Using jq to parse json output of AWS CLI tools with Lightsail - amazon-web-services

Question

I'm trying to modify a script to automate lightsail snapshots, and I am having trouble modifying the jq query.
I'm trying to parse the output of aws lightsail get-instance-snapshots
This is the original line from the script:
aws lightsail get-instance-snapshots | jq '.[] | sort_by(.createdAt) | select(.[0].fromInstanceName == "WordPress-Test-Instance") | .[].name'
which returns a list of snapshot names with one per line.
I need to modify the query so that is does not return all snapshots, but rather only ones where the name start with 'autosnap'. i'm doing this as the script rotates snapshots, but I don't want it to delete snapshots I manually create (which will not start with 'autosnap').
Here is a redacted sample output from aws lightsail get-instance-snapshots
{
"instanceSnapshots": [
{
"location": {
"availabilityZone": "all",
"regionName": "*****"
},
"arn": "*****",
"fromBlueprintId": "wordpress_4_9_2_1",
"name": "autosnap-WordPress-Test-Instance-2018-04-16_01.46",
"fromInstanceName": "WordPress-Test-Instance",
"fromBundleId": "nano_1_2",
"supportCode": "*****",
"sizeInGb": 20,
"createdAt": 1523843190.117,
"fromAttachedDisks": [],
"fromInstanceArn": "*****",
"resourceType": "InstanceSnapshot",
"state": "available"
},
{
"location": {
"availabilityZone": "all",
"regionName": "*****"
},
"arn": "*****",
"fromBlueprintId": "wordpress_4_9_2_1",
"name": "Premanent-WordPress-Test-Instance-2018-04-16_01.40",
"fromInstanceName": "WordPress-Test-Instance",
"fromBundleId": "nano_1_2",
"supportCode": "*****",
"sizeInGb": 20,
"createdAt": 1523842851.69,
"fromAttachedDisks": [],
"fromInstanceArn": "*****",
"resourceType": "InstanceSnapshot",
"state": "available"
}
]
}
I would have thought something like this would work, but I'm not having any luck after many attempts...
aws lightsail get-instance-snapshots | jq '.[] | sort_by(.createdAt) | select(.[0].fromInstanceName == "WordPress-Test-Instance") | select(.[0].name | test("autosnap")) |.[].name'
Any help would be greatly appreciated!

Answer 1

The basic query for making the selection you describe would be:
.instanceSnapshots | map(select(.name|startswith("autosnap")))
(If you didn't need to preserve the array structure, you could go with:
.instanceSnapshots[] | select(.name|startswith("autosnap"))
)
You could then perform additional filtering by extending the pipeline.
If you were to use test/1, the appropriate invocation would be test("^autosnap") or perhaps test("^autosnap-").
Example
.instanceSnapshots
| map(select(.name|startswith("autosnap")))
| map(select(.fromInstanceName == "WordPress-Test-Instance"))
| sort_by(.createdAt)
| .[].name
The two successive selects could of course be compacted into one. For efficiency, the sorting should be done as late as possible.
Postscript
Although you might indeed be able to get away with commencing the pipeline with .[] instead of .instanceSnapshots, the latter is advisable in case the JSON schema changes. In a sense, the whole point of data formats like JSON is to make it easy to write queries that are robust with respect to (sane) schema-evolution.