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

#!/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.