debian

Debian-specific material

apt

Search, install, remove and upgrade packages with apt, including how to see what it will do before it does it, and which commands are safe in a script.
apt-cache

Search the package cache, read a package's record, follow its dependencies, and use policy to see which version apt would install and from where.
apt-file

Tested apt-file examples: searching Debian's file index for a command you have not installed, and the substring trap in a plain pattern.
dpkg

Query what's installed, find which package owns a file, inspect a .deb before trusting it, and repair a half-finished install: the layer beneath apt.
sudo

Tested sudo and su examples: running one command as root, what the environment loses on the way, the redirect that still runs as you, and reading sudoers.
which

Tested which examples: how PATH order decides the answer, the shell script Debian ships in place of the GNU program, and the deprecation that was reversed.
Could not get lock /var/lib/dpkg/lock-frontend

apt refuses to run because something else is already installing. Modern apt tells you exactly which process, and deleting the lock file is the wrong fix.
The following packages have been kept back

apt-get upgrade refuses to install new packages, so an upgrade that needs one is held back instead. What the message means and the three things that cause it.
The repository is not signed

apt cannot verify a repository's signature: the NO_PUBKEY and Missing key errors. What Debian 13 changed, and why apt-key adv is no longer the fix.
sudo: command not found

On Debian, sudo is often simply not installed, and your user is not in the sudo group. Why the installer does that, and how to fix it from a root shell.
apt vs apt-get

The same package ships both, and they do the same job. What differs is who each is written for, and only one of them is safe in a script.
cron vs systemd timers

Debian ships both and uses both. What differs is where the output goes, what happens to a missed run, and whether you can test the schedule.
remove vs purge vs autoremove

Not three strengths of one operation. Two of them differ over configuration files; the third acts only on packages you never asked for.
sh vs bash vs dash

On Debian /bin/sh is dash, not bash. That one fact explains most scripts that work at your prompt and fail the moment something else runs them.
APT essentials

The apt, apt-get, and dpkg commands that cover almost everything you'll do to manage packages on Debian, and how remove differs from purge.
Installing a .deb by hand

Install a downloaded .deb the way that resolves its dependencies, see what dpkg -i leaves behind when you don't, and read the package before you trust it.
Listing what is installed

List installed packages reliably with dpkg-query, tell what you asked for from what came along, and find out when a package arrived from the logs.
Debian's release channels

What stable, testing, unstable and backports each mean, why the suite name in your sources file matters more than you think, and which to run.
Managing services with systemd

How Debian ships, enables and starts systemd services, where unit files live, and how to override a packaged unit without ever editing it.
Adding a third-party repository safely

Why apt-key is gone, how Signed-By scopes a vendor's key to their repository alone, and how to stop that repository replacing the rest of your system.
Which package provides a file

Trace a file to the package that ships it: dpkg -S for what is installed, apt-file for what is not, and the files that belong to no package at all.