Customizing a GCE Ubuntu VM image - google-compute-engine

Question

I have a Google Cloud Platform account that I access from a VirtualBox VM. I am using the Google Compute Engine for a project that I am currently working on, and I had to create a custom image based on the Ubuntu 14.04 image that's available there.
I made changes to the Ubuntu image by ssh'ing into an Ubuntu 14.04 instance, (from my Vbox VM terminal) installing the Matlab compiler runtime, and downloading some other files that I needed. I created the custom image by following the steps according to the documentation.
However, now the changes I made are only available to me when I SSH from my Vbox VM terminal. I need to be able to run a certain matlab program Via startup scripts, how can I make it so that all users using this image have access to the customizations I made? Is there a way I can do this without having to make the edits by ssh'ing from the developers console and redoing all the changes?
EDIT: I don't think I was very clear so Ill give some examples. say my Google account is alexanderlang. When I ssh into an instance created from my custom image from the developers console, bash prompt looks like:
alexanderlang#myinstance $
My Vbox username is alex, and when I ssh into the same instance from my vbox terminal, bash prompt looks like:
alex#myinstance $
alex#myinstance can run matlab programs, but alexanderlang#myinstance cannot. I'm talking about the same instance, created from the same image. I think this might have something to do with the ssh keys for my custom image, but I don't know how to change or remove those keys.

Answer 1

When you connect to your VM instance via ssh by using either Developers Console or gcloud, the user account is dynamically created (if it doesn't already exist) by setting metadata on the VM. The question is: how does each tool choose your username?
When you use Google Developers Console, the only information it knows about you is your Google Account name, so it uses that, e.g., <first-name>_<last-name> or similar.
When you connect to your instance via gcloud, it knows the value of $USER so it uses that instead.
Note that in either case, your account has passwordless sudo access, so if you want to switch from one account to the other, you can run:
sudo su alex
while logged in as alexanderlang and then you have access to all the programs that alex does.
Similarly, you can run:
sudo su alexanderlang
while logged in as alex to do the reverse.
Startup scripts run as root. To run commands as another user, you need to do two things:
change to that username
run commands as that user
sudo su alex will create a new shell and hence ignore the rest of the script (until you manually exit the user shell, which is not what you want).
You can use sudo su alex -c 'command to run' but since what you want to run is a complex script, you need to first save the script to a file, and then run it.
Your options are:
pre-create the shell script to run
dynamically generate it from the startup script
Doing (1) is easy if the script never changes. For frequently-changing scripts (and it sounds like, many dynamically created VMs), you want to use option (2).
Here's how to do this in a startup script:
cat > /tmp/startup-script-helper.sh <<EOF
# ... put the script contents here ...
EOF
sudo su alex -c '/tmp/startup-script-helper.sh'

Answer 2

You can use Packer to create a derived image from a stock GCE VM image. Packer will let you do the following very easily:
boot a GCE VM using an image you specify
run some customization step, e.g., shell script, or Chef/Puppet/etc.
save the resulting image in your Google Cloud Platform project
Then, you can boot any number of new VMs using your newly-created image.
Note that since your VM image will be stored on Google Cloud Storage, you will be charged for the space it uses. Current pricing for Google Cloud Storage standard class is USD $0.026 / GB / month. A typical VM image should be less than 1GB.
You can see a complete example of how I used Packer to build VMs and pre-installed Ambari on it via my GitHub repo.