How to get the most recently launched EC2 instance with AWS CLI? - amazon-web-services

Question

I'm currently using the following CLI command to get the instance PublicIPAddress and LaunchTime for a given instance Name tag, 'myInstanceName':
aws ec2 describe-instances --filters 'Name=tag:Name,Values=myInstanceName' \
--region us-east-1 \
--query 'Reservations[*].Instances[*].{PublicIpAddress: PublicIpAddress, LaunchTime: LaunchTime}'
This results in the following:
[
{
"LaunchTime": "2019-01-25T11:49:06.000Z",
"PublicIpAddress": "11.111.111.11"
}
]
This is fine, but if there are two instances with the same name I will get two results in my result JSON. I need to find a way to get the most recent instance for a given name.
Solution Update
This question is quite specific to EC2 instances. The issue can be resolved using two different methods, answered below:
Parsing Result with jq
Using JMESPath
Please see this related question for more general sorting by date with JMESPath, and for further reading.

Answer 1

Here's a method for finding the latest-launched instance, and displaying data about it:
aws ec2 describe-instances --query 'sort_by(Reservations[].Instances[], &LaunchTime)[:-1].[InstanceId,PublicIpAddress,LaunchTime]'
sort_by(Reservations[].Instances[], &LaunchTime)[:-1] will return the last instance launched. The fields are then retrieved from those instances.
To understand this type of fun, see:
JMESPath Tutorial — JMESPath
JMESPath Examples — JMESPath

Answer 2

Try using the jq utility. It's a command-line JSON parser. If you're not familiar with it then I'd recommend the jq playground for experimentation.
First flatten the awcli results, as follows:
aws ec2 describe-instances \
--query 'Reservations[].Instances[].{ip: PublicIpAddress, tm: LaunchTime}' \
--filters 'Name=tag:Name,Values= myInstanceName'
Note that I've aliased LaunchTime to tm for brevity. That will result in (unsorted) output like this:
[
{
"ip": "54.4.5.6",
"tm": "2019-01-04T19:54:11.000Z"
},
{
"ip": "52.1.2.3",
"tm": "2019-03-04T20:04:00.000Z"
}
]
Next, pipe this result into jq and sort by descending tm (the alias for LaunchTime), as follows:
jq 'sort_by(.tm) | reverse'
That will result in output like this:
[
{
"ip": "52.1.2.3",
"tm": "2019-03-04T20:04:00.000Z"
},
{
"ip": "54.4.5.6",
"tm": "2019-01-04T19:54:11.000Z"
}
]
Finally, use jq to filter out all but the first result, as follows:
jq 'sort_by(.tm) | reverse | .[0]'
This will yield one result, the most recently launched instance:
{
"ip": "52.1.2.3",
"tm": "2019-03-04T20:04:00.000Z"
}
Putting it all together, the final command is:
aws ec2 describe-instances \
--query 'Reservations[].Instances[].{ip: PublicIpAddress, tm: LaunchTime}' \
--filters 'Name=tag:Name,Values= myInstanceName' | \
jq 'sort_by(.tm) | reverse | .[0]'