It would be more correct to say that stdin, stdout, and stderr are "I/O streams" rather than files. As you've noticed, these entities do not live in the filesystem. But the Unix philosophy, as far as I/O is concerned, is "everything is a file". In practice, that really means that you can use the same library functions and interfaces (printf, scanf, read, write, select, etc.) without worrying about whether the I/O stream is connected to a keyboard, a disk file, a socket, a pipe, or some other I/O abstraction.

大多数程序都需要读取输入，写入输出，并记录错误，所以stdin, stdout， 和stderr是为您预定义的，以方便编程。这只是 一种约定，操作系统不强制执行。

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen() function creates certain descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Normally there are three open streams with constant pointers declared in the header and associated with the standard open files. At program startup three streams are predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device

https://www.mkssoftware.com/docs/man5/stdio.5.asp

下面是一篇关于stdin, stdout和stderr的长篇文章:

Linux中的stdin、stdout和stderr是什么?

总结:

Streams Are Handled Like Files Streams in Linux—like almost everything else—are treated as though they were files. You can read text from a file, and you can write text into a file. Both of these actions involve a stream of data. So the concept of handling a stream of data as a file isn’t that much of a stretch. Each file associated with a process is allocated a unique number to identify it. This is known as the file descriptor. Whenever an action is required to be performed on a file, the file descriptor is used to identify the file. These values are always used for stdin, stdout, and stderr: 0: stdin 1: stdout 2: stderr

具有讽刺意味的是，我在堆栈溢出和上面的文章中发现了这个问题，因为我正在搜索关于异常/非标准流的信息。所以我的探索还在继续。

作为上述答案的补充，以下是关于重定向的总结:

编辑:这张图并不完全正确。

第一个例子根本没有使用stdin，它将“hello”作为参数传递给echo命令。

图表还显示，2>&1与&>具有相同的效果

ls Documents ABC > dirlist 2>&1
#does not give the same output as 
ls Documents ABC > dirlist &>

这是因为&>需要重定向到一个文件，而2>&1只是将stderr发送到stdout

有关这些文件的权威信息，请查看手册页，在您的终端上运行该命令。

$ man stdout 

但简单来说，每个文件都是用于:

输出流的Stdout

流输入的Stdin

打印错误或日志消息的标准错误。

每个unix程序都有这些流中的每一个。

使用ps -aux显示当前进程，所有这些进程都列在/proc/ as /proc/(pid)/中，通过调用cat /proc/(pid)/fd/0，它会打印在该进程的标准输出中找到的任何东西。因此,或许,

/proc/(pid)/fd/0 -标准输出文件 /proc/(pid)/fd/1 -标准输入文件 /proc/(pid)/fd/2 -标准错误文件

例如

但只有在/bin/bash中才能很好地工作，其他进程通常在0中没有任何内容，但在2中有许多错误