Overview

If you are working in cloud and that too specifically in AWS you will be familiar with the AWS Commandline Interface . It is quite powerful and very useful to quickly accomplish something and also script it at the same time. It can return results in either text or JSON format. JSON is useful when you need to programmatically interpret the results. This post provides an example of how to filter and process the outputs in AWS Commandline Interface.

AWS Commandline Interface provides a --query option to filter the command results based on JMESPath . If you haven’t already, please take a moment to go through AWS documentation on Controlling command output from the AWS CLI.

Also, the blog post Advanced AWS CLI JMESPath Query Tricks talks about it really well.

The Problem

A few months ago, I faced a problem where I had to programmatically fetch the most recent object from an S3 bucket given the bucket prefix. First, I had to figure out how to get metadata about s3 objects, specifically the last modified timestamp. Then I had to figure out how to sort these objects based on the timestamp. I knew JMESPath had sort and sort_by methods. Having never used them before, it took me some time to figure out the correct command. Here, I have attempted to explain the thought flow and process I went through to figure out the solution

Let’s solve this one step at a time.

The base command

This is the raw AWSCLI command to fetch all s3 objects matching the given prefix:

aws s3api list-objects --bucket <your bucket name>  --prefix "<object prefix>"

This provides result as shown in AWSCLI reference. An example shown here as well for convenience:

{
    "Contents": [
        {
            "Key": "SomePrefix/MyObject1"
            "LastModified": "2019-02-01T00:00:04.000Z",
            "ETag": "\"445175bab9cfc988fbe495a2a49b65d5\"",
            "Size": 7889,
            "StorageClass": "STANDARD",
            "Owner": {
                "DisplayName": "my_owner-name",
                "ID": "9e6b02405e2933107e2d7489d7a22ca772ae1eecf4677a27517b9ec21d248d9b"
            }
        },
       ....
    ]
}

JMESPath Query

Now, all we have to do is sort the list under Contents based on LastModified timestamp. So we add --query 'sort_by(Contents, &LastModified). to the command. You can find examples for sort_by method at https://jmespath.org/examples.html#using-functions . This gives us a list of objects in a sorted order. The command is now :

aws s3api list-objects --bucket <your bucket name>  --prefix "<object prefix>"  --query 'sort_by(Contents, &LastModified)

But we only want the most recently modified key. Now that we have a sorted list, the most recently modified key will be the last item within the list. So we pipe the output of the previous query and get the last item using [-1]. Since we are only interested in the name of the s3 key we can refine this further to get only the key by [-1].Key.

Finally, we have our command!

aws s3api list-objects --bucket <your bucket name>  --prefix "<object prefix>"  --query 'sort_by(Contents, &LastModified) | [-1].Key' --output text

Conclusion

I have used this approach to get the desired results several times in various contexts. I hope this will be useful to you too.