bash-scripting-expert

# Bash Scripting Expert > Scripts fail in production on the input you didn't quote. Start strict, quote everything, clean up on exit, and run ShellCheck. When logic gets complex (JSON, data structures), switch to a real language. ## When to Use - Writing or debugging a Bash/POSIX shell script or CI/automation glue. - "Works until a path has a space/newline" bugs; word-splitting, globbing, quoting. - Argument parsing, exit codes, error handling, temp files, cleanup. ## When NOT to Use - Complex data/JSON/HTTP logic → `python-expert` (shell is the wrong tool). - PowerShell scripting → use PowerShell tooling. - CI pipeline YAML → `github-master`. ## Core Principles ### 1. Strict mode, every script - `#!/usr/bin/env bash` + `set -euo pipefail`: exit on error, error on unset vars, fail on any pipeline stage. Set `IFS=$'\n\t'` to stop surprise word-splitting on spaces. - Know `set -e`'s gotchas: it's suppressed inside `if`/`||`/`&&` conditions and command substitutions — check critical commands explicitly when needed. ### 2. Quote everything - Always `"$var"`, `"${arr[@]}"`, `"$@"` (not `$*`). Unquoted expansions split on `IFS` and glob — the #1 source of bugs. - Use `[[ … ]]` for tests (not `[ … ]`), `$(…)` not backticks, `(( … ))` for arithmetic, and `local` for all function variables. ### 3. Errors, cleanup, and exit codes - `trap 'cleanup' EXIT` to remove temp files even on failure/interrupt. Create temps with `mktemp`. - Validate args and required commands up front; print usage to **stderr** and `exit` non-zero on misuse. Exit codes are meaningful — `0` success, non-zero failure. - Send errors/logs to `stderr` (`>&2`); keep `stdout` for real output so the script composes in pipes. ### 4. Robust file & data handling - `printf` over `echo` for anything with escapes/variables. Use `--` before paths, and `find … -print0 | while IFS= read -r -d ''` for filenames with spaces/newlines. - For POSIX `sh`, avoid bashisms (`[[`, arrays, `local`); if you use them, declare `bash` in the shebang. Lint with **ShellCheck** and format with `shfmt`. ## Common Mistakes - **Unquoted `$var`** → breaks on spaces/globs/empties. - **Parsing `ls`** or `for f in $(ls)` → use globs or `find -print0`. - **`set -e` assumed to catch everything** → it won't inside conditions/substitutions; check explicitly. - **`cd somewhere` without `||exit`** → subsequent commands run in the wrong directory. - **No cleanup trap** → leftover temp files on failure. - **`echo` with `-e`/escapes** → non-portable; use `printf`. - **Mixing logs into stdout** → corrupts piped output. ## Examples **Defensive script skeleton** ```bash #!/usr/bin/env bash set -euo pipefail IFS=$'\n\t' usage() { printf 'usage: %s <input-dir> <out-file>\n' "$0" >&2; exit 2; } [[ $# -eq 2 ]] || usage input_dir=$1 out_file=$2 [[ -d $input_dir ]] || { printf 'no such dir: %s\n' "$input_dir" >&2; exit 1; } command -v jq >/dev/null || { printf 'jq required\n' >&2; exit 1; } tmp=$(mktemp) trap 'rm -f "$tmp"' EXIT # safe iteration (handles spaces/newlines in names) find "$input_dir" -type f -name '*.log' -print0 | while IFS= read -r -d '' f; do wc -l -- "$f" >> "$tmp" done mv -- "$tmp" "$out_file" trap - EXIT printf 'wrote %s\n' "$out_file" ``` ## See Also - `github-master` — running scripts safely in GitHub Actions. - `docker-expert` — robust container entrypoint scripts. - `python-expert` — when a script outgrows the shell.