Find number of addresses already attributed by docker engine

nb-addresses.sh

#!/bin/sh
# part by part:
# 1. "brctl showmacs docker0" show the MAC addresses attached to the docker0 bridge
# 2. "$(($(brctl showmacs docker0|wc -l) - 1))" returns the number of the lines returned by the brctl command minus 1 (to exclude the header, for cleanup)
# 3. "uniq" show only unique enteries
# 4. "wc -l" count the number of lines in the result. Which is also the number of running containers with an IP address behind the docker0 bridge.
# Next address given by the docker engine is: (@IP of docker0)+the found number+1
brctl showmacs docker0|tail -n $(($(brctl showmacs docker0|wc -l) - 1))|uniq|wc -l
#Alternative through docker
#1. the docker inspect command with its arguments returns allocated IP addresses
#2. the second part count the addresses returned by forst part
docker inspect  --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}'  $(docker ps -q)|wc -l
# PS. If a container is given multiple addresses, on the same subnet or different subnets... you have to adapt these commands (if necessary)