Write Bash scripts that fail clearly

Make assumptions explicit

This example targets Bash on Linux and reports local filesystem capacity. It does not modify services. Prefer small scripts with a documented purpose, required tools, expected privileges, and defined outputs.

Check the prerequisite

Strict mode helps expose failures, but it is not a substitute for understanding command exit codes. In particular, set -e has exceptions in conditionals and pipelines. Check important operations explicitly.

#!/usr/bin/env bash
set -euo pipefail
command -v df >/dev/null || { echo "df is required" >&2; exit 1; }
printf "Filesystem capacity on %s\n" "$(hostname)"
df -hP

Keep diagnostics useful

Write diagnostic messages to stderr and preserve a nonzero exit status when an operation fails. Avoid set -x around secrets. Quote variable expansions and use temporary files carefully if the script grows beyond this read-only example.

Test the failure path

Run ShellCheck when available, then test missing prerequisites, permission failures, and invalid inputs in a disposable environment. If a future version changes state, add a preview mode and document what is safe to rerun.