CLI: Get a free unused file descriptor

free_file_descriptor.sh

#!/usr/bin/env sh

# this works in sh, ash, bash, zsh
# and works in Linux and Cygwin and Busybox
# probably works in Mac too
# remember process subtitution is not POSIX
# comm would normally require --nocheck-order but busybox comm doesn't, so we just send error messages to /dev/null

# min_fd -> max_fd -> number_of_fds_to_return
get_free_fd () {
    seq "$1" "$2" > /tmp/free_fd
    echo -n /proc/$$/fd/[0-9]* | tr ' ' '\0' | xargs -0 -I '{}' basename '{}' | sort -n > /tmp/used_fd
    free_fd="$(comm -23 /tmp/free_fd /tmp/used_fd 2>/dev/null | head -n "$3")"
    rm /tmp/free_fd && rm /tmp/used_fd
    printf "$free_fd"
    [ -n "$free_fd" ] && return 0 || return 1
}

# use it like:
get_free_fd 0 $(ulimit -n) 1

# it will return a file descriptor or return nothing at all
# you can test for an available file descriptor using `test -n`

# if you're in bash 4.0 or latest ZSH, there's an easier ways to do this
# see: http://stackoverflow.com/a/17030546/582917