请谁解释一下>a.txt<b.txt cat是怎么运行的？

唐三葬

总是没想明白，哪位能解释一下吗？
  另外怎么样在机子里查看shell的命令帮助？
  谢谢！

KornLee

1,>(重定向输出符)a.txt是生成一个空文件a.txt,然后用<(重定向输入符)把b.txt的内容作为输入输入到a.txt文件中.
2,shell的帮助可以这样:man bash

唐三葬

怎么后面改成<cat a.txt就不对了？这样想起来好象更合理啊。

KornLee

也许是那人习惯那样写吧吧~,
>a.txt<b.txt cat ==cat b.txt>a.txt

idkey

奇怪的、没用的用法，呵呵：
UUOC [from the comp.unix.shell group on Usenet] stands for `Useless Use of cat'; the reference is to the Unix command cat(1), not the feline animal. As received wisdom on comp.unix.shell observes, "The purpose of cat is to concatenate (or `catenate') files. If it's only one file, concatenating it with nothing at all is a waste of time, and costs you a process." Nevertheless one sees people doing

     cat file | some_command and its args ...

instead of the equivalent and cheaper

     <file some_command and its args ...

or (equivalently and more classically)

     some_command and its args ... <file

see http://en.wikipedia.org/wiki/Cat_(Unix)