Duplication of file descriptors in redirection

Question

From the The GNU Bash Reference Manual, section 3.6 Redirections:

Bash handles several filenames specially when they are used in redirections, as described in the following table:

/dev/fd/fd

If fd is a valid integer, file descriptor fd is duplicated.

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

what does "duplicated" mean here? Can you give some examples?

^{(The above excerpt is from Edition 4.3, last updated 2 February 2014, of The GNU Bash Reference Manual, for Bash, Version 4.3.)}

Solution

Redirections are implemented via the dup family of system functions. dup is short for duplication and when you do e.g.:

3>&2

you duplicate (dup2 ) filedescritor 2 onto filedescriptor 3, possibly closing filedescriptor 3 if it's already open (which won't do a thing to your parent process, because this happens in a forked off child (if it does not (redirections on shell functions in certain contexts), the shell will make it look as if it did)).

When you do:

1<someFile

it'll open someFile on a new file descriptor (that's what the open syscall normally does) and then it'll dup2 that filedescriptor onto 1.

What the manual says is that if one of the special dev files listed takes the place of someFile, the shell will skip the open-on-a-new-fd step and instead go directly to dup2ing the matching filedescriptor (i.e., 1 for /dev/stdout, etc.) onto the target (filedescriptor on the left side of the redirection).