8.2. Basic Package Interaction
Armed with a basic understanding of the APT landscape, let's take a look at some basic package interactions including the initialization of APT; installation, removal, and purging of packages; and upgrading of the Kali Linux system. Then let's venture from the command line to take a look at some graphical APT tools.
APT is a vast project and tool set, whose original plans included a graphical interface. From a client perspective, it is centered around the command-line tool apt-get as well as apt, which was later developed to overcome design flaws of apt-get.
There are graphical alternatives developed by third parties, including synaptic and aptitude, which we will discuss later. We tend to prefer apt, which we use in the examples that follow. We will, however, detail some of the major syntax differences between tools, as they arise.
When working with APT, you should first download the list of currently available packages with apt update. Depending on the speed of your connection, this can take some time because various packages' list, sources' list and translation files have grown in size alongside Kali development. Of course, CD/DVD-ROM installation sets install much more quickly, because they are local to your machine.
Thanks to the thoughtful design of the Debian package system, you can install packages, with or without their dependencies, fairly easily. Let's take a look at package installation with dpkg and apt.
dpkg is the core tool that you will use (either directly or indirectly through APT) when you need to install a package. It is also a go-to choice if you are operating offline, since it doesn't require an Internet connection. Remember, dpkg will not install any dependencies that the package might require. To install a package with dpkg, simply provide the -i or --install option and the path to the .deb. This implies that you have previously downloaded (or obtained in some other way) the .deb file of the package to install.
# dpkg -i man-db_2.9.3-2_amd64.deb (Reading database ... 309317 files and directories currently installed.) Preparing to unpack man-db_2.9.3-2_amd64.deb ... Unpacking man-db (2.9.3-2) over (2.9.3-2) ... Setting up man-db (2.9.3-2) ... Updating database of manual pages ... man-db.service is a disabled or a static unit not running, not starting it. Processing triggers for kali-menu (2020.4.0) ... Processing triggers for mime-support (3.64) ...
We can see the different steps performed by dpkg and can see at what point any error may have occurred. The -i or --install option performs two steps automatically: it unpacks the package and runs the configuration scripts. You can perform these two steps independently (as apt does behind the scenes) with the --unpack and --configure options, respectively:
# dpkg --unpack man-db_2.9.3-2_amd64.deb (Reading database ... 309317 files and directories currently installed.) Preparing to unpack man-db_2.9.3-2_amd64.deb ... Unpacking man-db (2.9.3-2) over (2.9.3-2) ... Processing triggers for kali-menu (2020.4.0) ... Processing triggers for mime-support (3.64) ... # dpkg --configure man-db Setting up man-db (2.9.3-2) ... Updating database of manual pages ...
Note that the "Processing triggers" lines refer to code that is automatically executed whenever a package adds, removes, or modifies files in some monitored directories. For instance, the mime-support package monitors /usr/lib/mime/packages and executes the update-mime command whenever something changes in that directory (like /usr/lib/mime/packages/man-db in the specific case of man-db).
Sometimes dpkg will fail to install a package and return an error. However, you can order dpkg to ignore this and only issue a warning with various --force-* options. Issuing the dpkg --force-help command will display a complete list of these options. For example, you can use dpkg to forcibly install zsh:
$ dpkg -i --force-overwrite zsh_5.8-5+b1_amd64.deb
A frequent error, which you are bound to encounter sooner or later, is a file collision. When a package contains a file that is already installed by another package, dpkg will refuse to install it. The following types of messages will then appear:
Unpacking libgdm (from .../libgdm_3.8.3-2_amd64.deb) ... dpkg: error processing /var/cache/apt/archives/libgdm_3.8.3-2_amd64.deb (--unpack): trying to overwrite '/usr/bin/gdmflexiserver', which is also in package gdm3 3.4.1-9
In this case, if you think that replacing this file is not a significant risk to the stability of your system (which is usually the case), you can use --force-overwrite to overwrite the file.
While there are many available --force-* options, only --force-overwrite is likely to be used regularly. These options exist for exceptional situations, and it is better to leave them alone as much as possible in order to respect the rules imposed by the packaging mechanism. Do not forget, these rules ensure the consistency and stability of your system.
Although APT is much more advanced than dpkg and does a lot more behind the scenes, you will find that interacting with packages is quite simple. You can add a package to the system with a simple apt install package. APT will automatically install the necessary dependencies:
# apt install kali-tools-gpu Reading package lists... Done Building dependency tree Reading state information... Done The following additional packages will be installed: oclgausscrack truecrack The following NEW packages will be installed: Need to get 2,602 kB of archives. After this operation, 2,898 kB of additional disk space will be used. Get:1 http://kali.download/kali kali-rolling/main amd64 oclgausscrack amd64 1.3-1kali3 [30.7 kB] Get:2 http://kali.download/kali kali-rolling/main amd64 truecrack amd64 3.6+git20150326-0kali1 [2,558 kB] Get:3 http://kali.download/kali kali-rolling/main amd64 kali-tools-gpu amd64 2021.1.0 [12.6 kB] Fetched 2,602 kB in 1s (2,645 kB/s) Selecting previously unselected package oclgausscrack. (Reading database ... 108127 files and directories currently installed.) Preparing to unpack .../oclgausscrack_1.3-1kali3_amd64.deb ... Unpacking oclgausscrack (1.3-1kali3) ... Selecting previously unselected package truecrack. Preparing to unpack .../truecrack_3.6+git20150326-0kali1_amd64.deb ... Unpacking truecrack (3.6+git20150326-0kali1) ... Selecting previously unselected package kali-tools-gpu. Preparing to unpack .../kali-tools-gpu_2021.1.0_amd64.deb ... Unpacking kali-tools-gpu (2021.1.0) ... Setting up oclgausscrack (1.3-1kali3) ... Setting up truecrack (3.6+git20150326-0kali1) ... Setting up kali-tools-gpu (2021.1.0) ... Processing triggers for man-db (2.9.3-2) ... Processing triggers for kali-menu (2020.4.0) ...
You can also use apt-get install package, or aptitude install package. For simple package installation, they do essentially the same thing. As you will see later, the differences are more meaningful for upgrades or when dependencies resolution do not have any perfect solution.
If sources.list lists several distributions, you can specify the package version with apt install package=version, but indicating its distribution of origin (kali-rolling or kali-dev) with apt install package/distribution is usually preferred.
# apt install zsh=5.7.1-1 # apt install zsh/kali-dev
As with dpkg, you can also instruct apt to forcibly install a package and overwrite files with --force-overwrite, but the syntax is a bit strange since you are passing the argument through to dpkg:
# apt -o Dpkg::Options::="--force-overwrite" install zsh
As a rolling distribution, Kali Linux has spectacular upgrade capabilities. In this section, we will take a look at how simple it is to upgrade Kali, and we will discuss strategies for planning your updates.
We recommend regular upgrades, because they will install the latest security updates. To upgrade, use apt update followed by either apt upgrade, apt-get upgrade, or aptitude safe-upgrade. These commands look for installed packages that can be upgraded without removing any packages. In other words, the goal is to ensure the least intrusive upgrade possible. The apt-get command line tool is slightly more demanding than aptitude or apt because it will refuse to install packages that were not installed beforehand.
The apt tool will generally select the most recent version number.
To tell apt to use a specific distribution when searching for upgraded packages, you need to use the -t or --target-release option, followed by the name of the distribution you want (for example: apt -t kali-rolling upgrade). To avoid specifying this option every time you use apt, you can add APT::Default-Release "kali-rolling"; in the file /etc/apt/apt.conf.d/local.
For more important upgrades, such as major version upgrades, use apt full-upgrade. With this instruction, apt will complete the upgrade even if it has to remove some obsolete packages or install new dependencies. This is also the command that you should use for regular upgrades of your Kali Rolling system. It is so simple that it hardly needs explanation: APT's reputation is based on this great functionality.
Unlike apt and aptitude, apt-get doesn't know the full-upgrade command. Instead, you should use apt-get dist-upgrade (distribution upgrade), a well-known command that apt and aptitude also accept for backwards compatibility.
Since becoming a rolling distribution, Kali can receive upgrades several times a day. However, that might not be the best strategy. So, how often should you upgrade Kali Linux? There is no hard rule but there are some guidelines that can help you. You should upgrade:
- When you are aware of a security issue that is fixed in an update
- When you suspect that an updated version might fix a bug that you are experiencing
- Before reporting a bug to make sure it is still present in the latest version that you have available
- Often enough to get the security fixes that you have not heard about
There are also cases where it is best to not upgrade. For example, it might not be a good idea to upgrade:
- If you can't afford any breakage (for example, because you go offline, or because you are about to give a presentation with your computer); it is best to do the upgrade later, when you have enough time to troubleshoot any issue introduced in the process.
- If a disruptive change happened recently (or is still ongoing) and you fear that all issues have not yet been discovered. For example, when a new GNOME version is released, not all packages are updated at the same time and you are likely to have a mix of packages with the old version and the new version. Most of the time this is fine and it helps everybody to release those updates progressively, but there are always exceptions and some applications might be broken due to such discrepancies.
- If the apt full-upgrade output tells you that it will remove packages that you consider important for your work. In those cases, you want to review the situation and try to understand why apt wants to remove them. Maybe the packages are currently broken and in this case you might want to wait until fixed versions are available, or they have been obsoleted and you should identify their replacements and then proceed with the full upgrade anyway.
In general, we recommend that you upgrade Kali at least once a week. You can certainly upgrade daily but it doesn't make sense to do it more often than that. Even if our mirrors are synchronized four times a day, the updates coming from Debian usually land only once a day.
Removing a package is even simpler than installing one. Let's take a look at how to remove a package with dpkgand apt.
To remove a package with dpkg, supply the -r or --remove option, followed by the name of a package. This removal is not, however, complete: all of the configuration files, maintainer scripts, log files (system logs), data generated by the service (such as the content of an LDAP server directory or the content of a database for an SQL server), and most other user data handled by the package remain intact. The remove option makes it easy to uninstall a program and later re-install it with the same configuration. Also remember that dependencies are not removed. Consider this example:
# dpkg --remove kali-tools-gpu (Reading database ... 108172 files and directories currently installed.) Removing kali-tools-gpu (2021.1.0) ....
You can also remove packages from the system with apt remove package. APT will automatically delete the packages that depend on the package that is being removed. Like the dpkg example, configuration files and user data will not be removed.
Through the addition of suffixes to package names, you can use apt (or apt-get and aptitude) to install certain packages and remove others on the same command line. With an apt install command, add “-” to the names of the packages you wish to remove. With an apt remove command, add “+” to the names of the packages you wish to install.
The next example shows two different ways to install package1 and to remove package2.
# apt install package1 package2- [...] # apt remove package1+ package2 [...]
This can also be used to exclude packages that would otherwise be installed, for example due to a Recommends (discussed later in Section 18.104.22.168, “Recommends, Suggests, and Enhances Fields”). In general, the dependency solver will use that information as a hint to look for alternative solutions.
To remove all data associated with a package, you can purge the package with the dpkg -P package, or apt purge package commands. This will completely remove the package and all user data, and in the case of apt, will delete dependencies as well.
# dpkg -r debian-cd (Reading database ... 333606 files and directories currently installed.) Removing debian-cd (3.1.32) ... # dpkg -P debian-cd (Reading database ... 332950 files and directories currently installed.) Removing debian-cd (3.1.32) ... Purging configuration files for debian-cd (3.1.32) ...
Warning! Given the definitive nature of purge, do not execute it lightly. You will lose everything associated with that package.
Next, let's take a look at some of the tools that can be used to inspect Debian packages. We will learn of dpkg, apt, and apt-cache commands that can be used to query and visualize the package database.
We will begin with several dpkg options that query the internal dpkg database. This database resides on the filesystem at /var/lib/dpkg and contains multiple sections including configuration scripts (/var/lib/dpkg/info), a list of files the package installed (/var/lib/dpkg/info/*.list), and the status of each package that has been installed (/var/lib/dpkg/status). You can use dpkg to interact with the files in this database. Note that most options are available in a long version (one or more relevant words, preceded by a double dash) and a short version (a single letter, often the initial of one word from the long version, and preceded by a single dash). This convention is so common that it is a POSIX standard.
First, let's take a look at --listfiles package (or -L), which lists the files that were installed by the specified package:
$ dpkg -L base-passwd /. /usr /usr/sbin /usr/sbin/update-passwd /usr/share /usr/share/lintian /usr/share/lintian/overrides /usr/share/lintian/overrides/base-passwd /usr/share/doc-base /usr/share/doc-base/users-and-groups /usr/share/base-passwd /usr/share/base-passwd/group.master /usr/share/base-passwd/passwd.master /usr/share/man /usr/share/man/pl /usr/share/man/pl/man8 /usr/share/man/pl/man8/update-passwd.8.gz [...] /usr/share/doc /usr/share/doc/base-passwd /usr/share/doc/base-passwd/users-and-groups.txt.gz /usr/share/doc/base-passwd/changelog.gz /usr/share/doc/base-passwd/copyright /usr/share/doc/base-passwd/README /usr/share/doc/base-passwd/users-and-groups.html
Next, dpkg --search file (or -S), finds any packages containing the file or path passed in the argument. For example, to find the package containing /bin/date:
$ dpkg -S /bin/date coreutils: /bin/date
The dpkg --status package (or -s) command displays the headers of an installed package. For example, to search the headers for the coreutils package:
$ dpkg -s coreutils Package: coreutils Status: install ok installed Priority: required Section: utils Installed-Size: 13855 Maintainer: Michael Stone
Architecture: amd64 Multi-Arch: foreign Version: 8.23-3 Replaces: mktemp, realpath, timeout Pre-Depends: libacl1 (>= 2.2.51-8), libattr1 (>= 1:2.4.46-8), libc6 (>= 2.17), libselinux1 (>= 2.1.13) Conflicts: timeout Description: GNU core utilities This package contains the basic file, shell and text manipulation utilities which are expected to exist on every operating system. . Specifically, this package includes: arch base64 basename cat chcon chgrp chmod chown chroot cksum comm cp csplit cut date dd df dir dircolors dirname du echo env expand expr factor false flock fmt fold groups head hostid id install join link ln logname ls md5sum mkdir mkfifo mknod mktemp mv nice nl nohup nproc numfmt od paste pathchk pinky pr printenv printf ptx pwd readlink realpath rm rmdir runcon sha*sum seq shred sleep sort split stat stty sum sync tac tail tee test timeout touch tr true truncate tsort tty uname unexpand uniq unlink users vdir wc who whoami yes Homepage: http://gnu.org/software/coreutils
The dpkg --list (or -l) command displays the list of packages known to the system and their installation status. You can also use grep on the output to search for certain fields, or provide wildcards (such as b*) to search for packages that match a particular partial search string. This will show a summary of the packages. For example, to show a summary list of all packages that start with 'b':
$ dpkg -l 'b*' 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 backdoor-factory 3.4.2+dfsg-4 all Patch 32/64 bits ELF & win32/64 binaries with shellcode un backupninja
(no description available) un backuppc (no description available) un balsa (no description available) un base (no description available) [...]
The dpkg --contents file.deb (or -c) command lists all the files in a particular .deb file:
$ dpkg -c /var/cache/apt/archives/gpg_2.2.20-1_amd64.deb drwxr-xr-x root/root 0 2020-03-23 15:05 ./ drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/ drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/bin/ -rwxr-xr-x root/root 1062832 2020-03-23 15:05 ./usr/bin/gpg drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/ drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/doc/ drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/doc/gpg/ -rw-r--r-- root/root 677 2020-03-23 14:45 ./usr/share/doc/gpg/NEWS.Debian.gz -rw-r--r-- root/root 24353 2020-03-23 15:05 ./usr/share/doc/gpg/changelog.Debian.gz -rw-r--r-- root/root 384091 2020-03-20 11:38 ./usr/share/doc/gpg/changelog.gz -rw-r--r-- root/root 10555 2020-03-23 15:04 ./usr/share/doc/gpg/copyright drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/man/ drwxr-xr-x root/root 0 2020-03-23 15:05 ./usr/share/man/man1/ -rw-r--r-- root/root 46139 2020-03-23 15:05 ./usr/share/man/man1/gpg.1.gz
The dpkg --info file.deb (or -I) command displays the headers of the specified .deb file:
$ dpkg -I /var/cache/apt/archives/gpg_2.2.20-1_amd64.deb new Debian package, version 2.0. size 894224 bytes: control archive=1160 bytes. 1219 bytes, 25 lines control 374 bytes, 6 lines md5sums Package: gpg Source: gnupg2 Version: 2.2.20-1 Architecture: amd64 Maintainer: Debian GnuPG Maintainers
Installed-Size: 1505 Depends: gpgconf (= 2.2.20-1), libassuan0 (>= 2.5.0), libbz2-1.0, libc6 (>= 2.25), libgcrypt20 (>= 1.8.0), libgpg-error0 (>= 1.35), libreadline8 (>= 6.0), libsqlite3-0 (>= 3.7.15), zlib1g (>= 1:1.1.4) Recommends: gnupg (= 2.2.20-1) Breaks: gnupg (<< 2.1.21-4) Replaces: gnupg (<< 2.1.21-4) Section: utils Priority: optional Multi-Arch: foreign Homepage: https://www.gnupg.org/ Description: GNU Privacy Guard -- minimalist public key operations GnuPG is GNU's tool for secure communication and data storage. It can be used to encrypt data and to create digital signatures. It includes an advanced key management facility and is compliant with the proposed OpenPGP Internet standard as described in RFC4880. [...]
You can also use dpkg to compare package version numbers with the --compare-versions option, which is often called by external programs, including configuration scripts executed by dpkg itself. This option requires three parameters: a version number, a comparison operator, and a second version number. The different possible operators are: lt (strictly less than), le (less than or equal to), eq (equal), ne (not equal), ge (greater than or equal to), and gt (strictly greater than). If the comparison is correct, dpkg returns 0 (success); if not, it gives a non-zero return value (indicating failure). Consider these comparisons:
$ dpkg --compare-versions 1.2-3 gt 1.1-4 $ echo $? 0 $ dpkg --compare-versions 1.2-3 lt 1.1-4 $ echo $? 1 $ dpkg --compare-versions 2.6.0pre3-1 lt 2.6.0-1 $ echo $? 1
Note the unexpected failure of the last comparison: for dpkg, the string "pre" (usually denoting a pre-release) has no particular meaning, and dpkg simply interprets it as a string, in which case "2.6.0pre3-1" is alphabetically greater than "2.6.0-1". When we want a package's version number to indicate that it is a pre-release, we use the tilde character, "~":
$ dpkg --compare-versions 2.6.0~pre3-1 lt 2.6.0-1 $ echo $? 0
The apt-cache command can display much of the information stored in APT's internal database. This information is a sort of cache since it is gathered from the different sources listed in the sources.list file. This happens during the apt update operation.
The apt-cache command can do keyword-based package searches with apt-cache search keyword. It can also display the headers of the package's available versions with apt-cache show package. This command provides the package's description, its dependencies, and the name of its maintainer. This feature is particularly useful in determining the packages that are installed via meta-packages, such as kali-tools-wireless, kali-tools-web, and kali-tools-gpu. Note that apt search, apt show, aptitude search, and aptitude show work in the same way.
Some features are more rarely used. For instance, apt-cache policy displays the priorities of package sources as well as those of individual packages. Another example is apt-cache dumpavail, which displays the headers of all available versions of all packages. apt-cache pkgnames displays the list of all the packages that appear at least once in the cache.
Sooner or later, you will run into a problem when interacting with a package. In this section, we will outline some basic troubleshooting steps that you can take and provide some tools that will lead you closer to a potential solution.
In spite of the Debian or Kali maintainers' best efforts, a system upgrade isn't always as smooth as we would hope. New software versions may be incompatible with previous ones (for instance, their default behavior or their data format may have changed), or bugs may slip through the cracks despite the testing performed by package maintainers and Debian Unstable users.
You might sometimes find that a new version of software doesn't work at all. This generally happens if the application isn't particularly popular and hasn't been tested enough. The first thing to do is to have a look at the Kali bug tracker and at the Debian bug tracking system at https://bugs.debian.org/package , and check whether the problem has already been reported. If it hasn't, you should report it yourself (see Section 6.3, "Filing a Good Bug Report" for detailed instructions). If it is already known, the bug report and the associated messages are usually an excellent source of information related to the bug. In some cases, a patch already exists and has been made available in the bug report itself; you can then recompile a fixed version of the broken package locally (see Section 9.1, "Modifying Kali Packages"). In other cases, users may have found a workaround for the problem and shared their insights about it in their replies to the report; those instructions may help you work around the problem until a fix or patch is released. In a best-case scenario, the package may have already been fixed and you may find details in the bug report.
When the problem is a clear regression (where the former version worked), you can try to downgrade the package. In this case, you will need a copy of the old version. If you have access to the old version in one of the repositories configured in APT, you can use a simple one-liner command to downgrade (see Section 22.214.171.124, "Installing Packages with APT"). But with Kali's rolling release, you will usually only find a single version of each package at any one time.
You can still try to find the old .deb file and install it manually with dpkg. Old .deb files can be found in multiple places:
- in APT's cache in /var/cache/apt/archives/
- in the pool directory on your usual Kali mirror (removed and obsolete packages are kept for three to four days to avoid problems with users not having the latest package indices)
- in http://snapshot.debian.org if the affected package was provided by Debian and not by Kali; this service keeps historical versions of all Debian packages
Sometimes the upgrade gets interrupted because one of the package maintainer scripts fails (usually, it is the postinst). In those cases, you can try to diagnose the problem, and possibly work around it, by editing the problematic script.
Here we rely on the fact that maintainer scripts are stored in /var/lib/dpkg/info/ and that we can review and modify them.
Since maintainer scripts are usually simple shell scripts, it is possible to add a set -x line just after the shebang line and arrange them to be rerun (with dpkg --configure -a for postinst) to see precisely what is happening and where it is failing. This output can also nicely complement any bug report that you might file.
With this newly gained knowledge, you can either fix the underlying problem or transform the failing command into a working one (for example by adding || true at the end of the line).
Note that this tip does not work for a failing preinst since that script is executed even before the package gets installed so it is not yet in its final location. It does work for postrm and prerm although you will need to execute a package removal (respectively upgrade) to trigger them.
The dpkg tool keeps a log of all of its actions in /var/log/dpkg.log. This log is extremely verbose, since it details all the stages of each package. In addition to offering a way to track dpkg's behavior, it helps to keep a history of the development of the system: you can find the exact moment when each package has been installed or updated, and this information can be extremely useful in understanding a recent change in behavior. Additionally, with all versions being recorded, it is easy to cross-check the information with the changelog.Debian.gz for packages in question, or even with online bug reports.
# tail /var/log/dpkg.log 2021-01-06 23:16:37 status installed kali-tools-gpu:amd64 2021.1.0 2021-01-06 23:16:37 remove kali-tools-gpu:amd64 2021.1.0
2021-01-06 23:16:37 status half-configured kali-tools-gpu:amd64 2021.1.0 2021-01-06 23:16:37 status half-installed kali-tools-gpu:amd64 2021.1.0 2021-01-06 23:16:37 status config-files kali-tools-gpu:amd64 2021.1.0 2021-01-06 23:16:37 status not-installed kali-tools-gpu:amd64
When you mistakenly damage your system by removing or modifying certain files, the easiest way to restore them is to reinstall the affected package. Unfortunately, the packaging system finds that the package is already installed and politely refuses to reinstall it. To avoid this, use the --reinstall option of the apt and apt-getcommands. The following command reinstalls postfix even if it is already present:
apt --reinstall install postfix
The aptitude command line is slightly different but achieves the same result with aptitude reinstall postfix. The dpkg command does not prevent re-installation, but it is rarely called directly.
Remember that you can specify a specific distribution with apt as well, which means you can roll back to an older version of a package (if for instance you know that it works well), provided that it is still available in one of the sources referenced by the sources.list file:
apt install w3af/kali-rolling
If you are not careful, the use of a --force-* option or some other malfunction can lead to a system where the APT family of commands will refuse to function. In effect, some of these options allow installation of a package when a dependency is not met, or when there is a conflict. The result is an inconsistent system from the point of view of dependencies, and the APT commands will refuse to execute any action except those that will bring the system back to a consistent state (this often consists of installing the missing dependency or removing a problematic package). This usually results in a message like this one, obtained after installing a new version of rdesktop while ignoring its dependency on a newer version of libc6:
# apt full-upgrade [...] You might want to run 'apt-get -f install' to correct these. The following packages have unmet dependencies: rdesktop: Depends: libc6 (>= 2.14) but 2.3.6.ds1-13etch7 is installed E: Unmet dependencies. Try using -f.
If you are a courageous administrator who is certain of the correctness of your analysis, you may choose to ignore a dependency or conflict and use the corresponding --force-* option. In this case, if you want to be able to continue to use apt or aptitude, you must edit /var/lib/dpkg/status to delete or modify the dependency, or conflict, that you have chosen to override.
This manipulation is an ugly hack and should never be used, except in the most extreme case of necessity. Quite frequently, a more fitting solution is to recompile the package that is causing the problem or use a new version (potentially corrected) from a repository providing backports (backports are newer versions especially recompiled to work in an older environment).
APT is a C++ program whose code mainly resides in the libapt-pkg shared library. Thanks to this shared library, it opened the door for the creation of user interfaces (front-ends), since the shared library code can easily be reused. Historically, apt-get was only designed as a test front-end for libapt-pkg but its success tends to obscure this fact.
Over time, despite the popularity of command line interfaces like apt and apt-get, various graphical interfaces were developed. We will take a look at two of those interfaces in this section: aptitude and synaptic.
Aptitude, shown in Figure 8.1, "The aptitude package manager", is an interactive program that can be used in semi-graphical mode on the console. You can browse the list of installed and available packages, look up all the information, and select packages to install or remove. The program is designed specifically to be used by administrators so its default behavior is much more intelligent than APT's, and its interface much easier to understand.
Figure 8.1. The aptitude package manager
When you run aptitude, you are shown a list of packages sorted by state (installed, not-installed, or installed but not available on the mirrors), while other sections display tasks, virtual packages, and new packages that appeared recently on mirrors. To facilitate thematic browsing, other views are available.
In all cases, aptitude displays a list combining categories and packages on the screen. Categories are organized through a tree structure, whose branches can respectively be unfolded or folded with the Enter, [, and ] keys. The + key should be used to mark a package for installation, - to mark it for removal, and _ to purge it. Note that these keys can also be used for categories, in which case the corresponding actions will be applied to all the packages of the category. The u key updates the lists of available packages and Shift+u prepares a global system upgrade. The g key switches to a summary view of the requested changes (and typing g again will apply the changes), and q quits the current view. If you are in the initial view, this will close aptitude.
To search for a package, you can type / followed by a search pattern. This pattern matches the name of the package but can also be applied to the description (if preceded by ~d), to the section (with ~s), or to other characteristics detailed in the documentation. The same patterns can filter the list of displayed packages: type the l key (as in limit) and enter the pattern.
Managing the automatic flag of Debian packages (see Section 8.3.4, "Tracking Automatically Installed Packages") is a breeze with aptitude. It is possible to browse the list of installed packages and mark packages as automatic with Shift+m or you can remove the mark with the m key. Automatic packages are displayed with an "A" in the list of packages. This feature also offers a simple way to visualize the packages in use on a machine, without all the libraries and dependencies that you don't really care about. The related pattern that can be used with l (to activate the filter mode) is ~i!~M. It specifies that you only want to see installed packages (~i) not marked as automatic (!~M).
Another interesting feature of aptitude is the fact that it respects recommendations between packages while still giving users the choice not to install them on a case-by-case basis. For example, the gnome package recommends gdebi (among others). When you select the former for installation, the latter will also be selected (and marked as automatic if not already installed on the system). Typing g will make it obvious: gdebi appears on the summary screen of pending actions in the list of packages installed automatically to satisfy dependencies. However, you can decide not to install it by deselecting it before confirming the operations.
Note that this recommendation tracking feature does not apply to upgrades. For instance, if a new version of gnome recommends a package that it did not recommend formerly, the package won't be marked for installation. However, it will be listed on the upgrade screen so that the administrator can still select it for installation.
Suggestions between packages are also taken into account, but in a manner adapted to their specific status. For example, since gnome suggests dia-gnome, the latter will be displayed on the summary screen of pending actions (in the section of packages suggested by other packages). This way, it is visible and the administrator can decide whether to take the suggestion into account or not. Since it is only a suggestion and not a dependency or a recommendation, the package will not be selected automatically—its selection requires manual intervention (thus, the package will not be marked as automatic).
In the same spirit, remember that aptitude makes intelligent use of the concept of tasks. Since tasks are displayed as categories in the screens of packages lists, you can either select a full task for installation or removal or browse the list of packages included in the task to select a smaller subset.
To conclude this section, let's note that aptitude has more elaborate algorithms compared to apt when it comes to resolving difficult situations. When a set of actions is requested and when these combined actions would lead to an incoherent system, aptitude evaluates several possible scenarios and presents them in order of decreasing relevance. However, these algorithms are not foolproof. Fortunately, there is always the possibility to manually select the actions to perform. When the currently selected actions lead to contradictions, the upper part of the screen indicates a number of broken packages (you can directly navigate to those packages by pressing b). Then you can manually build a solution. In particular, you can get access to the different available versions by selecting the package with Enter. If the selection of one of these versions solves the problem, you should not hesitate to use the function. When the number of broken packages gets down to zero, you can safely go to the summary screen of pending actions for a last check before you apply them.
Synaptic is a graphical package manager that features a clean and efficient graphical interface (shown in Figure 8.2, “synaptic Package Manager”) based on GTK+. Its many ready-to-use filters give fast access to newly available packages, installed packages, upgradable packages, obsolete packages, and so on. If you browse through these lists, you can select the operations to be done on the packages (install, upgrade, remove, purge); these operations are not performed immediately, but put into a task list. A single click on a button then validates the operations and they are performed in one go.
Figure 8.2. synaptic Package Manager