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I've read multiple posts on running scripts on GCP VMs but unfortunately could not find an answer that would satisfy my needs.
I have a Go application and I'm looking for a way to run a bash script on a VM instance programatically.
I'm using a Google Cloud Golang SDK which allows me to fetch VM instance info. Unfortunately SDK does not contain a functionality that allows running a bash script on a specific instance(unlike an Azure Cloud SDK for example).

Options I've found:

  1. Google Cloud Compute SDK has an option to set a startup script, that will run every time an instance is restarted.
  2. Add instance-level public SSH key. Establish an SSH connection and run a script using Go SSH client.

Problems:

  1. Obviously startup script will require an instance reboot and this is not possible in my use case.
  2. SSH might be also problematic, in case instance is not running SSH daemon or SSH port is not open. Also, SSH daemon config does not permit root login by default(PermitRootLogin might be false), thus script might be running on a non privileged user, making this option not suitable either.

I should probably note that I am not authorised to change configuration of those VMs (for example change ssh daemon conf to permit root login), I can just use a token based authentication to access them, preferably through SDK, though other options are also possible as long as I am not exposing the instance to additional risks.
What options do I have? Is this even doable? Am I missing something?

Thanks!

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2 Answers 2

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As said by Kolban, there is no such API to trigger from outside a bash inside the VM. The best solution is to deploy a webserver (a REST API) that call the bash and to expose it (externally or internally).

But you can also cheat. You can create a daemon on your VM that you run with a startup script and that listen a custom metadata; let's say check it every seconds.

When the metadata is updated, the daemon can perform actions. You can imagine that the metadata contain the script to run with the parameters. At the end of the run, the metadata is cleaned by the daemon.

So now, to run your bash, call the setMetadata Api. It's not out of the box, but you can have something similar of what you expected.

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8 Comments

WOW!!! That is clever!! That is a beautiful polling based solution to sending a signal to the inside of a Compute Engine in a clean and elegant fashion!
It's indeed a clever solution. But in order to run a startup script, I need to reboot a VM and I can't reboot the instance unfortunately. Or am I missing something?
No, you don't need to reboot your VM, you have to define a new metadata with your startup script and then you can test it -> Run it on the VM already running.
@guillaumeblaquiere But how can I rerun it, when rerunning requires connecting to the instance? If I could connect to the instance in the first place I would just run the script. I'm quoting the docs: "You can rerun your startup scripts on your VM instance by connecting to the instance and running one of the following commands."
I understand, thanks for your patience and detailed answers!
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Think of GCP as providing the virtual machine infrastructure such as compute, memory, disk and networking. What runs when the machine boots is between you and the machine image. I am hearing you say that you want to run a bash script within the VM. That is outside of the governance of GCP. GCP will only affect the operation and existence of the environment. If what you want to happen is run a script within the VM programatically you will need to run some form of demon inside the VM that can be signaled to run such a script. This could be a web server such as flask or express, it could be your SSH server or it could be some other technology you choose.

The core thing I think you were looking for was some GCP API that, when called, would run a script within the Compute Engine. I'm going to say that there is no such API.

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