Bash

Default: #!/usr/bin/env bash, set -euo pipefail, ShellCheck clean, functions over long straight-line scripts, and a move to Python once logic needs data structures.

Script skeleton

#!/usr/bin/env bash
set -euo pipefail
usage() { echo "usage: ${0##*/} [--dry-run] <target>" >&2; exit 2; }
dry=0
while [[ $# -gt 0 ]]; do
  case $1 in
    --dry-run) dry=1 ;;
    -h|--help) usage ;;
    *) target=$1 ;;
  esac
  shift
done
[[ -n ${target:-} ]] || usage
tmp=$(mktemp -d); trap 'rm -rf "$tmp"' EXIT

Rules that prevent most bugs

• Quote every expansion: "$var", "${arr[@]}". Unquoted variables split on spaces and expand globs.
• Use [[ ... ]] for tests and $(...) for command substitution.
• set -e does not fire inside if conditions, &&/|| chains or command substitutions in some contexts; check critical commands explicitly.
• pipefail makes a pipeline fail if any stage fails — but grep returning 1 on "no match" then becomes an error; handle it (grep ... || true).
• Read files line by line with while IFS= read -r line; do ...; done < file.

Running safely and repeatedly

• One instance at a time: exec 9>"$lockfile"; flock -n 9 || { echo "already running" >&2; exit 0; }.
• Idempotent: check state before changing it (does the branch exist, is the file already current) so a rerun after a failure is safe.
• Write atomically: write to a temp file in the same directory and mv it into place.
• Retries: loop with a cap and growing sleep for network commands; never retry forever.
• Dry run first for anything destructive, and print exactly what would happen.

Timers, hooks and CI

• Timers and hooks run with a minimal environment: no interactive profile, a short PATH, a different working directory. Use absolute paths and set PATH at the top.
• Log with timestamps to a file or the journal (logger -t name), since nobody watches the terminal.
• In git hooks, read the message file passed as $1, do not assume the working directory, and chain to any pre-existing hook instead of replacing it.

Verification loop

1. shellcheck script.sh — zero findings, or each suppressed with a reason.
2. bash -n script.sh for syntax.
3. Run with --dry-run, then for real, then run it again to prove idempotency.

Pitfalls

• cd in a script without || exit continues in the wrong directory.
• rm -rf "$dir/"* with an empty $dir targets /; guard with ${dir:?}.
• local var=$(cmd) hides cmd's exit status; declare and assign separately.
• echo interprets options and escapes inconsistently; use printf '%s\n'.
• Arithmetic with leading zeros (08) is treated as octal; force base 10 with 10#$n.
• sed -i differs between GNU and BSD; on macOS use sed -i '' or prefer Python for edits.