Debian Cleanup Tip #1: Get rid of useless configuration files

If you like to keep your place clean, you probably want to do the same with your computer. I’m going to show you a few tips over the next 4 weeks so that you can keep your Debian/Ubuntu system free of dust!

Over time the set of packages that is installed on your system changes, either because you install and remove stuff, or because the distribution evolved (and you upgraded your system to the latest version).

But the Debian packaging system is designed to keep configuration files when a package is removed. That way if you reinstall it, you won’t have to redo the configuration. That’s a nice feature but what if you will never reinstall those packages?

Then those configuration files become clutter that you would rather get rid of. In some cases, those files lying around might have unwanted side-effects (recent example: it can block the switch to a dependency-based boot sequence because obsolete init scripts without the required dependencies are still present).

The solution is to “purge” all packages which are in the “config-files” state. With aptitude you can do aptitude purge ~c (or aptitude purge ?config-files). Replace “purge” by “search” if you only want to see a list of the affected packages.

If you want a machine-friendly list of the packages in that state, you could use one of those commands (and then pass the result to apt-get if you don’t have aptitude available):

$ grep-status -n -sPackage -FStatus config-files
$ dpkg-query -f '${Package} ${Status}\n' -W | grep config-files$ | cut -d" " -f1

Note that grep-status is part of the dctrl-tools package.

Of course you can also use graphical package managers, like Synaptic. Click on the “Status” button on the bottom left, then on “Not installed (residual config)” and you have a list of packages that you can purge. You can select them all, right click and pick “Mark for Complete Removal”. See the screenshot below. The last step is to click on “Apply” to get the packages purged.

Synaptic purging residul config files

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5 reasons why a Debian package is more than a simple file archive

Folder with gearsYou’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 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

data.tar.gz contains the real files of the package, the content of that archive gets installed in your root directory when you run “dpkg --unpack“.

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 -

2. It contains meta-information defining the package and its relationships

The 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:

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 control.tar.gz:

  • debsums use the md5sums file to ensure no files were accidentally modified
  • dpkg-shlibdeps uses shlibs and symbols files to generate dependencies on libraries
  • debconf uses config scripts 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.

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Correctly renaming a conffile in Debian package maintainer scripts

After having dealt with the removal of obsolete conffiles, I’ll now explain what you should do when a configuration file managed by dpkg must be renamed.

The problem

Let’s suppose that version 1.2 of the software stopped providing /etc/foo.conf. Instead it provides /etc/bar.conf because the configuration file got renamed. If you do nothing special, the new conffile will be installed with the default configuration, and the old one will stay around. Any customization made by the administrator are lost in the process (in fact they are not lost, they are still in foo.conf but they are unused).

Of course, you could do mv /etc/foo.conf /etc/bar.conf in the pre-installation script. But that’s not satisfactory: it will generate a spurious conffile prompt that the end-user will not understand.

The solution

In the preinst script, you have to verify if the old conffile has been modified by the administrator. If yes, you want to keep the file around. Otherwise you know you will be able to ditch it once the upgrade is over, and you rename it to /etc/foo.conf.dpkg-remove to remember this fact.

In the postinst script, you remove /etc/foo.conf.dpkg-remove. If the old conffile (/etc/foo.conf) still exists, it’s because it was modified by the administrator. You make a backup of the new conffile in /etc/bar.conf.dpkg-dist and rename the old one into /etc/bar.conf.

In the postrm, when called to abort an upgrade, you move /etc/foo.conf.dpkg-remove back to its original name.

In practice, use dpkg-maintscript-helper

dpkg-maintscript-helper can automate all those tasks. You just have to put the following snippet in the maintainer scripts (postinst, postrm, preinst):

if dpkg-maintscript-helper supports mv_conffile 2>/dev/null; then
    dpkg-maintscript-helper mv_conffile /etc/foo.conf /etc/bar.conf 1.1-3 -- "$@"

In this example, I assumed that version 1.1-3 was the last version of the package that contained /etc/foo.conf (i.e. the last version released before 1.2-1 was packaged).

You can avoid the preliminary test if you pre-depend on “dpkg (>=” or if enough time has passed to assume that everybody has a newer version anyway. You can learn all the details in dpkg-maintscript-helper’s manual page.

Debhelper support

Debhelper makes it easy to inject those commands in the maintainer scripts. You just have to provide debian/*.maintscript files. See dh_installdeb’s manual page for details.

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The right way to remove an obsolete conffile in a Debian package

A conffile is a configuration file managed by dpkg, I’m sure you remember the introductory article about conffiles. When your package stops providing a conffile, the file stays on disk and it’s recorded as obsolete by the package manager. It’s only removed during purge. If you want the file to go away, you have to remove it yourself within your package’s configuration scripts. You will now learn how to do this right.

When is that needed?

dpkg errs on the side of safety by not removing the file until purge but in most cases it’s best to remove it sooner so as to not confuse the user. In some cases, it’s even required because keeping the file could break the software (for example if the file is in a .d configuration directory, and if it contains directives that are either no longer supported by the new version or in conflict with other new configuration files).

What’s complicated in “rm”?

So you want to remove the conffile. Adding an “rm” command in debian/postinst sounds easy. Except it’s not the right thing to do. The conffile might contain customizations made by the administrator and you don’t want to wipe those. Instead you want to keep the file around so that he can get his changes back and do whatever is required with those.

The correct action is thus to move the file away in the prerm, to ensure it doesn’t disturb the new version. At the same time, you need to verify whether the conffile has been modified by the administrator and remember it for later. In the postinst, you need to remove the file if it’s unmodified, or keep it under a different name that doesn’t interfere with the software. In many cases adding a simple .dpkg-bak suffix is enough. For instance, run-parts ignore files that contain a dot, and many other software are configured to only include files with a certain extension—say *.conf. In the postrm, you have to remove the obsolete conffiles that were kept due to local changes and you should also restore the original conffile in case the upgrade obsoleting the conffile is aborted.

Automating everything with dpkg-maintscript-helper

Phewww… that’s a lot of things to do for a seemingly simple task. Fortunately everything can be automated with dpkg-maintscript-helper. Let’s assume you want to remove /etc/foo/conf.d/bar because it’s obsolete and you’re going to prepare a new version 1.2-1 with the appropriate code to remove the file on upgrade. You just have to put this snippet in the 3 relevant scripts (preinst, postinst, postrm):

if dpkg-maintscript-helper supports rm_conffile 2>/dev/null; then
    dpkg-maintscript-helper rm_conffile /etc/foo/conf.d/bar 1.2-1 -- "$@"

You can avoid the preliminary test if you pre-depend on “dpkg (>=” or if enough time has passed to assume that everybody has a newer version anyway. You can learn all the details in dpkg-maintscript-helper’s manual page.

Debhelper integration

debhelper makes it easy to inject those commands for you. You can provide debian/*.maintscript files. See dh_installdeb’s manual page for details.

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