List running EC2 instances with a given tag older than a certain number of hours

too-old.sh

# too-old.sh
# Author: David Adams
# License: Public Domain
#
# Provides a bash function for listing EC2 instances with a given tag value older than a
# certain number of hours. This script is not particularly robust to unexpected error
# conditions. This is just a quick example.
#
# PREREQUISITES
#
# You must have the AWS CLI installed and in your path as `aws` and you must have jq 1.6+
# installed and in your path as `jq`. This function also assumes you have appropriate AWS
# credentials configured with access to run ec2:DescribeInstances. Finally, you should also
# have your AWS region configured either via your AWS CLI config or by setting the AWS_REGION
# environment variable.
#
# USAGE
#
# Load this file into your shell by running `source too-old.sh`.
#
# To list all instances with tag Role=worker launched more than 72 hours ago, run:
#
#   $ list-old-instances Role worker 72
#   i-010102930184dab
#   [... etc ...]
#
# If no instances are found a message will be written to stderr:
#
#   $ list-old-instances Role none 100000
#   No instances found with tag 'Role'='none' running longer than 100000 hours.
#

list-old-instances() {
    if (( # < 3 )); then
        echo "USAGE: list-old-instances <tag-name> <tag-value> <hours>" >&2
        return 1
    fi

    local tagname=$1
    local tagvalue=$2
    local hours=$3

    local result=$(
        aws ec2 describe-instances \
            --output json \
            --filters \
            Name=tag:"$tagname",Values="$tagvalue" \
            Name=instance-state-name,Values=running \
        | jq -r --argjson hours "$hours" '
            .Reservations[].Instances[]
            |select((.LaunchTime|sub("\\+00:00$";"Z")|fromdate) < (now - ($hours*60*60)))
            |.InstanceId
        '
    )

    if [[ -z $result ]]; then
        echo "No instances found with tag '$tagname'='$tagvalue' running longer than $hours hours." >&2
    else
        echo "$result"
    fi
}