EC2 Tag conditionals

This is a python library and shell command that answers the question:

"Is this instance tagged with the given tag and have a given value"

It is designed to be run on AWS's EC2 instances.

It will always fail if it's not on AWS, so tags should only be tested for truthiness, not falseness.

As a Library

from ec2_tag_conditional import InstanceTags

tags = InstanceTags()

if tags['Env'] == 'prod':
    do_prod_thing()
 else:
    do_other_thing()

As a command line script

> instance-tags "Env=prod"
> echo $?
0

> instance-tags "Madeup=NotThere"
> echo $?
1

> instance-tags "Env=prod" && do_prod_thing

Example use cases

This code was written with the following use case in mind:

You have n servers in a auto scaling group, launched from a custom AMI (golden image). The nature of the application running on the servers is that, for some functions to work (backup, reporting), a given set of tasks should only be run by one server.

This server is called a 'controller'. The script that created the ASG also tags (in the AWS metadata) one (and only one) of the servers with controller=True.

When the AMI is baked, the images don't need to know if they are a controller or not, as cron tasks can be written like:

instance-tags "controller=True" && do_controller_only

Or for controllers in production (rather than dev or staging environments):

instance-tags "controller=True" && instance-tags "Env=prod" && do_controller_only

Because the exit code of the instance-tags script is 1 if the tag with the given value isn't found on the instance, the script wont run on any server that isn't an EC2 instance with the given values.