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.
blkid

Tested blkid examples: reading a filesystem's UUID, label and type, single-value output for scripts, searching by tag, and probing past the cache.
chage

Tested chage examples: reading the ageing settings, expiring passwords on a schedule, forcing a change, and expiring the whole account on a date.
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.
date

Tested date examples: formatting with a + string, parsing a date with -d, arithmetic that crosses a month boundary, file timestamps, and time zones.
dd

Tested dd examples: making disk images, choosing a block size, seek and skip, conv=notrunc, sparse files, and why the output argument is the dangerous one.
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.
env and printenv

Tested env and printenv examples: reading a variable, running one command with a changed environment, and what -S fixes on a shebang line.
free

Tested free examples: which column to read, why free memory is small on a healthy machine, units, swap and commit, and why a container sees its host.
fsck

Tested fsck examples: checking without repairing, forcing a check past the clean flag, reading the exit status, and repairing a damaged filesystem.
fuser

Tested fuser examples: which processes hold a file, what is keeping a filesystem busy, reading the access letters, and killing the holders with -k.
getent

Tested getent examples: passwd, group, hosts and shadow through NSS, the exit status a script reads, and the members a group listing leaves out.
id

Tested id examples: the three parts of the line, real against effective uid under a setuid binary, and the group a running shell has not picked up.
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.
lsof

Tested lsof examples: what has a file open, which process holds a port, finding a deleted file that is still using disk space, and narrowing the output.
man

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

Tested examples for adduser, useradd, usermod, deluser and the group commands: why Debian ships two of each, and the -aG that silently drops every other group.