sysadmin

System administration tasks

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.
chown

Tested chown examples: setting owner and group, -R over a tree, why a glob misses the dotfiles, and the reason you cannot give away a file you own.
crontab

Tested crontab examples: schedule syntax, @reboot/@daily shortcuts, per-user crontabs, and the PATH gotcha that breaks jobs that work fine in your shell.
df

Tested df examples: reading the output, asking about the filesystem a path is on, choosing columns and units, and the full disk that still reports free space.
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.
du

Tested du examples: -sh totals, --max-depth, apparent size versus allocated blocks, sparse files, hard links, excludes, and sorting the output.
job control

Tested jobs, fg, bg, disown and nohup examples: the jobs table, job specs like %1, suspending and resuming, and why none of it works in a script.
journalctl

Tested journalctl examples: filtering by unit, priority and time, following logs live, output formats, and controlling journal disk usage.
kill

Tested kill, pkill and killall examples: choosing a signal, why -9 is a last resort, matching by name or command line, and signalling a whole process group.
man

Tested man and apropos examples: what the section numbers mean, searching by description, finding which package shipped a page, and reading a synopsis.
ps

Tested ps examples: aux versus -ef, choosing columns with -o, selecting by user or name, sorting, the process tree, and pgrep instead of grep.
ss

Tested ss examples: listing listening sockets, finding the process holding a port, filtering by state and address, and reading the queue columns.
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.
systemctl

Tested systemctl examples: starting and stopping services, enabling them at boot, reading unit state, drop-in overrides, and diagnosing failures.
tail

Tested tail examples: -n and -c, the +N form that counts from the start, following a growing file with -f, and why log rotation needs -F instead.
File permissions explained

The rwx/owner-group-other model, numeric vs symbolic chmod, umask, and why setuid/setgid on directories behave differently than you'd expect.
Processes and signals

How processes are born, orphaned and reaped, what a signal actually does, why kill -9 is a last resort, and why Ctrl-C stops a whole pipeline at once.
Add a directory to your PATH permanently

How to put a directory on PATH for good on Debian, why ~/.local/bin needs no configuration at all, and what to do for any other directory.
Find and replace across many files

Replace a string across a directory tree with grep -rl and sed -i, safely for filenames containing spaces, with a dry run and a backup first.
Find files without the permission denied noise

Why 2>/dev/null is the wrong fix for find's permission denied errors, and what -readable -prune does instead, including what happens to the exit status.
Keep a program running after you log out

How to stop a long job dying when an ssh session ends, what each approach really does about SIGHUP, and an honest recommendation between them.
Find and kill whatever is using a port

Identify the process bound to a TCP port that's refusing to let a service start, then stop it.