What is the best way to write an VM's external IP to a .env file for a Node.js project running on GCP Compute Engine (Managed Instance Group) - node.js

Question

Background Info
I have a Node.js app running on a Managed Instance Group of VM's on GCP's Compute Engine.
New VM's are generated from a template with a startup script. The script does the usual stuff, install Node.js, curl, git clone the app code, etc.
This application is set to auto-scale, which is why I need configurations to happen pro grammatically - namely setting host and port in the .env file for the Node.js project.
How I have tried to solve the problem
The only way I can think about doing this programmatically in my startup.sh script is by running the command: gcloud compute instaces list
This returns something like this
NAME ZONE MACHINE_TYPE PREEMPTIBLE INTERNAL_IP EXTERNAL_IP STATUS
VM-1 us-central1-a n1-standard-1 XX.XXX.X.X XX.XXX.XXX.XXX RUNNING
VM-2 us-central1-a n1-standard-1 XX.XXX.X.X XX.XXX.XXX.XXX RUNNING
VM-3 us-central1-a n1-standard-1 XX.XXX.X.X XX.XXX.XXX.XXX RUNNING
Then from that, I would want to pull out the EXTERNAL_IP of the current machine the script is running on, and somehow write it to an environment variable / write it to the .env file for my project.
The .env file only has this information in it:
{
"config" : {
"host" : "VM_EXTERNAL_IP"
"port" : 3019
}
}
This method, I think, would require some sort of regex to grab the correct IP out of the commands output, then stored in Environment variable, then written to .env.
This seems like unnecessary work as I surely not the first person to want to do something like this. Is there a standard way of doing this? I'm not a Node.js expert by any means, and even less of a GCP expert. Perhaps there is a mechanism on GCP to deal with this? - some metadata API that can easily grab the IP to use in code? Maybe on the Node.js side there is a better way of configuring the host? Any recommendations is appreciated.

Answer 1

There are many methods to determine the external IP address. Note that your instance does not have an external public IP address. Google implements a 1-to-1 NAT which handles IP address translation which maps the public IP address to the private IP address.
The CLI supports the command line option --format json. You can use tools that parse json such as jq.
gcloud compute instances list --format json | jq -r ".[].networkInterfaces[0].accessConfigs[0].natIP"
Get your public IP address from several sources, which may or may not be the same as your instance:
https://checkip.amazonaws.com/.
curl https://checkip.amazonaws.com/
Use the CLI with options to get just the external address
gcloud compute instances describe [INSTANCE_NAME] --format='get(networkInterfaces[0].accessConfigs[0].natIP)'
Read the metadata server from inside the instance:
curl http://metadata/computeMetadata/v1/instance/network-interfaces/0/access-configs/0/external-ip -H "Metadata-Flavor: Google"
Once you have determined the method to get your external IP address, you can use tools such as awk to replace the value in your file.
https://www.grymoire.com/Unix/Awk.html