What does exec 3<&1 do?

Question

I understand that exec can do I/O redirection on the current shell, but I only sees usage like:

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

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

From that I understand that < is for input stream, > is for output stream. So what does exec 3<&1 do?

PS: I found this from Bats source code

Solution

From 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.

I did some debugs with strace:

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

For 3<&1:

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

For 3>&1:

dup2(1, 3)                              = 3

For 2>&1:

dup2(1, 2)                              = 2

It seems that 3<&1 do exactly the same as 3>&1, duplicating stdout to file descriptor 3.

Comments

[orion]

From the manpage, I would expect it to throw a redirection error because stdout is not open for input. However, it really does duplicate stdin (which is &0). How?

[user1686]

@orion: Internally, the same dup2() syscall is used for any kind of file descriptor; bash's x>&y vs x<&y is just syntax sugar. Also, when stdio is attached to a tty, the tty device is very often opened for read+write and just duplicated from 0 to 1 and 2.

[Zhenkai]

@grawity so exec 3<&1 is same as exec >&3 ?

[user1686]

No, but it is the same as exec 3>&1.