JMESPath descend in tree for filter

Question

What I'm trying to achieve

I want to extract the volume ID for the root block device using describe-instances.

What I tried so far

aws ec2 describe-instances --filters "Name=tag:Backup,Values=True" --query 'Reservations[].Instances[].{Name: Tags[?Key==`Name`].Value | [0], Id: InstanceId, Block: BlockDeviceMappings[?DeviceName==RootDeviceName].Ebs.VolumeId, Test: RootDeviceName}'

What's not working

Several things:

1. Ebs.VolumeId is not the direct descendant of DeviceName, it is descending from BlockDeviceMappings.
2. RootDeviceName is not a descendant of BlockDeviceMappings.

So when I'm trying to pull the RootDeviceName and search the VolumeId accordingly I'm getting a blank field (Block: is for testing and irrelevant to the case).

The first 2 fields are correct.

Thanks in advance!

Answer 1

Yes, that is quite an ask!

The closest I got working was to specify the actual value for DeviceName:

aws ec2 describe-instances --query 'Reservations[].Instances[].[InstanceId, BlockDeviceMappings[?DeviceName==`/dev/xvda`].Ebs.VolumeId]'

(This syntax worked on a Mac.)

Frankly, I'd recommend using a language to make the call (eg Python) and then apply your own logic, rather than trying to convince JMESPath to extract the correct values.

It would be something like this:

import boto3

ec2_client = boto3.client('ec2', region_name = 'ap-southeast-2')

response = ec2_client.describe_instances(
    Filters=[
        {
            'Name': 'tag:Backup',
            'Values': ['True']
        }
    ]
)

for r in response['Reservations']:
    for i in r['Instances']:
        name = [t['Value'] for t in i['Tags'] if t['Key'] == 'Name'][0]
        for b in i['BlockDeviceMappings']:
            if b['DeviceName'] == i['RootDeviceName']:
                print (i['InstanceId'], name, b['Ebs']['VolumeId'])