Ubuntu – Page 5

Debian Cleanup Tip #5: identify cruft that can be removed from your Debian system

February 28, 2011 by Raphaël Hertzog

Last week we learned how to identify and restore packages whose files have been corrupted. This time we’ll concentrate ourselves on the non-packaged files…

Non-packaged files

They are files which are not provided by a Debian package, or in other words, files where dpkg --search finds no associated package:

$ dpkg --search /srv/cvs
dpkg-query: no path found matching pattern /srv/cvs

You always have such files on your system, at least all your own files in /home. But many daemons also create files as part of their work (and they are usually stored in /var): internal files for a database server, mail spool for a mail server, etc. Those are normal and you want to leave them alone.

But you might have non-packaged files in /usr and that should not be the case if you install everything from packages. It would thus be useful to be able to list those files in order to detect a software that has been manually installed.

Manually installed software is not a good idea

Such an installation might cause troubles for example by taking precedence over the same software provided in a Debian package. Over time the local installation will not be upgraded while the packaged one will.

The other packages which depend on this software will believe they have the latest version since their dependency is satisfied but in fact they are using the older version since it takes precedence.

So you want to get rid of those? Let’s see how we can find them.

Use cruft to identify non-packaged files

As I explained above, there are many non-packaged files that are legitimate and that you don’t want to remove. That’s why cruft does something more elaborated than a scan of the filesystem and a check of dpkg’s database.

It provides a way for packages to say which files they might legitimately create during run-time and that cruft should not report. And it knows of many such files. But it’s far from exhaustive and definitely not up-to-date.

So you should always take its output with suspicion and consider twice where the file came from. Do not trust it blindly to remove the files… you have been warned.

How to use cruft

You should give it a list of directories to ignore to reduce the noise in the output, for example like this:

$ sudo cruft -d / -r report --ignore /home --ignore /var --ignore /tmp
$ less report
cruft report: mercredi 23 février 2011, 15:45:34 (UTC+0100)

---- missing: ALTERNATIVES ----
        /etc/alternatives/cli-csc.1.gz
        /usr/share/man/man1/cli-csc.1.gz
---- missing: dpkg ----
        /etc/xdg/autostart/gnome-power-manager.desktop
        /usr/lib/libpython2.6_d.so.1.0-gdb.py
        /usr/share/fonts/X11/100dpi
        /usr/share/fonts/X11/75dpi
---- unexplained: / ----
        /boot
        /dev
        /etc/.java
        /etc/.java/.systemPrefs
[...]
        /usr/lib/pymodules/python2.6
        /usr/lib/pymodules/python2.6/.path
        /usr/lib/pymodules/python2.6/Brlapi-0.5.5.egg-info
[...]

Note that it doesn’t traverse filesystems so if your /usr is on another partition than /, you will need to use the option -d "/ /usr" to have it scan both.

Analyze the report

Now you can quietly go through the report that has been generated and decide which files need to be removed or not. The report also contains missing files (files which should exist according to the dpkg database but which are not there) but the bulk of the listing will be in the “unexplained” section: files which are not part of any package (and whose presence is not explained by any other explain script that packages can ship).

Again take this with great suspicion, and you should rather not delete a file if you don’t know it got there in the first place. For instance, on my system it lists many files below /usr/lib/pymodules/ and those are legitimate: they come from Debian packages but they are copied there dynamically from /usr/{lib,share}/pyshared in order to support multiple python versions. If you remove those files, you effectively break your system.

You will also find many .pyc files created by python packages, they are a byte-compiled version of the corresponding .py file. Removing them breaks nothing but you loose a bit of performance.

On the opposite, most of the files below /usr/local/ are likely the result of some manual software installation and those should be safe to remove (if you know that you are not using the corresponding software).

Conclusion: useful but needs work

In summary, you can use cruft to identify non-packaged files and maybe learn a bit more about what got manually installed on the system, but it requires some patience to go through the report as many of the files reported are false positives.

Yes, cruft badly needs supplementary volunteers to cope with the many ways packages legitimately generate non-packaged files. It’s not even complicated work: the package is mostly in shell and in Perl, and /usr/share/doc/cruft/README.gz explains how it all works.

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Debian Cleanup Tip #4: find broken packages and reinstall them

February 21, 2011 by Raphaël Hertzog

Last week, we learned to get rid of third-party packages, now we’re going one step further: we’ll verify if the files of the installed packages are still exactly like they were when they got installed.

If you’re a tinkerer and hand-edit some files for some quick tests, or if you tend to re-install newer versions of some packages from the sources, you might have overwritten some packaged files and it would be good to be able to detect this (and remedy to the problem). debsums is the tool that makes it possible.

Use debsums to identify modified files

I often use debsums when I take over the maintenance of a Debian server because I want to verify which files have been modified by the former administrator.

Without any argument, debsums is very verbose, it will list every installed file (except configuration files) and tells whether it’s unmodified (“OK”) or not (“FAILED”).

$ sudo debsums
/usr/bin/a2ps                                               OK
[...]

With the --all option, it will verify all files including configuration files. With --config it will verify only the configuration files.

With the --changed option, debsums will only list modified files among those inspected. The following invocation will thus list all files which have been modified on the system and which are not configuration files.

$ sudo debsums --changed
/usr/lib/perl5/AptPkg/Config.pm
/usr/lib/perl5/AptPkg.pm
[...]

Find out the package affected and reinstall it

debsums told me that /usr/lib/perl5/AptPkg.pm was modified. Indeed I remember having manually installed a modified version of that perl module for a quick test.

I find out the affected package with dpkg --search /usr/lib/perl5/AptPkg.pm: it’s libapt-pkg-perl.

Now I just have to reinstall this package to overwrite the modified files with the original ones:

$ sudo aptitude reinstall libapt-pkg-perl
[...]
# Or with apt-get
$ sudo apt-get --reinstall install libapt-pkg-perl
[...]

You might have to repeat the process until debsums no longer reports any modified file.

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Debian Cleanup Tip #3: get rid of third-party packages

February 14, 2011 by Raphaël Hertzog

Last week, we learned how to get rid of obsolete packages. This time, we’re going to learn how to bring back your computer to a state close to a “pure” Ubuntu/Debian installation.

Thanks to the power of APT, it’s easy to add new external repositories and install supplementary software. Unfortunately some of those are not very well maintained. They might contain crappy packages or they might simply not be updated. An external package which was initially working well, can become a burden on system maintenance because it will be interfering with regular updates (for example by requiring a package that should be removed in newer versions of the system).

So my goal for today is to teach you how to identify the packages on your system that are not coming from Debian or Ubuntu. So that you can go through them from time to time and keep only those that you really need. Obsolete packages are a subset of those, but I’ll leave them alone. We took care of them last week.

Each (well-formed) APT repository comes with a “Release” file describing it (example). They provide some values that can be used by APT to identify packages contained in the repository. All official Debian repositories are documented with Origin=Debian (and Origin=Ubuntu for Ubuntu). You can verify the origin value associated to each repository (if any) in the output of apt-cache policy:

[...]
 500 http://ftp.debian.org/debian/ lenny/main i386 Packages
     release v=5.0.8,o=Debian,a=stable,n=lenny,l=Debian,c=main
     origin ftp.debian.org
[...]

From there on, we can simply ask aptitude to compute a list of packages which are both installed and not available in an official Debian repository:

$ aptitude search '?narrow(?installed, !?origin(Debian))!?obsolete'
or
$ aptitude search '~S ~i !~ODebian !~o'

You can replace “search” with “purge” or “remove” if you want to get rid of all the packages listed. But you’re more likely to want to remove only a subset of carefully chosen packages… you’re probably still using some of the software that you installed from external repositories.

With synaptic, you can also browse the content of each repository. Click on the “Origin” button and you have a list of repositories. You can go through the non-Debian repositories and look which packages are installed and up-to-date.

But you can do better, you can create a custom view. Click on the menu entry “Settings > Filter”. Click on “New” to create a new filter and name it “External packages”. Unselect everything in the “Status” tab and keep only “Installed”.

Go in the “Properties” tab and here add a new entry “Origin” “Excludes” “ftp.debian.org”. In fact you must replace “ftp.debian.org” with the hostname of your Debian/Ubuntu mirror. The one that appears on the “origin” line in the output of apt-cache policy (see the excerpt quoted above in this article).

Note that the term “Origin” is used to refer to two different things, a field in the release file but also the name of the host for an APT repository. It’s a bit confusing if you don’t pay attention.

Close the filters window with OK. You now have a new listing of “External packages” under the “Custom Filters” screen. You can see which packages are installed and up-to-date and decide whether you really want to keep it. If the package is also provided by Debian/Ubuntu and you want to go back to the version provided by your distribution, you can use the “Package > Force version…” menu entry.

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Debian Cleanup Tip #2: Get rid of obsolete packages

February 7, 2011 by Raphaël Hertzog

Last week, we learned to remove useless configuration files. This week, we’re going to take care of obsolete packages.

An obsolete package is a package who is no longer provided by any of the APT repositories listed in /etc/apt/source.lists (and /etc/apt/sources.list.d/). There can be multiple reasons why a package is no longer available in the repository (or at least not under the same name) :

the upstream author stopped maintaining the software a long time ago, nobody else took over and the Debian maintainer preferred to remove the package from Debian. Usually there are alternatives in the Debian archive.
the package was orphaned in Debian since a long time, nobody took over and it had very few users. The Debian QA team might have asked its removal.
the latest version of the software might have been packaged under a new package name. Either because the amount of changes was so important that it was preferred to not upgrade automatically to the latest version (it has been the case with request-tracker and nagios, they both embed a version number in their package names), or simply because the maintainer wants to let the user install several versions at the same time (that’s the case for example with the Linux kernel, the python interpreter and many libraries).
the software has been renamed, the maintainer renamed the packages and kept transitional packages under the old name for one release. Then the transitional packages have been removed.

In any case, it’s never a good idea to keep obsolete packages around: they do not benefit from security updates and they might cause problems during upgrades if they depend on other packages that should be removed to complete the upgrade.

You could blindly remove them with aptitude purge ~o (or aptitude purge ?obsolete) but you might want to first verify what those package are. There might be some packages that you have manually installed, that are not part of any current APT repository, and that you want to keep around nevertheless (I have skype, dropbox and a few personal packages for example). You can get the list with aptitude search ?obsolete…

With the graphical package manager (Synaptic), you can find the list of obsolete packages by clicking on the “Status” button and selecting “Installed (local or obsolete)”. You can then go through the list and decide for each package whether you want to keep it or not.

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