watch是周期性的执行下个程序，并全屏显示执行结果

用法：

vm@fedora:~$ watch --help

Usage:
 watch [options] command

Options:
  -b, --beep             beep if command has a non-zero exit
  -c, --color            interpret ANSI color and style sequences
  -d, --differences[=<permanent>]
                         highlight changes between updates
  -e, --errexit          exit if command has a non-zero exit
  -g, --chgexit          exit when output from command changes
  -q, --equexit <cycles>
                         exit when output from command does not change
  -n, --interval <secs>  seconds to wait between updates
  -p, --precise          attempt run command in precise intervals
  -r, --no-rerun         do not rerun program on window resize
  -t, --no-title         turn off header
  -w, --no-wrap          turn off line wrapping
  -x, --exec             pass command to exec instead of "sh -c"

 -h, --help     display this help and exit
 -v, --version  output version information and exit

For more details see watch(1).

用例：

每隔两秒执行一次uptime查看系统负载：

watch -n 2 uptime