ubuntu1        amd64               X terminal emulator
philip@ubuntu:~$        

We can verify whether or not a package is installed with --get-selections:

philip@ubuntu:~$ dpkg --get-selections
a11y-profile-manager-indicator    install
account-plugin-facebook           install
account-plugin-flickr             install
account-plugin-google             install
accountsservice                   install
acl                               install
acpi-support                      install
acpid                             install
activity-log-manager              install
adduser                           install
adium-theme-ubuntu                install
adwaita-icon-theme                install
aisleriot                         install
alsa-base                         install
alsa-utils                        install
amd64-microcode                   install
anacron                           install
apg                               install
app-install-data                  install
app-install-data-partner          install
apparmor                          install
appmenu-qt:amd64                  install
appmenu-qt5                       install
apport                            install

We can view the locations in which a package is installed by using the L option. Let's continue with our example:

philip@ubuntu:~$ dpkg -L xterm
/.
/etc
/etc/X11
/etc/X11/app-defaults
/etc/X11/app-defaults/UXTerm-color
/etc/X11/app-defaults/UXTerm
/etc/X11/app-defaults/KOI8RXTerm-color
/etc/X11/app-defaults/KOI8RXTerm
/etc/X11/app-defaults/XTerm-color
/usr/share/man/man1/koi8rxterm.1.gz
/usr/share/man/man1/resize.1.gz
/usr/share/man/man1/xterm.1.gz
/usr/share/man/man1/lxterm.1.gz
philip@ubuntu:~$

We can search for a particular package in this system by using the s option:

philip@ubuntu:~$ dpkg -s apache
dpkg-query: package 'apache' is not installed and no information is available
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
philip@ubuntu:~$ dpkg --info apache
dpkg-deb: error: failed to read archive 'apache': No such file or directory
philip@ubuntu:~$

In this case, Apache is not installed by default on this system.

I've downloaded a tftp client for this demonstration. Let's verify that the tftp client isn't installed on this system:

philip@ubuntu:~/Downloads$ dpkg -l tftp
dpkg-query: no packages found matching tftp
philip@ubuntu:~/Downloads$

Now we'll install a package using the dpkg command. Let's try to install the tftp client package using the i option:

philip@ubuntu:~/Downloads$ dpkg -i tftp_0.17-18_i386.deb
dpkg: error: requested operation requires superuser privilege
philip@ubuntu:~/Downloads$

From the preceding output, you can see that we would need root privileges to install or remove a package. Let's retry as root: 

root@ubuntu:/home/philip/Downloads# ls -l | grep tftp
-rw-rw-r-- 1 philip philip  17208 Jul 18 08:15 tftp_0.17-18_i386.deb
root@ubuntu:/home/philip/Downloads#                              
root@ubuntu:/home/philip/Downloads# dpkg -i tftp_0.17-18_i386.deb
Selecting previously unselected package tftp:i386.
(Reading database ... 241431 files and directories currently installed.)
Preparing to unpack tftp_0.17-18_i386.deb ...
Unpacking tftp:i386 (0.17-18) ...
Setting up tftp:i386 (0.17-18) ...
Processing triggers for man-db (2.7.5-1) ...
root@ubuntu:/home/philip/Downloads#

Great! Now, let's retry the dpkg command with the l option:

root@ubuntu:/home/philip/Downloads# dpkg -l tftp
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                          Version             Architecture        Description
+++-=============================-===================-===================-================================================================
ii  tftp:i386                     0.17-18             i386                Trivial file transfer protocol client
root@ubuntu:/home/philip/Downloads#

Awesome! We can now see that our tftp client is listed. We can also run the dpkg with the --get-selections to verify the following:

root@ubuntu:/home/philip/Downloads# dpkg --get-selections | grep tftp
tftp:i386                                                            install
root@ubuntu:/home/philip/Downloads#

We can also remove a package using the dpkg command. Let's remove the tftp package which we installed in the previous example. We will use the -r option for this:

root@ubuntu:/home/philip/Downloads# dpkg -r tftp
(Reading database ... 241438 files and directories currently installed.)
Removing tftp:i386 (0.17-18) ...
Processing triggers for man-db (2.7.5-1) ...
root@ubuntu:/home/philip/Downloads#

Now, let's verify that the tftp package has indeed been uninstalled:

root@ubuntu:/home/philip/Downloads# dpkg -l tftp
dpkg-query: no packages found matching tftp
root@ubuntu:/home/philip/Downloads#

Great! However, when we use the -r option, it does not remove the configuration file(s). In order to remove the package, along with the configuration files, we should use the -P (purge) option. Here is how that works:

root@ubuntu:/home/philip/Downloads# dpkg -P tftp
(Reading database ... 241438 files and directories currently installed.)
Removing tftp:i386 (0.17-18) ...
Processing triggers for man-db (2.7.5-1) ...
root@ubuntu:/home/philip/Downloads#

We can also extract the content of a package without installing it. We should use the -x option for this:

root@ubuntu:/home/philip/Downloads# dpkg -x tftp_0.17-18_i386.deb ./tftp_0.17-18_i386
root@ubuntu:/home/philip/Downloads# ls
root@ubuntu:/home/philip/Downloads# ls tftp_0.17-18_i386
usr
root@ubuntu:/home/philip/Downloads# ls tftp_0.17-18_i386/usr/
bin  share
root@ubuntu:/home/philip/Downloads#
root@ubuntu:/home/philip/Downloads# ls tftp_0.17-18_i386/usr/bin/
tftp
root@ubuntu:/home/philip/Downloads# ls tftp_0.17-18_i386/usr/share/
doc/ man/
root@ubuntu:/home/philip/Downloads# ls tftp_0.17-18_i386/usr/share/
doc  man
root@ubuntu:/home/philip/Downloads#

Before we can download any packages and install them using the dpkg utility, we need to know the correct hardware architecture of the system. Fortunately, we can use the dpkg-architecture command:

root@ubuntu:/home/philip/Downloads# dpkg-architecture
DEB_BUILD_ARCH=amd64
DEB_BUILD_ARCH_BITS=64
DEB_BUILD_ARCH_CPU=amd64
DEB_BUILD_ARCH_ENDIAN=little
DEB_BUILD_ARCH_OS=linux
DEB_BUILD_GNU_CPU=x86_64
DEB_BUILD_GNU_SYSTEM=linux-gnu
DEB_BUILD_GNU_TYPE=x86_64-linux-gnu
DEB_TARGET_ARCH_CPU=amd64
DEB_TARGET_ARCH_ENDIAN=little
DEB_TARGET_ARCH_OS=linux
DEB_TARGET_GNU_CPU=x86_64
DEB_TARGET_GNU_SYSTEM=linux-gnu
DEB_TARGET_GNU_TYPE=x86_64-linux-gnu
DEB_TARGET_MULTIARCH=x86_64-linux-gnu
root@ubuntu:/home/philip/Downloads#

Based on the preceding output, we can see that this system supports either 32-bit or 64-bit packages. We can also garner useful information regarding the purpose of a package. We need to use the dpkg-query command with the -s option here:

root@ubuntu:/home/philip/Downloads# dpkg-query -s tftp
Package: tftp
Status: install ok unpacked
Priority: optional
Section: net
Installed-Size: 80
Maintainer: Alberto Gonzalez Iniesta <agi@inittab.org>
Architecture: i386
Source: netkit-tftp
Version: 0.17-18
Config-Version: 0.17-18
Replaces: netstd
Depends: netbase, libc6 (>= 2.3)
Description: Trivial file transfer protocol client
Tftp is the user interface to the Internet TFTP (Trivial File Transfer
Protocol), which allows users to transfer files to and from a remote machine.
The remote host may be specified on the command line, in which case tftp uses
host as the default host for future transfers.
root@ubuntu:/home/philip/Downloads#

From the preceding output, we are given a description at the bottom regarding the use of the tftp package.