Testing if a file descriptor is valid

In ksh (both AT&T and pdksh variants) or zsh, you can do:

if print -nu3; then
  echo fd 3 is writeable
fi

They won't write anything on that fd, but still check if the fd is writable (using fcntl(3, F_GETFL)) and report an error otherwise:

$ ksh -c 'print -nu3' 3< /dev/null
ksh: print: -u: 3: fd not open for writing

(which you can redirect to /dev/null).

With bash, I think your only option is to check if a dup() succeeds like in your approach, though that won't guarantee that the fd is writable (or call an external utility (zsh/perl...) to do the fcntl()).

Note that in bash (like most shells), if you use (...) instead of {...;}, that will fork an extra process. You can use:

if { true >&3; } 2<> /dev/null

instead to avoid the fork (except in the Bourne shell where redirecting compound commands always causes a subshell). Don't use : instead of true as that's a special builtin, so would cause the shell to exit when bash is in POSIX compliance mode.

You could however shorten it to:

if { >&3; } 2<> /dev/null

In the POSIX command Application Usage description you'll find the following:

There are some advantages to suppressing the special characteristics of special built-ins on occasion. For example:

command exec > unwritable-file

does not cause a non-interactive script to abort, so that the output status can be checked by the script.

This is why you can just do:

if    command >&3
then  echo 3 is open >&3
else  ! echo 3 is not open
fi    2<>/dev/null

Or...

{ command >&3
  printf %s\\n%.0d  string "0$(($??8:0))" >&"$(($??1:3))"
} 2<>/dev/null

Which will write string followed by a \newline either to stdout or 3 and still pass on a non-zero exit status when 3 is not open because the math done on $? winds up failing to convert the octal 08 to %decimal but truncates to nothing at all the octal 00.

Or...

command exec >&3 || handle_it

But if you're using ksh93, you can just do:

fds

For a list of of open file descriptors. Add -l to see where they go.

Your trick looks cute; but for an idiomatic way I wonder why you didn't use:

if ( exec 1>&3 ) 2>&-