man watch:
WATCH(1) Linux User's Manual WATCH(1) NAME watch - execute a program periodically, showing output fullscreen SYNOPSIS watch [-bdehpvtx] [-n seconds] [--beep] [--color] [--differences[=cumu‐ lative]] [--errexit] [--exec] [--help] [--interval=seconds] [--no-title] [--precise] [--version] command DESCRIPTION watch runs command repeatedly, displaying its output and errors (the first screenfull). This allows you to watch the program output change over time. By default, the program is run every 2 seconds; use -n or --interval to specify a different interval. Normally, this interval is interpreted as the amout of time between the completion of one run of command and the beginning of the next run. However, with the -p or --precise option, you can make watch attempt to run command every interval seconds. Try it with ntptime and notice how the fractional seconds stays (nearly) the same, as opposed to normal mode where they continuously increase. The -d or --differences flag will highlight the differences between successive updates. Using --differences=cumulative makes highlighting "sticky", presenting a running display of all positions that have ever changed. The -t or --no-title option turns off the header showing the interval, command, and current time at the top of the display, as well as the following blank line. The -b or --beep option causes the com‐ mand to beep if it has a non-zero exit. watch will normally run until interrupted. If you want watch to exit on an error from the program running use the -e or --errexit options, which will cause watch to exit if the return value from the program is non-zero. By default watch will normally not pass escape characters, however if you use the --c or --color option, then watch will interpret ANSI color sequences for the foreground. NOTE Note that command is given to "sh -c" which means that you may need to use extra quoting to get the desired effect. You can disable this with the -x or --exec option, which passes the command to exec(2) instead. Note that POSIX option processing is used (i.e., option processing stops at the first non-option argument). This means that flags after command don't get interpreted by watch itself. EXAMPLES To watch for mail, you might do watch -n 60 from To watch the contents of a directory change, you could use watch -d ls -l If you're only interested in files owned by user joe, you might use watch -d 'ls -l | fgrep joe' To see the effects of quoting, try these out watch echo $$ watch echo '$$' watch echo "'"'$$'"'" To see the effect of precision time keeping, try adding -p to watch -n 10 sleep 1 You can watch for your administrator to install the latest kernel with watch uname -r (Note that -p isn't guaranteed to work across reboots, especially in the face of ntpdate or other bootup time-changing mechanisms) BUGS Upon terminal resize, the screen will not be correctly repainted until the next scheduled update. All --differences highlighting is lost on that update as well. Non-printing characters are stripped from program output. Use "cat -v" as part of the command pipeline if you want to see them. Combining Characters that are supposed to display on the character at the last column on the screen may display one column early, or they may not display at all. Combining Characters never count as different in --differences mode. Only the base character counts. Blank lines directly after a line which ends in the last column do not display. --precise mode doesn't yet have advanced temporal distortion technology to compensate for a command that takes more than interval seconds to execute. watch also can get into a state where it rapid-fires as many executions of command as it can to catch up from a previous executions running longer than interval (for example, netstat taking ages on a DNS lookup).
1．命令格式：
watch[参数][命令]
2．命令功能：
可以将命令的输出结果输出到标准输出设备，多用于周期性执行命令/定时执行命令
3．命令参数：
-n或--interval watch缺省每2秒运行一下程序，可以用-n或-interval来指定间隔的时间。
-d或--differences 用-d或--differences 选项watch 会高亮显示变化的区域。 而-d=cumulative选项会把变动过的地方(不管最近的那次有没有变动)都高亮显示出来。
-t 或-no-title 会关闭watch命令在顶部的时间间隔,命令，当前时间的输出。
-h, --help 查看帮助文档
4．使用实例：
实例1：
命令：每隔一秒高亮显示网络链接数的变化情况
watch -n 1 -d netstat -ant
说明：
其它操作：
切换终端： Ctrl+x
退出watch：Ctrl+g
实例2：每隔一秒高亮显示http链接数的变化情况
命令：
watch -n 1 -d 'pstree|grep http'
说明：
每隔一秒高亮显示http链接数的变化情况。 后面接的命令若带有管道符，需要加''将命令区域归整。
实例3：实时查看模拟攻击客户机建立起来的连接数
命令：
watch 'netstat -an | grep:21 | grep<模拟攻击客户机的IP>| wc -l'
说明：
实例4：监测当前目录中 scf' 的文件的变化
命令：
watch -d 'ls -l|grep scf'
(如：
-rwxr-xr-x 1 ms sumng 45 2月 12 19:32 scf'
只要我们已修改scf'，相关的修改就会高亮显示）。
实例5：10秒一次输出系统的平均负载
命令：
watch -n 10 'cat /proc/loadavg'