Which package provides a file

A command you were told to run reports command not found, or a build stops on a header file that is not where it expected. Both need the same answer, the name of a package that would put the file there.

Debian answers that in two places, and which one to ask depends on whether the file is already on the machine. dpkg keeps a record of every path it has unpacked, so searching it is instant and it knows nothing beyond this machine. The archive's index covers every file in every package Debian carries, and has to be downloaded before you can search it.

dpkg -S: the file is already on the machine

dpkg -S $(command -v ls)

coreutils: /usr/bin/ls

Nothing is downloaded and no index is needed. When a package is unpacked, dpkg writes the list of paths it placed into /var/lib/dpkg/info/<package>.list, and -S searches those files. It is the same record dpkg -L reads in the other direction.

The $(command -v ls) is not decoration. Pass a bare name instead and dpkg -S matches it as a substring, against every path it knows about:

dpkg -S ls | wc -l

Locale files, manual pages and any path with those two letters in it. Passing the absolute path narrows it to the one line you wanted, and command -v is how to get the absolute path of something already on your PATH. For a manual page, man -w prints the path to feed in. A relative path never matches at all, because the lists hold absolute ones.

apt-file: the file is still in the archive

ifconfig is not installed on a modern Debian, and the guide telling you to run it is usually old enough not to mention that:

dpkg -S /usr/sbin/ifconfig

dpkg-query: no path found matching pattern /usr/sbin/ifconfig

That says no package on this machine claims the path, which is not the same as no package shipping it. For the archive-wide question, install apt-file and let apt update fetch the Contents index it searches:

apt-file search bin/ifconfig

net-tools: /usr/sbin/ifconfig

The pattern is bin/ifconfig rather than a full path on purpose. ifconfig lived in /sbin for twenty years and moved to /usr/sbin in the /usr merge, so searching the directory you remember can come back empty while the file sits there under the other prefix.

Add -l when the answer is going straight into an install command:

apt-file search -l bin/ifconfig

net-tools

The same index answers the reverse question, which is worth asking before installing a package you have not met:

apt-file list net-tools | head -4

net-tools: /usr/bin/netstat
net-tools: /usr/sbin/arp
net-tools: /usr/sbin/ifconfig
net-tools: /usr/sbin/ipmaddr

Files that belong to no package

Some of what is on a Debian system never came out of a .deb, and both commands above will deny all knowledge of it:

dpkg -S /etc/passwd

dpkg-query: no path found matching pattern /etc/passwd

/etc/passwd is built during installation from a template the base-passwd package ships as /usr/share/base-passwd/passwd.master, and it changes every time an account is added, so shipping it as a packaged file would mean overwriting your accounts on every upgrade. Log files and anything else generated at first boot are absent for the same sort of reason.

The same is true of a command that is definitely installed:

readlink /usr/bin/editor

/etc/alternatives/editor

dpkg -S /usr/bin/editor

dpkg-query: no path found matching pattern /usr/bin/editor

apt-file has nothing for it either:

apt-file search -x 'bin/editor$' || echo "no package in the archive ships that path"

no package in the archive ships that path

The symlink is created by update-alternatives when a package registers itself as a candidate editor, which happens in a maintainer script rather than in the package's file list. Ask the alternatives system instead:

update-alternatives --query editor | head -3

Name: editor
Link: /usr/bin/editor
Slaves:

/usr/bin/vi, /usr/bin/awk, /usr/bin/pager and the rest of /etc/alternatives behave the same way.

command-not-found does the lookup for you

Debian packages the whole exercise above as command-not-found, which is not installed by default:

/usr/lib/command-not-found ifconfig

Command 'ifconfig' not found, but can be installed with:
apt install net-tools

You would not normally type that. In an interactive shell, bash calls it through the command_not_found_handle function defined in /etc/bash.bashrc, so the suggestion appears by itself when you mistype a command or ask for one you have not installed.

It reads a database of its own rather than the Contents index directly, and that database is built by update-command-not-found. On a fresh install it is empty, and the hook stays quiet until you run that once.