监测一个命令的运行结果
很有用的功能 
top  saidar 等 定时刷新结果

-d, --differences[=cumulative]        highlight changes between updates
                (cumulative means highlighting is cumulative)
-n, --interval=seconds            seconds to wait between updates
-t, --no-title                    turns off showing the header
  
  
watch -n 1  -d  free
watch -n 1  -d  'free -h'

watch -n 1 -d "uptime"

watch -n 1 -d -t "uptime"