Parsing AWS instance data with jq

Semi-random amusing code snippet of the day:

aws ec2 describe-instances | jq << EOF
    .[][].Instances[]
    | select(.Tags[]?.Value == "production")
    | .PrivateIpAddress
EOF

Basically, it’s combining the AWS command line tools and the excellent jq tool for parsing JSON to extract a field from all instances with a particular tag on your AWS account (whatever account you have configured in your ~/.aws/ directory).

To describe it a little bit more, the data is structured as a list of Instance objects. The first line of the jq query loops over each instance object.

Next, each of those has zero or more Tags (the []? is to not fail if the tag object is empty), with Key and Value entries. select is a new feature I hadn’t seen before which will pass along an object if the condition holds. These are essentially equivalent:

select(condition)
if condition then . else empty end

After that, we extract a given field. In this particular case, I wanted IP addresses, but there are a bunch of other fields you can access. Here are a few other interesting ones:

  • AmiLaunchIndex
  • Architecture
  • ImageId
  • InstanceId
  • InstanceType
  • LaunchTime
  • PrivateDnsName
  • PrivateIpAddress
  • PublicDnsName
  • PublicIpAddress
  • SecurityGroups
  • State
  • SubnetId
  • Tags

The beauty of doing this directly in the shell is that you can then chain it to something else. For example, what if I wanted to log into every production server in turn and ask how much free disk space they have:

for IP in aws ec2 describe-instances | jq << EOF
    .[][].Instances[]
    | select(.Tags[]?.Value == "production")
    | .PrivateIpAddress
EOF
do
    echo $IP
    ssh $IP du -h
    echo
done

I’m really starting to admire the ‘Do One Thing and Do It Well’ philosophy of Unix and chaining things together.

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