Q:

How can I get the RDS endpoint for use in Userdata

I am trying to get the RDS endpoint to use in user data with cli but unable to figure it out.
I need to get the RDS endpoint to inject into a php file but when I try the following I get:
Unable to locate credentials. You can configure credentials by running "aws configure".

I am building the ec2 and vpc using CLI and need to be able to get RDS endpoint as part of the Userdata.
I tried the following on the EC2 instance itself and I get the above error.
aws rds --region ca-central-1 describe-db-instances --query "DBInstances[*].Endpoint.Address"

Even if I am able to resolve that, I need to be able to get the endpoint to pass as part of the userdata. Is that even possible?

A:

The Unable to locate credentials error says that the AWS Command-Line Interface (CLI) does not have any credentials to call the AWS APIs.
You should assign a role to the EC2 instance with sufficient permission to call describe-db-instances on RDS. See: IAM Roles for Amazon EC2
Then, your User Data can include something like:
#!
RDS=`aws rds --region ca-central-1 describe-db-instances --query "DBInstances[*].Endpoint.Address"`
echo >file $RDS

Or pass it as a parameter:
php $RDS

