How do I pass the input of a step function startExecution to the executed function

Question

I'm trying to use a step function startExecution state to break up a state machine into 2 parts and I want to pass the input of the startExecution function to the executed function, e.g.,

```
"Process chunk": {
            "Type": "Task",
            "Resource": "arn:aws:states:::states:startExecution",
            "Parameters": {
              "StateMachineArn": "arn:aws:states:us-west-1:12345:stateMachine:Ingestion-process-parts",
              "Input": "$.input.MessageDetails"  
            },
            "End": true,
            "ResultPath": "$.input.MessageDetails"
          }

```

where the startExecution input is

```
"input": {
    "MessageNumber": 0,
    "MessageDetails": ""
  },
```

but no matter what I try for the input to the executed state machine it just passes a literal value, i.e., in the case above, instead of passing the input of the startExecution  state to the executed function it passes

```
"input": {
    "MessageDetails": "$.input.MessageDetails",
    "AWS_STEP_FUNCTIONS_STARTED_BY_EXECUTION_ID": "arn:aws:states:us-west-1:1234:execution:Ingestion-Chunk-CSV:055ede1a-dc03-4412-a341-d3b6611dbed4"
  },

```
Note that the same input parsing Does work for the Result:

```
{
  "name": "Process chunk",
  "output": {
    "MessageNumber": 0,
    "MessageDetails": "",

```

It doesn't appear the the normal approach of parsing the input using $... works for this purpose.  Is it possible to do what I want?

thanks

Accepted Answer

As documented [here](https://docs.aws.amazon.com/step-functions/latest/dg/input-output-inputpath-params.html), 
> For key-value pairs where the value is selected using a path, the key name must end in .$

What you need to do is the following:
```
"Process chunk": {
     "Type": "Task",
     "Resource": "arn:aws:states:::states:startExecution",
     "Parameters": {
          "StateMachineArn": "arn:aws:states:us-west-1:12345:stateMachine:Ingestion-process-parts",
          "Input.$": "$.input.MessageDetails"  
     },
     "End": true,
     "ResultPath": "$.input.MessageDetails"
 }
```

How do I pass the input of a step function startExecution to the executed function

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