beginner

Good starting points for newcomers

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.
cat

Tested cat examples: joining rotated logs, numbering lines with -n and -b, and using cat -A to find the carriage returns stopping a script from running.
cd

Tested cd examples: why no program could do this job, what cd - and $OLDPWD hold, and the two different answers cd .. gives under a symlink.
cp

Tested cp examples: copying files and whole trees, why a second cp -r nests the directory inside itself, and what -a preserves that a plain copy loses.
less

Tested less examples: the keys that move and search, why a piped less turns into cat, and how lesspipe lets it open a compressed file directly.
ln

Tested ln examples: why a symlink's target is resolved from the link's own directory, what a hard link shares, and the -n that stops a relink landing inside.
ls

Tested ls examples: the long format column by column, hidden files, sorting by time and size, symlinks, time styles, and why piped output looks different.
man

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

Tested mkdir examples: the two separate things -p does, why an existing file still fails it, and why -m never reaches the parents it invents.
mv

Tested mv examples: renaming and moving files, why a second mv nests a directory inside itself, and what changes once the destination is another filesystem.
rm

Tested rm examples: -r and -rf, why a directory's permissions decide what you may delete, and three ways a command removes more than the files you named.
touch

Tested touch examples: creating a file without truncating an existing one, setting access and modification times separately, and the timestamp nothing can set.
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.
Exit codes and error handling

How $?, PIPESTATUS, set -e, and pipefail combine to tell a script what succeeded and what failed, and where each one falls short.
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.
Pipes and redirection

How | connects commands together, and how >, >>, 2>&1, and <() route data to and from files, other commands, and each other.
Terminal, shell and tty

What a terminal, a tty, a shell and a console each actually are, why Ctrl-D is not a signal, and why Ctrl-S appears to freeze everything.
Your first script

Write, mark executable, and run your first bash script, plus what the shebang line and PATH lookup do.
Variables and quoting

Why unquoted variables break bash scripts, when to use double vs single quotes, and how glob characters make it worse.
Conditionals and test

The difference between [ and [[ in bash, why unquoted variables crash [ but not [[, and pattern matching with case.
Loops

for, while and until in bash, iterating over files without breaking on spaces, and why a while read loop in a pipeline forgets everything it did.
Script arguments

How a script reads what it was called with: positional parameters, why "$@" and "$*" are not the same, shift, and parsing real flags with getopts.
Functions

Defining bash functions, why return gives a status not a value, how to hand data back, and why local separates a helper from a landmine.
Arrays

Indexed and associative arrays in bash: building them, why the @ and * subscripts differ, and filling one from command output without splitting.