Shell script function for logging with log levels and custom formatting (POSIX compatible)

log.sh

##############################################################################
# Logs messages with a configurable logging level and format, accepting input
# from either stdin (such as a pipe) or as arguments
#
# Author: Stephen Roberts <stephenroberts@gmail.com>
#
# Globals:
#   LOGLEVEL - arbitrary integer for desired log output level (default: 0)
#
# Arguments:
#   --level         [optional] - integer of log level for the message
#                                (default: 0)
#   --format STRING [optional] - printf format string (default: "%b\n")
#   ...message(s)   [optional] - messages to log (read from stdin if no args)
#
# Examples:
#   LOGLEVEL=3
#   log --loglevel 3 --format "\e[33m%b\e[0m\n" "Hello, world!" # logged
#   log --loglevel 4 "Debug message"                            # not logged
#   echo "Error message" | log --loglevel 0                     # logged
##############################################################################
log() {
  loglevel=0
  logfmt="%b\n"
  LOGLEVEL=${LOGLEVEL:-0}
  while [ $# -gt 0 ]; do
    case $1 in
      --level)
        loglevel=$2
        shift
        shift
        ;;
      --format)
        logfmt=$2
        shift
        shift
        ;;
      *)
        logoutput="$1"
        shift
        ;;
    esac
  done

  if [ -z "$logoutput" ]; then
    # Read from stdin
    while read -r logoutput; do
      if [ "$loglevel" -le "$LOGLEVEL" ]; then
        # shellcheck disable=SC2059
        printf "$logfmt" "$logoutput"
      fi
    done
  else
    # Log from args
    if [ "$loglevel" -le "$LOGLEVEL" ]; then
      # shellcheck disable=SC2059
      printf "$logfmt" "$logoutput"
    fi
  fi
  unset logoutput
}