scripting

Shell scripting techniques

jq

Tested jq examples: picking values out of nested JSON, filtering with select, reshaping objects, and getting raw strings back out into a shell script.
tee

Tested tee examples: saving a pipeline's output while still seeing it, appending with -a, and the sudo tee trick for writing files your shell cannot.
xargs

Tested xargs examples: batching with -n, placeholders with -I, NUL-separated input from find -print0, parallel runs with -P, and what its exit codes mean.
Environment variables and PATH

Why a program you can see and execute is still not found, how the environment is copied into each new process, and where PATH is really set on Debian.
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.
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.
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.
Parameter expansion

Bash parameter expansion: default values with :-, trimming paths with # and %, replacing text with //, and slicing, length and case conversion.
Where a script lives

What $0 really contains, why a script cannot find a file next to itself with a relative path, and the BASH_SOURCE line that works through a symlink.
Arithmetic

Integer arithmetic in bash with $(( )) and (( )), -eq against =, why a leading zero is an error, and the awk one-liner for sums bash cannot do.
Here-docs and generating files

Here-documents in bash: feeding a block of text to a command, quoting the terminator to stop expansion, generating config files, and here-strings.
Traps and cleanup

The mktemp and trap EXIT pattern for scripts that clean up after themselves, why the trap body needs single quotes, and what set -e does not cover.
Debugging and robustness

Reading what a script actually did with bash -x and PS4, checking syntax without running it, making unset variables fatal, and what shellcheck catches.
A real script

A capstone backup script using arguments, arrays, parameter expansion, a trap and real exit codes, and how to schedule it with cron or a systemd timer.
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.
Bulk rename a batch of files

Rename many files at once with a plain bash loop, safely handling spaces and previewing changes before committing.
sh vs bash vs dash

On Debian /bin/sh is dash, not bash. That one fact explains most scripts that work at your prompt and fail the moment something else runs them.
which vs type vs command -v

which searches PATH and nothing else, so it reports that cd does not exist. What type and command -v answer instead, and which one belongs in a script.