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'