You’re probably manipulating Debian packages everyday, but do you know what those files are? This article will show you their bowels… Surely they are more than file archives otherwise we would just use TAR archives (you know those files ending with .tar.gz). Let’s have a look!
1. It’s two TAR file archives in an AR file archive!
A .deb file is actually an archive using the AR format, you can manipulate it with the ar command. This archive contains 3 files, you can check it yourself, download any .deb file and run “ar t” on it:
$ ar t gwibber_2.31.91-1_all.deb debian-binary control.tar.gz data.tar.gz
debian-binary is a text file indicating the version of the format of the .deb file, the current version is “2.0”.
$ ar p gwibber_2.31.91-1_all.deb debian-binary 2.0
data.tar.gz contains the real files of the package, the content of that archive gets installed in your root directory when you run “
But the most interesting part—which truly makes .deb files more than a file archive—is the last file.
control.tar.gz contains meta-information used by the package manager. What are they?
$ ar p gwibber_2.31.91-1_all.deb control.tar.gz | tar tzf - ./ ./postinst ./prerm ./preinst ./postrm ./conffiles ./md5sums ./control
2. It contains meta-information defining the package and its relationships
control file within the
control.tar.gz archive is the most fundamental file. It contains basic information about the package like its name, its version, its description, the architecture it runs on, who is maintaining it and so on. It also contains dependency fields so that the package manager can ensure that everything needed by the package is installed before-hand. If you want to learn more about those fields, you can check Binary control files in the Debian Policy.
Those information end up in
/var/lib/dpkg/status once the package is installed.
3. It contains maintainer scripts so that everything can just work out of the box
At various steps of the installation/upgrade/removal process, dpkg is executing the maintainer scripts provided by the package:
postinst: after installation
preinst: before installation
postrm: after removal
prerm: before removal
Note that this description is largely simplified. In fact the scripts are executed on many other occasions with different parameters. There’s an entire chapter of the Debian Policy dedicated to this topic. But you might find this wiki page easier to grasp: http://wiki.debian.org/MaintainerScripts.
While this looks scary, it’s a very important feature. It’s required to cope with non-backwards compatible upgrades, to provide automatic configuration, to create system users on the fly, etc.
4. Configuration files are special files
Unpacking a file archive overwrites the previous version of the files. This is the desired behavior when you upgrade a package, except for configuration files. You prefer not to loose your customizations, don’t you?
That’s why packages can list configuration files in the
conffiles file provided by
control.tar.gz. That way dpkg will deal with them in a special way.
5. You can always add new meta-information
And in fact many tools already exploit the possibility to provide supplementary files in
- debsums use the
md5sumsfile to ensure no files were accidentally modified
- dpkg-shlibdeps uses
symbolsfiles to generate dependencies on libraries
- debconf uses
configscripts to collect configuration information from the user
Once installed, those files are kept by dpkg in
/var/lib/dpkg/info/package.* along with maintainer scripts.