Bash & Zsh

Single vs double quotes, ${var} braces, and parameter expansion that replaces half your sed scripts.

Indexed arrays, associative arrays, slicing, length — and where zsh's array semantics quietly win.

if / while / for / case — and the three different test brackets ([, [[, and (() — that mean similar things and don't.

`local`, return values vs exit codes, scope rules — and why bash functions don't return what you think they return.

fd 0/1/2 manipulation, here-docs, here-strings, tee, process substitution — the full I/O toolkit.

Globs, brace expansion, tilde, command substitution, process substitution — and the ORDER they happen in.

& background, wait, jobs, fg/bg, trap on EXIT/ERR/INT — making your scripts behave like good UNIX citizens.

set -euo pipefail, IFS, set -x — the four-line opener that turns a fragile bash script into one you can trust.

Compatibility, key zsh wins (extended globbing, prompts, completions, arrays), when each shell is right.