"Good enough" leader election script for AWS AutoScalingGroups

is_leader.sh

#!/usr/bin/env bash

function echoerr { echo "$@" 1>&2; }
function error_exit { echoerr "$1"; exit 1; }

if [ -z "${REGION}" ] ; then error_exit "No REGION set"; fi
command -v jq >/dev/null 2>&1 || error_exit "'jq' not found"
command -v aws >/dev/null 2>&1 || error_exit "'aws' not found"
command -v tee >/dev/null 2>&1 || error_exit "'tee' not found"
command -v ec2metadata >/dev/null 2>&1 || error_exit "'ec2metadata' not found"

function getAsgName() {
    MAX_CACHE_AGE=86400
    INSTANCE_ID=$1
    CACHE_FILENAME="/tmp/asglookup_${INSTANCE_ID}"
    if [ -f ${CACHE_FILENAME} ] ; then
        LAST_MODIFIED=$(stat -c %Y ${CACHE_FILENAME})
        NOW=$(date +%s)
        AGE=$((NOW-LAST_MODIFIED))
        if [ "${AGE}" -lt "${MAX_CACHE_AGE}" ] ; then
            cat "${CACHE_FILENAME}"
            return;
        fi
    fi
    ASG_NAME=$(aws --output text --region "${REGION}" ec2 describe-tags --filters "Name=resource-id,Values=${INSTANCE_ID}" "Name=key,Values=aws:autoscaling:groupName" --query "Tags[0].Value")
    echo "${ASG_NAME}" | tee ${CACHE_FILENAME}
}

function getOldestInstance() {
    MAX_CACHE_AGE=60
    ASG_NAME=$1
    CACHE_FILENAME="/tmp/oldestinstancelookup_${ASG_NAME}"
    if [ -f ${CACHE_FILENAME} ] ; then
        LAST_MODIFIED=$(stat -c %Y ${CACHE_FILENAME})
        NOW=$(date +%s)
        AGE=$((NOW-LAST_MODIFIED))
        if [ "${AGE}" -lt "${MAX_CACHE_AGE}" ] ; then
            cat "${CACHE_FILENAME}"
            return;
        fi
    fi
    OLDEST_INSTANCE_ID=$(aws --region "${REGION}" ec2 describe-instances --filters "Name=tag:aws:autoscaling:groupName,Values=${ASG_NAME}" "Name=instance-state-name,Values=running" | jq -r '[.Reservations[].Instances[]] | sort_by(.LaunchTime)[0] | .InstanceId')
    echo "${OLDEST_INSTANCE_ID}" | tee ${CACHE_FILENAME}
}

THIS_INSTANCE_ID=$(ec2metadata --instance-id)
ASG_NAME=$(getAsgName ${THIS_INSTANCE_ID})
OLDEST_INSTANCE_ID=$(getOldestInstance ${ASG_NAME})

if [ "${THIS_INSTANCE_ID}" != "${OLDEST_INSTANCE_ID}" ] ; then
    exit 1;
fi

Usage:

is_leader.sh && echo "Yes, I am the leader"
is_leader.sh || echo "No, I'm not the leader"