1.top命令
输出的第一行就是系统当前时间和已运行的时间长度，如
top
top - 23:15:59 up 9 days, 8:45, 1 user, load average: 1.23, 0.87, 0.58
2.uptime命令
输出和top命令的第一行类似：
uptime
23:16:58 up 9 days, 8:46, 1 user, load average: 1.57, 1.02, 0.64
3.查看/proc/uptime/
cat /proc/uptime
809318.30 767841.27
运用系统工具date即可算出系统启动时间
date -d "$(awk -F. '{print $1}' /proc/uptime) second ago" +"%Y-%m-%d %H:%M:%S"
2009-06-02 14:30:36
Let life be beautiful like summer flowers and death like autumn leaves.