Safely shutdown a preemptible GCE instance (Bash)

gce_preemptible_safety_shutdown_script.sh

#!/bin/bash

set -u

export PATH="/usr/local/bin:/sbin:/usr/sbin:$PATH"

instance_name=$(curl -sSfL -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/name)
zone_info=$(curl -sSfL -H "Metadata-Flavor: Google" http://metadata.google.internal/computeMetadata/v1/instance/zone)
zone=${zone_info##*/}
region=${zone:0:-2}
created_by=$(gcloud compute instances describe ${instance_name} --zone ${zone} --format "value[](metadata.items.created-by)")
instance_group=${created_by##*/}

preempted=$(curl -sSfL http://metadata.google.internal/computeMetadata/v1/instance/preempted -H "Metadata-Flavor: Google")
if [ "${preempted}" = "TRUE" ]; then
    # Delete instance from regional instance group
    gcloud compute instance-groups managed delete-instances ${instance_group} --instances ${instance_name} --region ${region}

    # Delete instance from zonal instance group
    if [ $? -ne 0 ]; then
        gcloud compute instance-groups managed delete-instances ${instance_group} --instances ${instance_name} --zone ${zone}
    fi
fi