EBS storage partition using a custom script in EC2 "User data" during launch.

Yes, it is possible to partition an EC2 instance's EBS volume during launch using a custom script in the "User data" section. This approach can be used to create the specific partitions you've outlined for your CIS Level 1 compliant RHEL 9 instance.

Here's a high-level overview of how you can achieve this:

1. Create a shell script that includes the necessary commands to partition the EBS volume according to your specifications.

2. This script would typically use tools like parted or fdisk to create the partitions, then use mkfs to format them, and finally update the /etc/fstab file to ensure the partitions are mounted correctly on subsequent boots.

3. Include this script in the "User data" section when launching your EC2 instance.

4. The script will run automatically when the instance first boots up, creating and configuring the partitions as specified.

However, it's important to note that this approach requires careful scripting and testing, as errors in the partitioning process could render the instance unusable. Also, this method is best suited for new instances; modifying existing instances' partitions can be risky and may lead to data loss.

If you're looking for alternatives that don't involve custom scripting or the AWS CLI, you might consider:

1. Creating a custom AMI: You could launch an EC2 instance, manually partition it as needed, then create an AMI from that instance. Future instances launched from this AMI would have the desired partition structure.

2. Using configuration management tools: Tools like Ansible, Chef, or Puppet can be used to configure instances post-launch, including partitioning.

3. Leveraging AWS Systems Manager: You could use Systems Manager to run commands or scripts on your instances after they've launched, which could include partitioning commands.

These alternatives might be easier to manage and less error-prone than custom User Data scripts, especially if you're not comfortable with low-level disk partitioning commands. However, they may require additional setup or have associated costs.

Remember to thoroughly test any partitioning solution in a non-production environment before implementing it in your production setup.
Sources
Run commands when you launch an EC2 instance with user data input - Amazon Elastic Compute Cloud
Example: Run additional scripts for AWS PCS from an S3 bucket - AWS PCS
Automating EC2 Instance Naming with Terraform and User Data: A Solution for Dynamic Environments | AWS re:Post