Below is a simple shell script that assists in creating a custom AMI from an existing EC2 instance using the AWS CLI [1][2]:

#!/bin/bash

# AWS Region
region="us-east-1"  # Replace with your desired region

# EC2 Instance ID
instance_id="your-instance-id"  # Replace with the ID of your EC2 instance

# AMI Name and Description
ami_name="MyCustomAMI"
ami_description="Custom Amazon Machine Image created using a script"

# Create the AMI
echo "Creating a custom Amazon Machine Image (AMI)..."
aws ec2 create-image \
  --instance-id $instance_id \
  --name "$ami_name" \
  --description "$ami_description" \
  --block-device-mappings "[{\"DeviceName\":\"/dev/sda1\",\"Ebs\":{\"VolumeType\":\"gp2\"}}]" \
  --region $region

# Wait for the AMI creation to complete
echo "Waiting for the AMI creation to complete..."
aws ec2 wait image-available --image-ids $ami_id --region $region

echo "Custom AMI created with ID: $ami_id"

Before running the script, make sure you have the AWS CLI configured with your AWS access and secret keys using aws configure. Also, replace "your-instance-id" with the actual instance ID and set the ami_name and ami_description as desired.
This script creates an AMI from an existing EC2 instance and waits for the AMI creation to complete before displaying the AMI ID. It's a simple example that can be expanded upon based on your specific requirements.