Listar las actualizaciones de paquetes pendientes en Linux

#!/bin/bash
if command -v apt-get >/dev/null 2>&1; then
  apt-get update -qq
  apt list --upgradable 2>/dev/null | tail -n +2
elif command -v dnf >/dev/null 2>&1; then
  dnf -q check-update
  rc=$?
  # dnf returns 100 when updates are available
  [ $rc -eq 100 ] && exit 0
  exit $rc
else
  echo "No supported package manager found"
  exit 2
fi