Shell重定向i/o命令 [英] Shell redirection i/o order

问题描述

我在玩 i/o shell 重定向.我尝试过的命令(在 bash 中):

ls -al *.xyz 2>&1 1>文件.lst

和

ls -al *.xyz 1>文件.lst 2>&1

当前文件夹中没有任何*.xyz文件.

这些命令给了我不同的结果.第一个命令在屏幕上显示错误消息 ls: *.xyz: No such file or directory.但是第二个会将此错误消息打印到文件中.为什么第一个命令没有将错误输出写入文件?

解决方案

这个错误:

ls: *.xyz: 没有那个文件或目录

正在由 ls 二进制写入 stderr.

但是在这个命令中:

ls -al *.xyz 2>&1 1>文件.lst

您首先将 stderr 重定向到 stdout，默认情况下会转到 tty(终端)

然后您将 stdout 重定向到文件 files.lst，但是请记住 stderr 不会重定向到文件，因为您有 stderr 到 stdout 重定向 before stdout 到 file 重定向.在这种情况下，您的 stderr 仍会写入 tty.

但是，在第二种情况下，您更改重定向的顺序(首先 stdout 到 file，然后 stderr 到 stdout>) 并且正确地将 stderr 重定向到一个 file 也被 stdout 使用.

I'm playing with i/o shell redirection. The commands I've tried (in bash):

ls -al *.xyz 2>&1 1> files.lst

and

ls -al *.xyz 1> files.lst 2>&1

There is no any *.xyz file in current folder.

These commands gives me the different results. The first command shows an error message ls: *.xyz: No such file or directory on the screen. But the second one prints this error message to the file. Why did the first command failed to write an err output to the file?

解决方案

This error:

ls: *.xyz: No such file or directory

is being written on stderr by ls binary.

However in this command:

ls -al *.xyz 2>&1 1> files.lst

You're first redirecting stderr to stdout which by default goes to tty (terminal)

And then you're redirecting stdout to a file files.lst, however remember that stderr doesn't redirected to file since you have stderr to stdout redirection before stdout to file redirection. Your stderr still gets written to tty in this case.

However in 2nd case you change the order of redirections (first stdout to file and then stderr to stdout) and that rightly redirects stderr to a file which is also being used by stdout.

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