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.