How to filter for multiple identifier values in AWS CLI

When filtering multiple identifier values in the AWS CLI, the results are the same with or without a period. Is the period optional?

The official references does not mention that it can be omitted.

• AWS Command Line Interface: Filtering for multiple identifier values
• JMESPath: SubExpressions

The output of the following two queries will be the same:

aws ec2 describe-instances \ 
  --query 'Reservations[].Instances[].[InstanceId,InstanceType]'

Returns

[
    [
        "i-0f74894a273e9a76f",
        "t2.micro"
    ],
    [
        "i-007598b789e08ca19",
        "t2.micro"
    ],
    [
        "i-0e2defdabf5771dfa",
        "t2.micro"
    ],
    [
        "i-07db1ecca76653ae5",
        "t2.micro"
    ]
]

And

aws ec2 describe-instances \
  --query 'Reservations[].Instances[][InstanceId,InstanceType]'

Returns

[
    [
        "i-0f74894a273e9a76f",
        "t2.micro"
    ],
    [
        "i-007598b789e08ca19",
        "t2.micro"
    ],
    [
        "i-0e2defdabf5771dfa",
        "t2.micro"
    ],
    [
        "i-07db1ecca76653ae5",
        "t2.micro"
    ]
]

Why is that?

The two are equivalent because the result of Reservations[].Instances[] is a JSON array.
So you can either do a Multiselect on top of this array [].[InstanceId,InstanceType] or do a projection of the fields in the array itself [][InstanceId,InstanceType].

So, indeed on a simple JSON:

{
  "foo": [{
    "bar": "baz",
    "qux": "quux"
  }]
}

These expressions:

• foo[].[bar, qux]
  

• foo[][bar, qux]
  

are the same and both yields

[
  [
    "baz",
    "quux"
  ]
]

This said, you should probably favour the first form foo[].[bar, qux], not only because this is the syntax used in the tutorials, but also because it will work in all the situation, so, also, if the parent JSON node is an object and not an array.

Here is a example where the two syntax are not identical, given the JSON:

{
  "foo": {
    "bar": "baz",
    "qux": "quux"
  }
}

• The expression:

  foo.[bar, qux]
  

  will work as expected and return

  [
    "baz",
    "quux"
  ]
  

• While

  foo[bar, qux]
  

  will just error out and gives you nothing in return.