O que o exec 3 & 1 faz?

Question

Eu entendo que exec pode fazer redirecionamento de E / S no shell atual, mas só vejo o uso como:

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

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

A partir disso, entendo que < é para fluxo de entrada, > é para fluxo de saída. Então, o que faz exec 3<&1 ?

PS: Eu encontrei isso de código fonte do Bats

    

Answer 1

De 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.

Eu fiz alguns debugs com strace :

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

Para 3<&1 :

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

Para 3>&1 :

dup2(1, 3)                              = 3

Para 2>&1 :

dup2(1, 2)                              = 2

Parece que 3<&1 faz exatamente o mesmo que 3>&1 , duplicando stdout para o descritor de arquivo 3.