less /var/log/secure |grep 'Accepted'

less /var/log/secure |grep 'Failed'

less /var/log/secure |grep 'closed'

查看当前在线用户,顺便可查看机器负载情况
w 或 whoo

清理日志

cat /dev/null > /var/log/wtmp
cat /dev/null > /var/log/btmp
cat /dev/null > /var/log/lastlog
cat /dev/null > /var/log/secure