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

LOG_FILE="${1:?log file required}"
MAX_BYTES="${2:-20971520}"
BACKUPS="${3:-5}"

mkdir -p "$(dirname "$LOG_FILE")"
touch "$LOG_FILE"

rotate_logs() {
  local size
  size="$(stat -c%s "$LOG_FILE" 2>/dev/null || echo 0)"
  if [[ "$size" -lt "$MAX_BYTES" ]]; then
    return
  fi

  local index
  for ((index=BACKUPS; index>=1; index--)); do
    if [[ -f "${LOG_FILE}.${index}" ]]; then
      if [[ "$index" -eq "$BACKUPS" ]]; then
        rm -f "${LOG_FILE}.${index}"
      else
        mv "${LOG_FILE}.${index}" "${LOG_FILE}.$((index + 1))"
      fi
    fi
  done
  mv "$LOG_FILE" "${LOG_FILE}.1"
  : > "$LOG_FILE"
}

while IFS= read -r line || [[ -n "$line" ]]; do
  rotate_logs
  printf '%s\n' "$line" | tee -a "$LOG_FILE"
done