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 --helpUsage:watch [options] commandOptions:-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 exitFor 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