Scripting

A guided course in bash scripting, one lesson at a time.

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.