如随机选取train.txt中的200行保存在新文件 test.txt中
shuf train.txt -n 200 -o test.txt
查看 shuf 命令帮助 “shuf --help”
用法： shuf [选项]... [文件]
或者: shuf -e [选项]... [参数]...
或者: shuf -i LO-HI [选项]...
Write a random permutation of the input lines to standard output.
如果没有指定文件，或者文件为"-"，则从标准输入读取。
必选参数对长短选项同时适用。
-e, --echo treat each ARG as an input line
-i, --input-range=LO-HI treat each number LO through HI as an input line
-n, --head-count=COUNT output at most COUNT lines
-o, --output=FILE write result to FILE instead of standard output
--random-source=FILE get random bytes from FILE
-r, --repeat output lines can be repeated
-z, --zero-terminated line delimiter is NUL, not newline
--help显示此帮助信息并退出
