watch 命令使用
1.watch命令简介
watch [options] command
The watch program executes a given command at regular inter-vals; the default is every two seconds. The command is passed to the shell (so be sure to quote or escape any special characters), and the results are displayed in a full-screen mode, so you can observe the output conveniently and see what has changed. For example,
每隔一段时间重复运行一个命令，默认间隔时间是2秒。要运行的命令直接传给shell（注意引用和转义特殊字符）。结果会展示为全屏模式，这样你可以很方便的观察改变。例如:
watch -n 60 date
executes the date command once a minute, sort of a poor man’s clock. Type ^C to exit.
执行date命令每分钟一次，输入^C 退出。
2.Useful options
常用选项
-n seconds Set the time between executions, in seconds.
设置运行时间间隔，以秒为单位
-d Highlight differences in the output, to emphasize what has changed from one execution to the next.
高亮输出不同，强调两次执行的不同。
3.watch 手册
NAME
watch - execute a program periodically, showing output fullscreen
watch - 定期执行程序，输出全屏
SYNOPSIS
watch [-dhvt] [-n <seconds>] [--differences[=cumulative]] [--help]
[--interval=<seconds>] [--no-title] [--version] <command>
DESCRIPTION
watch runs command repeatedly, displaying its output (the first screen-full). 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.
监视重复运行命令，展示输出（全屏）。让你监视程序输出变化。默认程序2s运行一次；使用-n或--interval 指定不同的间隔时间
The -d or --differences flag will highlight the differences between successive updates. The --differences[=cumulative] option 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.
-d 或 --differences 会高亮显示新内容。--cumulative 选项让高亮保持，展示运行中所有的修改的内容。 -t 或 --no-title 选项关闭头部展示，只展示空白行。
watch will run until interrupted.
watch会一直运行，知道中断。
NOTE
Note that command is given to "sh -c" which means that you may need to use extra quoting to get the desired effect.
注意，命令"sh -c" 你需要应用起来才能达到预期的效果。
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.
注意，使用POSIX选项（选项会停在第一个非选项参数）意味着你不能在命令后再设置间隔。
EXAMPLES
To watch for mail, you might do
查看邮件
watch -n 60 from
To watch the contents of a directory change, you could use
查看目录变化：
watch -d ls -l
If you’re only interested in files owned by user joe, you might use
想找joe用户的文件，是下面的命令
watch -d ’ls -l | fgrep joe’
To see the effects of quoting, try these out
看引号的使用
watch echo $$ watch echo ’$$’ watch echo "’"’$$’"’"
结束