Working with many docker containers on one machine I needed to get list of free IP addresses in specific docker network. This is the way:

comm -3 <(nmap -sL 172.30.0.10/29 | awk '/Nmap scan report/{print $NF}' | sort) <(nmap -sn 172.30.0.10/29 | awk '/Nmap scan report/{print $NF}' | sort) | sort -V
172.30.0.9
172.30.0.10
172.30.0.11
172.30.0.12
172.30.0.13
172.30.0.14
172.30.0.15

Let me explain.

This generates the list of IPs in subnetwork:

nmap -sL 172.30.0.10/29 | awk '/Nmap scan report/{print $NF}'
172.30.0.8
172.30.0.9
172.30.0.10
172.30.0.11
172.30.0.12
172.30.0.13
172.30.0.14
172.30.0.15

This generates list of used IPs in subnetwork:

nmap -sn 172.30.0.10/29 | awk '/Nmap scan report/{print $NF}' 
172.30.0.8

The comm compares two lists and suppress common lines

comm - compare two sorted files line by line
       -3     suppress column 3 (lines that appear in both files)

One more example:

cat ip_pool.txt
172.30.0.8
172.30.0.9
172.30.0.10
172.30.0.11
172.30.0.12
172.30.0.13
172.30.0.14
172.30.0.15

cat ip_used.txt
172.30.0.8
172.30.0.10

comm -3 <(cat ip_pool.txt | sort) <(cat ip_used.txt | sort) | sort -V
172.30.0.9
172.30.0.11
172.30.0.12
172.30.0.13
172.30.0.14
172.30.0.15

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My main background for a recent years is MySQL DEV/DBA engineer but I’m also participating in wider range of tasks which could be likely called as DataOps/DevOps tasks. You can see my online CV here: http://catyellow.net

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