How to get Task ID from within ECS container?

The technique I'd use is to set the environment variable in the container definition.

If you're managing your tasks via Cloudformation, the relevant yaml looks like so:

  Taskdef:
    Type: AWS::ECS::TaskDefinition
    Properties:
      ...
      ContainerDefinitions:
        - Name: some-name
          ...
          Environment:
            - Name: AWS_DEFAULT_REGION
              Value: !Ref AWS::Region
            - Name: ECS_ENABLE_CONTAINER_METADATA
              Value: 'true'

This technique helps you keep everything straightforward and reproducible.

If you need metadata programmatically and don't have access to the metadata file, you can query the agent's metadata endpoint:

curl http://localhost:51678/v1/metadata

Note that if you're getting this information as a running task, you may not be able to connect to the loopback device, but you can connect to the EC2 instance's own IP address.

Previous answers are correct, here is another way of doing this:

From the ec2 instance where container is running, run this command

curl http://localhost:51678/v1/tasks | python -mjson.tool |less

This doesn't work for newer Amazon ECS container versions anymore, and in fact it's now much simpler and also enabled by default. Please refer to this docu, but here's a TL;DR:

If you're using Amazon ECS container agent version 1.39.0 and higher, you can just do this inside the docker container:

curl -s "$ECS_CONTAINER_METADATA_URI_V4/task" \
  | jq -r ".TaskARN" \
  | cut -d "/" -f 3

Here's a list of container agent releases, but if you're using :latest – you're definitely fine.