# Amazon ECS environment variables
<a name="ecs-environment-variables"></a>

Environment variables control a task's behavior such as the capacity used to run the task. Amazon ECS sets the following environment variables for your tasks:
+ `AWS_CONTAINER_CREDENTIALS_RELATIVE_URI` - The full HTTP URL endpoint for the SDK to use when making a request for credentials. This includes both the scheme and the host. For more information, see [Container credential provider](https://docs.aws.amazon.com/sdkref/latest/guide/feature-container-credentials.html) in the *AWS SDKs Reference Guide*.
+ `ECS_CONTAINER_METADATA_URI_V4` - The address of the task metadata version 4. For more information, see [Amazon ECS task metadata endpoint version 4](task-metadata-endpoint-v4.md).
+ `ECS_CONTAINER_METADATA_URI` - The address of the task metadata version 3. For more information, see [Amazon ECS task metadata endpoint version 3](task-metadata-endpoint-v3.md).
+ `ECS_AGENT_URI` - The base address for different endpoints supported by Fargate. For more information, see:
  +  [Amazon ECS task scale-in protection endpoint](task-scale-in-protection-endpoint.md)
  +  [Amazon ECS fault injection endpoints](fault-injection-endpoints.md)
+ `AWS_EXECUTION_ENV` - The information about the compute option the task runs on.
  + For Fargate, Amazon ECS sets this to `AWS_ECS_FARGATE`. 
  + For EC2, Amazon ECS sets this to `AWS_ECS_EC2`. 
+ `AWS_DEFAULT_REGION` – The default AWS Region for your AWS account. This is the default Region where your task runs.
+ `AWS_REGION` – The Region where the task runs. If defined, this value overrides the `AWS_DEFAULT_REGION`.

You can view the environment variables in the task metadata. For more information, see one of the following topics:
+ Fargate - [Amazon ECS task metadata available for tasks on Fargate](fargate-metadata.md)
+ EC2 - [Task metadata available for Amazon ECS tasks on EC2](ec2-metadata.md)