sysadmin
System administration tasks
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Monitor a log file in real time
Watch a log file as new lines are written, filter it live, and keep watching across log rotation.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.