Explanation:

CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and a decimal number. The number is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks. CIDR was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.

IPv4 CIDR blocks interpretation :

IPv4 : a.b.c.d
Bits : 8.8.8.8 --> 32 bits

CIDR block Notation : a.b.c.d/prefix where prefix is the number of IP address the network can have.

Example : 192.168.100.0/22

IPs calculation for the above CIDR block will be [192(8bits).168(8bits).100(8bits).0(8bits)] = 2^32-22 = 2^10 = 1,024
where 32 -> 8 * 4 bits , 22 -> prefix

Total number of IPs for 192.168.100.0/22 CIDR block will be 1,024

192.168.100.0 -> 255
192.168.101.0 -> 255
192.168.102.0 -> 255
192.168.103.0 -> 255
where 0 -> 255 = 256 IPs

Total IPs = 256 * 4 = 1,024

By default, AWS uses 5 IPs out of these 256 IPs, So the number of IPs released will be 256 - 5 = 251

IPs used by AWS:

0 -> Network Address
1 -> Amazon VPC Routing
2 -> Domain Name System (DNS)
3 -> Future use by AWS
255 -> Broadcasting

In AWS, a subnet can have 1 primary CIDR block and 4 secondary CIDR blocks

Details:

 Security Group vs Network ACL (Network Access Control List)

Examples:

1) List of EC2 instance IDs:
aws ec2 describe-instances | grep InstanceId | tr '",' ' '

2) List of Load Balancers by Name:
aws elbv2 describe-load-balancers | grep LoadBalancerName | tr '",' ' '

3) List of Target Groups by Name:
aws elbv2 describe-target-groups | grep TargetGroupName | tr '",' ' '

4) List of Autoscale Groups by Name:
aws autoscaling describe-auto-scaling-groups | grep AutoScalingGroupName | tr '",' ' '

5) List of Launch configurations by Name:
aws autoscaling describe-launch-configurations | grep LaunchConfigurationName | tr '",' ' '

 6) List of available IAM users by Name:
aws iam list-users | grep UserName | tr '",' ' '

Reference 

Solution:

The procedure to run the .sh file shell script on Linux is as follows:

Set execute permission on your script:
chmod +x <script-name>.sh

To run your script, enter:
./<script-name>.sh OR sh <script-name>.sh OR bash script-name>.sh

Quick fix:

Adding "#cloud-boothook" command to the script fixes the issue.

Example:

#cloud-boothook
#!/bin/bash
echo 'User-data bootstrap script invoked' > /home/ec2-user/bootstrap-script-output.txt