Bash robustness

Bash robustness

Bash's defaults are optimized for interactive forgiveness, not script correctness: errors continue, unset variables expand to nothing, and words split themselves. A robust script overrides all three and proves itself with shellcheck.

Method

1. Open with the strict preamble, knowing its limits. set -euo pipefail (exit on error, unset-is-error, pipeline fails on any stage). Know the honest caveats: -e is disabled inside if conditions and &&/|| chains, and command substitution in an assignment (local x=$(cmd)) masks failure: split declaration and assignment. Where a command may legitimately fail, say so: cmd || true.
2. Quote every expansion, always. "$var", "$(cmd)", "$@" for argument passthrough: unquoted expansions word-split and glob, which is the root of most "works until a space or * shows up" bugs. Filenames from the world get -- guards (rm -- "$file") and find -print0 | xargs -0 for iteration.
3. Clean up with trap, not hope. Temp files via mktemp, removed in trap 'rm -rf "$tmpdir"' EXIT; the trap runs on error and Ctrl-C too, which is the point. Multi-resource scripts compose cleanup functions; a script that leaves debris on failure is a slow disk-full incident (see graceful-shutdown's ethic at script scale).
4. Fail with context, log to stderr. die() { echo "error: $*" >&2; exit 1; } and use it with specifics ("config $f not readable"); progress messages to stderr so stdout stays clean for data (the python-cli-tools stream contract, shell edition). Check preconditions early: required commands (command -v jq >/dev/null || die ...), required variables, expected file existence (see guard-clauses).
5. Run shellcheck as a gate, not a suggestion. In CI and pre-commit (see git-hooks-automation); fix or explicitly directive-suppress every finding with a reason. Shellcheck catches the quoting, -e, and subshell traps that reviews miss; a script that cannot pass it is not done. Format with shfmt for diff stability (see code-formatting).
6. Know when to leave bash. Past ~100 lines, needing real data structures, JSON manipulation beyond one jq, error handling with context, or unit tests: switch to Python (see python-cli-tools) or another real language. Bash excels at orchestrating processes and files; it punishes application logic (see shell-portability for the dialect question).

Boundaries

• set -e is a tripwire, not error handling; money-path scripts (deploys, data mutation) check each critical step explicitly and report what succeeded before the failure (see script-idempotency for rerun safety).
• Parsing ls, HTML, or human-formatted output is fragile by construction; use the tool's machine format (--json, -0, --porcelain) or a real parser (see text-processing).
• Concurrency in bash (background jobs, wait, flock) is workable for simple fan-out but has no supervision story; parallel pipelines belong in an orchestrator or a language with one (see background-jobs).