What are the main use cases for using Amazon EFS?

High scalibility
High availability
Web hosting
Content management system (CMS)
Strong data consistency
Integration with AWS services like AWS EC2, AWS Lambda

Does Amazon EFS support encryption at rest and in transit? If yes, then how can it be configured?

When you create a new file system using the Amazon EFS console, encryption at rest is enabled by default
Open the Amazon Elastic File System console.
To open the file system creation wizard, click Create file system.
Select Enable encryption.
To enable encryption using your own KMS CMK key, from the KMS master key list select the name of your AWS Key.

Is there any way to encrypt data stored on an EFS file system without having to re-encrypt all existing files?

Use AWS Command Line Interface (AWS CLI) or AWS SDK to create a cross-account Amazon S3 location in DataSync.
Create a DataSync task that transfers data from the source bucket to the destination bucket.

How do we access data stored on an EFS file system from an EC2 instance?

Create Your Security Groups.
Create our EFS file system.
Migrate to the EC2 management console, launch our EC2 instance and mount our filesystem, then check this connection.
Configure Security Group

Can you explain about any 5 aws S3 cli commands?

1.aws s3 mb [--options]
This command is used to make a bucket. Bucket names must be globally unique (unique across all of Amazon S3) and should be DNS compliant.
2.aws s3 ls [--options]
To list your buckets, folders, or objects, use the s3 ls command. Using the command without a target or options lists all buckets.
3.aws s3 rb [--options]
To delete a bucket, use the s3 rb command.
4.aws s3 rm [--options]
To delete objects in a bucket or your local directory, use the s3 rm command.
5.aws s3 mv [--options]
Use the s3 mv command to move objects from a bucket or a local directory.