Filtering AWS CLI output by tags using jq

Recently I was writing a shell script to deal with the AWS CLI, and I wanted to filter the list of results using jq. Specifically, I wanted to filter using some of the AWS tags, which are a bit unwieldy – although the tags form a set of key/value pairs, they’re returned as a list of objects with Key/Value keys.

For example, given this list of two instances, how do I get the JSON object for the bastion host?

[
  {
    "InstanceId": "i-123456789",
    "Tags": [
      {
        "Key": "Environment",
        "Value": "Production"
      },
      {
        "Key": "Name",
        "Value": "container-host_a88676"
      }
    ]
  },
  {
    "InstanceId": "i-987654321",
    "Tags": [
      {
        "Key": "Environment",
        "Value": "Production"
      },
      {
        "Key": "Name",
        "Value": "bastion-host_517e67"
      }
    ]
  }
]

I found a few snippets around the Internet, but they all did a complicated combination of map and select. I cobbled this together, which seemed to work, but I had to really stare at it to understand what it was doing:

jq 'map(select(.Tags[] | select(.Key=="Name") | .Value | startswith("bastion-")))'

I started reading the jq documentation to understand exactly how this worked, when I stumbled upon a much better way to do this:

I can use the from_entries filter as an intermediate transformation step to turn the tags into an object. This simplifies the structure of Tags, for example:

jq 'map(.Tags |= from_entries)'

transforms the JSON into:

[
  {
    "InstanceId": "i-123456789",
    "Tags": {
      "Environment": "Production",
      "Name": "container-host_a88676"
    }
  },
  {
    "InstanceId": "i-987654321",
    "Tags": {
      "Environment": "Production",
      "Name": "bastion-host_517e67"
    }
  }
]

I can then work with the tags as an object, and add my filter by name:

jq 'map(.Tags |= from_entries) | map(select(.Tags.Name | startswith("bastion-")))'

or I can combine it into a single step, like so:

jq 'map(select(.Tags | from_entries | .Name | startswith("bastion-")))'

I find both of those easier to understand than the doubly-nested select() I had in my first snippet.