Was macht Exec 3 <& 1?

Ich verstehe, dass execdie I / O-Umleitung für die aktuelle Shell ausgeführt werden kann, sehe aber nur die Verwendung wie folgt:

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

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

Daraus verstehe ich, dass dies <für den Eingabestream, >für den Ausgabestream ist. Also, was macht exec 3<&1man?

PS: Ich habe das aus dem Bats-Quellcode heraus gefunden

Von 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.

Ich habe ein paar Debugs gemacht mit strace:

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

Für 3<&1:

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

Für 3>&1:

dup2(1, 3)                              = 3

Für 2>&1:

dup2(1, 2)                              = 2

Dies entspricht anscheinend 3<&1genau dem 3>&1Kopieren von stdout in Dateideskriptor 3.