HOWTO: Convert android phone into headless linux machine

Question

I am looking for a way to convert my old android smartphones into headless 'over the network' linux devices. What I am looking for is, I should be able to

1. SSH onto the device
2. Use linux command-line (BASH preferably), once I have SSHed onto the device
3. Run programs on the device (let us say java microserices to begin with)

Why do I want to do this?

1. Because I have 2-3 old android smartphones lying around. I could leverage their hardware instead of buying a new laptop.
2. Because I am curious about it.

Any help is much appreciated.

Thanks.