[Cialug] JSON Querying

[Todd Walton]

I'm having trouble figuring out this JSON query using jq. Maybe someone
here can help?

I do a 'aws ec2 describe-instances' and I get back a JSON document (pasted
in below, truncated for clarity). So now I want to use jq to return the
InstanceId of any instance with less than 2 BlockDeviceMappings. You'll
notice that the first instance, i-0f7771a48c88ec8fc, has 3 block device
mappings: /dev/sda1, /dev/xvdcp, and /dev/xvdcc, while the second instance,
i-0fcf111d64bc97558, only has 1, /dev/sda1. I want to pass all that through
jq such that I return the InstanceId of the second one.

Any suggestions? I'd take a non-jq suggestion if it were possible.

References:
https://stedolan.github.io/jq/
https://jqplay.org/

The doc:

{
  "Reservations": [
    {
      "Instances": [
        {
          "Monitoring": {
            "State": "enabled"
          },
          "PublicDnsName": "ec2-xx-xx-x-xx.name-1.amazonaws.com",
          "State": {
            "Code": 16,
            "Name": "running"
          },
          "LaunchTime": "2018-06-13T12:00:20.000Z",
          "PublicIpAddress": "xx.xx.x.xx",
          "PrivateIpAddress": "xx.xx.x.xx",
          "VpcId": "vpc-rjf271d9",
          "CpuOptions": {
            "CoreCount": 2,
            "ThreadsPerCore": 2
          },
          "StateTransitionReason": "",
          "InstanceId": "i-0f7771a48c88ec8fc",
          "ImageId": "ami-60812a1d",
          "PrivateDnsName": "ip-xx-xx-x-xx.ec2.internal",
          "SubnetId": "subnet-xxxxxxxxxx",
          "InstanceType": "m4.xlarge",
          "SourceDestCheck": true,
          "Hypervisor": "xen",
          "BlockDeviceMappings": [
            {
              "DeviceName": "/dev/sda1",
              "Ebs": {
                "Status": "attached",
                "DeleteOnTermination": true,
                "VolumeId": "vol-xxxxxxxxxxx",
                "AttachTime": "2018-04-09T19:41:10.000Z"
              }
            },
            {
              "DeviceName": "/dev/xvdcp",
              "Ebs": {
                "Status": "attached",
                "DeleteOnTermination": false,
                "VolumeId": "vol-xxxxxxxxxxx",
                "AttachTime": "2018-05-17T12:03:23.000Z"
              }
            },
            {
              "DeviceName": "/dev/xvdcc",
              "Ebs": {
                "Status": "attached",
                "DeleteOnTermination": false,
                "VolumeId": "vol-xxxxxxxxxxx",
                "AttachTime": "2018-05-18T19:34:56.000Z"
              }
            }
          ],
          "Architecture": "x86_64",
          "RootDeviceType": "ebs",
          "RootDeviceName": "/dev/sda1",
          "VirtualizationType": "hvm",
          "AmiLaunchIndex": 0
        },
        {
          "Monitoring": {
            "State": "enabled"
          },
          "PublicDnsName": "ec2-xx-xx-x-xx.name-1.amazonaws.com",
          "State": {
            "Code": 16,
            "Name": "running"
          },
          "LaunchTime": "2018-06-13T12:00:20.000Z",
          "PublicIpAddress": "xx.xx.x.xx",
          "PrivateIpAddress": "xx.xx.x.xx",
          "VpcId": "vpc-rjf271d9",
          "CpuOptions": {
            "CoreCount": 2,
            "ThreadsPerCore": 2
          },
          "StateTransitionReason": "",
          "InstanceId": "i-0fcf111d64bc97558",
          "ImageId": "ami-60812a1d",
          "PrivateDnsName": "ip-xx-xx-x-xx.ec2.internal",
          "SubnetId": "subnet-xxxxxxxxxx",
          "InstanceType": "m4.xlarge",
          "SourceDestCheck": true,
          "Hypervisor": "xen",
          "BlockDeviceMappings": [
            {
              "DeviceName": "/dev/sda1",
              "Ebs": {
                "Status": "attached",
                "DeleteOnTermination": true,
                "VolumeId": "vol-xxxxxxxxxxx",
                "AttachTime": "2018-04-09T19:41:10.000Z"
              }
            }
          ],
          "Architecture": "x86_64",
          "RootDeviceType": "ebs",
          "RootDeviceName": "/dev/sda1",
          "VirtualizationType": "hvm",
          "AmiLaunchIndex": 1
        }
      ]
    }
  ]
}

--
Todd