sysadmin

System administration tasks

mkfs

Tested mkfs examples: making an ext4 filesystem, setting a label and UUID, choosing block and inode counts, and checks before formatting.
mount

Tested mount examples: attaching a filesystem by device, label or UUID, reading the options in force, bind mounts, fstab, and busy unmounts.
passwd

Tested passwd examples: setting a password from a script, reading the status line, what a locked password does not stop, and expiring an account instead.
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.
rsync

Tested rsync examples: the trailing-slash rule, dry runs, itemised output, --delete, exclude patterns, hard links and --link-dest snapshots.
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.
top

Tested top examples: the summary lines, the process columns, sorting and filtering in batch mode, threads, and reading a process's memory and CPU.
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.
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.
apt update failed

apt update reports a failure, exits 0 and carries on with an index fetched days ago. How to read the transcript and find the repository that broke.
command not found

The shell says this for a command that is missing, one that is not on PATH, one it remembers in the wrong place, and a script whose interpreter is gone.
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.
No space left on device

A write failed for want of space. Find which filesystem is full, whether it ran out of blocks or inodes, and why deleting the big file often frees no space.
Package installation failed

apt install stops before dpkg runs. Telling a package apt has never heard of from one it cannot download, and why --fix-missing is rarely the answer.
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.