find-free-nbd-device.sh

#!/bin/bash
#
# requires:
#  bash
#
set -e
set -o pipefail

function nbd_used?() {
  local devpath=${1}

  local find=0 line=
  while read line; do
    [[ ${line} == "" ]] && continue

    set ${line}
    [[ "${1}" == "43" ]] || continue

    if [[ ${devpath} == /dev/${4} ]]; then
      find=1
    fi
  done < /proc/partitions

  [[ "${find}" == "1" ]]
}

function find_free_nbd_device() {
  local nbd=
  for nbd in /dev/nbd*; do
    case "${nbd}" in
      *p[0-9]) continue ;;
    esac

    nbd_used? ${nbd} && continue

    echo ${nbd}
    break
  done

  # free nbd device not found
  return 1
}

find_free_nbd_device