Error Handling: set -euo pipefail

The defensive header

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

Most professional bash scripts start with these two lines. Without them, errors get swallowed silently and bugs go unnoticed.

What each flag does

• -e (errexit) — exit immediately when any command returns non-zero (with carve-outs for tests and pipelines).
• -u (nounset) — exit when an undefined variable is referenced. Catches typos like $PROOJECT.
• -o pipefail — pipeline exit code is the first non-zero stage's, not just the last command's.
• IFS=$'\n\t' — restrict word splitting to newlines and tabs (not spaces). Helps with filenames and multi-word values.

Carve-outs you'll need

set -e doesn't fire if the command is part of an if, a && chain, or piped into |. Use that to your advantage:

# OK — caller checks the exit code
if grep -q ERROR build.log; then ...

# Tolerate failure with || true
rm -rf .cache || true

Default with -u

With -u, referencing an unset variable explodes. The ${var:-} form gives an empty default, ${var:-fallback} gives a real one. Always defensive-default user-supplied env vars: "${DEBUG:-0}".

Custom error reporting

err() { echo "$0:${LINENO}: $*" >&2; exit 1; }
[[ -d "$dir" ]] || err "missing $dir"

A small err helper makes script failures actionable. Pair with trap (next lesson) for cleanup on the way out.