Work with instance user data - Amazon Elastic Compute Cloud
Specify instance user data at launchModify instance user dataRetrieve instance user data

Work with instance user data

When working with instance user data, keep the following in mind:

  • User data must be base64-encoded. The Amazon EC2 console can perform the base64-encoding for you or accept base64-encoded input.

  • User data is limited to 16 KB, in raw form, before it is base64-encoded. The size of a string of length n after base64-encoding is ceil(n/3)*4.

  • User data must be base64-decoded when you retrieve it. If you retrieve the data using instance metadata or the console, it's decoded for you automatically.

  • User data is treated as opaque data: what you give is what you get back. It is up to the instance to be able to interpret it.

  • If you stop an instance, modify its user data, and start the instance, the updated user data is not run when you start the instance.

Specify instance user data at launch

You can specify user data when you launch an instance. You can specify that the user data is run one time at launch, or every time you reboot or start the instance. For more information, see Run commands on your Linux instance at launch.

Modify instance user data

You can modify user data for an instance in the stopped state if the root volume is an EBS volume. For more information, see View and update the instance user data.

Retrieve instance user data

Note

The examples in this section use the IPv4 address of the IMDS: 169.254.169.254. If you are retrieving instance metadata for EC2 instances over the IPv6 address, ensure that you enable and use the IPv6 address instead: fd00:ec2::254. The IPv6 address of the IMDS is compatible with IMDSv2 commands. The IPv6 address is only accessible on Instances built on the Nitro System.

To retrieve user data from within a running instance, use the following URI.

http://169.254.169.254/latest/user-data

A request for user data returns the data as it is (content type application/octet-stream).

This example returns user data that was provided as comma-separated text.

IMDSv2
[ec2-user ~]$ TOKEN=`curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"` \
&& curl -H "X-aws-ec2-metadata-token: $TOKEN" -v http://169.254.169.254/latest/user-data
1234,john,reboot,true | 4512,richard, | 173,,,
IMDSv1
[ec2-user ~]$ curl http://169.254.169.254/latest/user-data
1234,john,reboot,true | 4512,richard, | 173,,,

This example returns user data that was provided as a script.

IMDSv2
[ec2-user ~]$ TOKEN=`curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"` \
&& curl -H "X-aws-ec2-metadata-token: $TOKEN" -v http://169.254.169.254/latest/user-data
#!/bin/bash
yum update -y
service httpd start
chkconfig httpd on
IMDSv1
[ec2-user ~]$ curl http://169.254.169.254/latest/user-data
#!/bin/bash
yum update -y
service httpd start
chkconfig httpd on

To retrieve user data for an instance from your own computer, see User data and the AWS CLI.