In Bash, how to find the lowest-numbered unused file descriptor?

Question

In a Bash-script, is it possible to open a file on \"the lowest-numbered file descriptor not yet in use\"?

I have looked around for how to do this, but it seems that Bas

Answer 1

I know this thread is old, but believe that the best answer is missing, and would be useful to others like me who come here searching for a solution.

Bash and Zsh have built in ways to find unused file descriptors, without having to write scripts. (I found no such thing for dash, so the above answers may still be useful.)

Note: this finds the lowest unused file descriptor > 10, not the lowest overall.

$ man bash /^REDIRECTION (paragraph 2)
$ man zshmisc /^OPENING FILE DESCRIPTORS

Example works with bsh and zsh.

Open an unused file descriptor, and assign the number to $FD:

$ exec {FD}>test.txt
$ echo line 1 >&$FD
$ echo line 2 >&$FD
$ cat test.txt
line 1
line 2
$ echo $FD
10  # this number will vary

Close the file descriptor when done:

$ exec {FD}>&-

The following shows that the file descriptor is now closed:

$ echo line 3 >&$FD
bash: $FD: Bad file descriptor
zsh: 10: bad file descriptor