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Some useful APT commands

The APT package manager used in the Debian and Ubuntu Linux distros keeps track of installed packages, but also which files they installed. Here are four package management commands I have ended up using regularly.

What package does this file belong to?

If you know the path to a file (for example a configuration file) you can find name of the package it belongs to by running dpkg:

$ dpkg -S /etc/smartd.conf
smartmontools: /etc/smartd.conf

In this example the smartmontools package provides the /etc/smartd.conf file.

You might want to know which package provides a tool you have been using. A common scenario is when you have a tool available on your computer, you want to run it on another computer, and you don't recall how you installed the tool. Simply pass the command name through the which command to get the full path and use dpkg again to find the owning package:

$ dpkg -S `which sendmail`
msmtp-mta: /usr/sbin/sendmail

In this example the msmtp-mta package provides the sendmail command. In general, the package name might not be obvious, especially if multiple packages can provide the same file.

Which package version do I have?

Checking which version of a package you gave installed is useful for knowing which features are supported by that version. The version is easily checked check using apt-cache when you have the package name:

$ apt-cache policy msmtp-mta
msmtp-mta:
  Installed: 1.8.6-1
  Candidate: 1.8.6-1
  Version table:
 *** 1.8.6-1 500
        500 http://se.archive.ubuntu.com/...
        100 /var/lib/dpkg/status

In this example the version of the installed msmtp-mta package is 1.8.6-1.

What files did the package install?

Sometimes you know that a package did install something, but not exacly where. This could be an example file, documentation, a utility command, an init script, a library, etc. Given a package name the dpkg command can list the files that the package installed:

$ dpkg -L endlessh
/.
/etc
/etc/init.d
/etc/init.d/endlessh
/lib
/lib/systemd
/lib/systemd/system
/lib/systemd/system/endlessh.service
/usr
/usr/bin
/usr/bin/endlessh
/usr/share
/usr/share/doc
/usr/share/doc/endlessh
/usr/share/doc/endlessh/README.Debian
/usr/share/doc/endlessh/changelog.Debian.gz
/usr/share/doc/endlessh/copyright
/usr/share/man
/usr/share/man/man1
/usr/share/man/man1/endlessh.1.gz

From this output we can see that the endlessh package provides an init script, a systemd service unit file, a binary, a readme file, a changelog, a license file, and a manpage.

How do I keep track of manually installed files?

Sometimes you need to install something that is not available as a package. Luckily, there's a tool called checkinstall that acts as a glue between a command that install files and the APT package manager. You typically invoke it like this:

$ cd some-package
$ ./configure
$ make
$ sudo checkinstall make install

Here the checkinstall command wraps the make install command one would usually run. The file system is scanned for new files created by the installation command. The created files are then added to a new APT package. When you run checkinstall you get to fill in the details of the package (name, version, maintainer, etc).

The source directory can be deleted at this point. Once the files are installed and owned properly by a package it is now easy to uninstallpreviously installed files - simply uninstall the package!

-- raek, 2020-07-24

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