terminate ECS instances in an auto-scaling group

terminate-draining-instances.sh

#!/bin/bash
# inspects EC2 instances in an ECS cluster and terminates instances that are in a DRAINING state
# the instances are terminated via autoscaling, and the desired capacity is decremented
#
# this is the proper way to terminate EC2 instances in an ECS cluster because:
#    - if you just decrement the desired capacity, instances with running tasks may be terminated, and you may have an outage
#    - if you terminate instances with zero tasks, the autoscaling group will just replace them
#
# USAGE
#     DRY RUN
#       terminate-draining-instances my-cluster
#     FOR REAL
#       terminate-draining-instances my-cluster -y
arns=$(aws ecs list-container-instances --cluster $1 --status DRAINING | jq '[.containerInstanceArns[]] | join(" ")' -j)

if [ "$2" = "-y" ]; then
  for instanceid in `aws ecs describe-container-instances --cluster $1 --container-instances $arns | jq '.containerInstances[].ec2InstanceId' --raw-output`; do
    echo terminating $instanceid
    aws autoscaling terminate-instance-in-auto-scaling-group --instance-id $instanceid --should-decrement-desired-capacity
  done
else
  echo these instances will be terminated:
  aws ecs describe-container-instances --cluster $1 --container-instances $arns | jq '.containerInstances[].ec2InstanceId' --raw-output
fi