How to find next available file descriptor in Bash?

Question

How can I figure out if a file descriptor is currently in use in Bash? For example, if I have a script that reads, writes, and closes fd 3, e.g.

exec 3<          


        

Answer 1

The other answer that uses pre-bash-4.1 syntax does a lot unnecessary subshell spawning and redundant checks. It also has an arbitrary cut-off for the maximum FD number.

The following code should do the trick with no subshell spawning (other than for a ulimit call if we want to get a decent upper limit on FD numbers).

fd=2 max=$(ulimit -n)

while ((++fd < max)); do
   ! true <&$fd && break
done 2>/dev/null && echo $fd

• Basically we just iterate over possible FDs until we reach one we can't dupe.
• In order to avoid the Bad file descriptor error message from the last loop iteration, we redirect stderr for the whole while loop.