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Chapter 5. Packaging System: Tools and Fundamental Principles

5.1. Structure of a Binary Package
5.2. Package Meta-Information
5.2.1. Description: the control File
5.2.2. Configuration Scripts
5.2.3. Checksums, List of Configuration Files
5.3. Structure of a Source Package
5.3.1. Format
5.3.2. Usage within Debian
5.4. Manipulating Packages with dpkg
5.4.1. Installing Packages
5.4.2. Package Removal
5.4.3. Querying dpkg's Database and Inspecting .deb Files
5.4.4. dpkg's Log File
5.4.5. Multi-Arch Support
5.5. Coexistence with Other Packaging Systems
As a Debian system administrator, you will routinely handle .deb packages, since they contain consistent functional units (applications, documentation, etc.), whose installation and maintenance they facilitate. It is therefore a good idea to know what they are and how to use them.
This chapter describes the structure and contents of “binary” and “source” packages. The former are .deb files, directly usable by dpkg, while the latter contain the source code, as well as instructions for building binary packages.

5.1. Structure of a Binary Package

The Debian package format is designed so that its content may be extracted on any Unix system that has the classic commands ar, tar, and gzip (sometimes xz or bzip2). This seemingly trivial property is important for portability and disaster recovery.
Imagine, for example, that you mistakenly deleted the dpkg program, and that you could thus no longer install Debian packages. dpkg being a Debian package itself, it would seem your system would be done for... Fortunately, you know the format of a package and can therefore download the .deb file of the dpkg package and install it manually (see sidebar TOOLS dpkg, APT and ar). If by some misfortune one or more of the programs ar, tar or gzip/xz/bzip2 have disappeared, you will only need to copy the missing program from another system (since each of these operates in a completely autonomous manner, without dependencies, a simple copy will suffice). If your system suffered some even more outrageous fortune, and even these don't work (maybe the deepest system libraries are missing?), you should try the static version of busybox (provided in the busybox-static package), which is even more self-contained, and provides subcommands such as busybox ar, busybox tar and busybox gunzip.
Have a look at the content of a .deb file:
$ ar t dpkg_1.17.23_amd64.deb
$ ar x dpkg_1.17.23_amd64.deb
$ ls
control.tar.gz  data.tar.gz  debian-binary  dpkg_1.17.23_amd64.deb
$ tar tzf data.tar.gz | head -n 15
$ tar tzf control.tar.gz
$ cat debian-binary
As you can see, the ar archive of a Debian package is comprised of three files:
  • debian-binary. This is a text file which simply indicates the version of the .deb file used (in 2015: version 2.0).
  • control.tar.gz. This archive file contains all of the available meta-information, like the name and version of the package. Some of this meta-information allows package management tools to determine if it is possible to install or uninstall it, for example according to the list of packages already on the machine.
  • data.tar.gz. This archive contains all of the files to be extracted from the package; this is where the executable files, documentation, etc., are all stored. Some packages may use other compression formats, in which case the file will be named differently (data.tar.bz2 for bzip2, data.tar.xz for XZ).