Proper Signal Handling and Cleanup

This rule applies to files matching the patterns above.

# Proper Signal Handling and Cleanup

## Rule
Every CLI tool that creates temporary resources MUST install signal handlers to clean up on SIGINT (Ctrl+C), SIGTERM (kill), and EXIT. Never leave orphaned temp files or held locks.

## Format
```bash
#!/usr/bin/env bash
set -euo pipefail

TEMP_DIR=""

cleanup() {
  local exit_code=$?
  if [[ -n "${TEMP_DIR}" && -d "${TEMP_DIR}" ]]; then
    rm -rf "${TEMP_DIR}"
  fi
  exit "${exit_code}"
}
trap cleanup EXIT INT TERM

main() {
  TEMP_DIR="$(mktemp -d)"
  # Work with temp files safely...
}

main "$@"
```

## Good Examples
```bash
#!/usr/bin/env bash
set -euo pipefail

LOCK_FILE=""
PID_FILE=""

cleanup() {
  local exit_code=$?
  # Remove lock file
  [[ -f "${LOCK_FILE}" ]] && rm -f "${LOCK_FILE}"
  # Remove PID file
  [[ -f "${PID_FILE}" ]] && rm -f "${PID_FILE}"
  # Kill background processes
  jobs -p | xargs -r kill 2>/dev/null || true
  exit "${exit_code}"
}
trap cleanup EXIT

on_sigint() {
  echo "" >&2
  echo "Interrupted by user" >&2
  exit 130  # 128 + SIGINT(2)
}
trap on_sigint INT

main() {
  LOCK_FILE="/tmp/mytool.lock"
  if [[ -f "${LOCK_FILE}" ]]; then
    echo "Error: Another instance is running" >&2
    exit 1
  fi
  echo $$ > "${LOCK_FILE}"
  # Long-running operation...
}

main "$@"
```

## Bad Examples
```bash
# BAD: No cleanup — Ctrl+C leaves temp files
TMPFILE=$(mktemp)
process_data > "${TMPFILE}"
mv "${TMPFILE}" output.txt
# If interrupted between mktemp and mv, temp file is orphaned

# BAD: Trap with expanding variables
trap "rm -f $TMPFILE" EXIT  # $TMPFILE expanded NOW, not at trap time
# Should use single quotes: trap 'rm -f "${TMPFILE}"' EXIT
```

## Enforcement
- Code review checklist: verify trap handlers for temp file usage
- ShellCheck SC2064 catches variable expansion in trap strings
- Test interruption: run tool and send SIGINT, verify cleanup occurred