exec 3<&1 有什么作用？

Question

我知道exec可以在当前 shell 上进行 I/O 重定向，但我只看到如下用法：

exec 6<&0   # Link file descriptor #6 with stdin.
            # Saves stdin.

exec 6>&1   # Link file descriptor #6 with stdout.
            # Saves stdout.

据我了解，这<是用于输入流，>用于输出流。那么有什么作用exec 3<&1呢？

PS：我从Bats 源代码中找到了这个

Answer 1

来自bash manpage：

Duplicating File Descriptors
       The redirection operator

              [n]<&word

       is used to duplicate input file descriptors.  If word expands to one or
       more  digits,  the file descriptor denoted by n is made to be a copy of
       that file descriptor.  If the digits in word  do  not  specify  a  file
       descriptor  open for input, a redirection error occurs.  If word evalu?
       ates to -, file descriptor n is closed.  If n  is  not  specified,  the
       standard input (file descriptor 0) is used.

       The operator

              [n]>&word

       is  used  similarly  to duplicate output file descriptors.  If n is not
       specified, the standard output (file descriptor 1)  is  used.   If  the
       digits  in word do not specify a file descriptor open for output, a re?
       direction error occurs.  As a special case, if n is omitted,  and  word
       does not expand to one or more digits, the standard output and standard
       error are redirected as described previously.

我做了一些调试strace：

sudo strace -f -s 200 -e trace=dup2 bash redirect.sh

对于3<&1：

dup2(3, 255)                            = 255
dup2(1, 3)                              = 3

对于3>&1：

dup2(1, 3)                              = 3

对于2>&1：

dup2(1, 2)                              = 2

似乎与3<&1完全相同3>&1，将标准输出复制到文件描述符 3。