AWS cli JMESpath query tag values

Question

As a follow-up to my previous question, I am having trouble getting tags to work right in this type of output. I want to print a table with each property as a row. This is how I expect it to look:

% aws --output table ec2 describe-instances --instance-id $id --query "Reservations[].Instances[0].[{ Property: 'Type', Value: InstanceType }]"         
-------------------------------
|      DescribeInstances      |
+-----------+-----------------+
| Property  |      Value      |
+-----------+-----------------+
|  Type     |  g4dn.12xlarge  |
+-----------+-----------------+

But with tag names it looks like this:

% aws --output table ec2 describe-instances --instance-id $id --query "Reservations[].Instances[0].[{ Property: 'Name', Value: Tags[?Key =='Name'].Value }]"   
-------------------
|DescribeInstances|
+-----------------+
|    Property     |
+-----------------+
|  Name           |
+-----------------+
||     Value     ||
|+---------------+|
||  Elliott-TKD  ||
|+---------------+|

The tag value is correct, but the formatting is weird, and when combined with more other rows the table gets really ugly.

Tags is an array, how do you expect it to render? Multiple rows, comma separated, else? — β.εηοιτ.βε, Feb 18 '21 at 17:44
I'm filtering the array to get a a single string with `.Value` — Elliott B, Feb 18 '21 at 17:45
Oh! Ok I see you're right. So is there a way to turn it into a string? — Elliott B, Feb 18 '21 at 17:47

β.εηοιτ.βε · Accepted Answer · 2021-02-18T20:03:54.040

The filter part of your query ([?Key == 'Name']) is creating what JMESPath is describing as a projection.
You will have to reset this projection in order to extract a single string out of it.
Resetting a projection can be achieved using pipes.

Projections are an important concept in JMESPath. However, there are times when projection semantics are not what you want. A common scenario is when you want to operate of the result of a projection rather than projecting an expression onto each element in the array. For example, the expression people[*].first will give you an array containing the first names of everyone in the people array. What if you wanted the first element in that list? If you tried people[*].first[0] that you just evaluate first[0] for each element in the people array, and because indexing is not defined for strings, the final result would be an empty array, []. To accomplish the desired result, you can use a pipe expression, <expression> | <expression>, to indicate that a projection must stop.

^{Source: https://jmespath.org/tutorial.html#pipe-expressions}

So your issue is very close to what they are describing here in the documentation and the reset of that projection can be achieved using:

Tags[?Key =='Name']|[0].Value

or, with:

Tags[?Key =='Name'].Value | [0]

which are two strictly identical queries.

Given the JSON:

{
  "Reservations": [
    {
      "Instances": [
        {

          "Tags": [
            {
              "Key": "Name",
              "Value": "Elliott-TKD"
            },
            {
              "Key": "Foo",
              "Value": "Bar"
            }
          ]
        }
      ]
    }
  ]
}

The query

Reservations[].Instances[0].[{ Property: `Name`, Value: Tags[?Key == `Name`]|[0].Value }]

Will give the expected

[
  [
    {
      "Property": "Name",
      "Value": "Elliott-TKD"
    }
  ]
]

So it will render properly in your table

------------------------------
|      DescribeInstance      |
+------------+---------------+
|  Property  |     Value     |
+------------+---------------+
|    Name    |  Elliott-TKD  |
+------------+---------------+

Thank you! I started out trying to write a very simple bash script, but now I'm starting to think it would have been easier to write it in Python with boto3. — Elliott B, Feb 18 '21 at 21:47

AWS cli JMESpath query tag values

1 Answers1