Run a Throwaway VM
cubic run --rm creates a VM instance, opens a shell in it and deletes the
whole VM instance when you leave the shell. It is the quick way to try a command
in a clean guest without anything left on your host afterwards.
Start One
$ cubic run --rm --image ubuntu
info: Using generated name grumpy-dragon
alice@grumpy-dragon:~$ python3 --version
Python 3.13.7
alice@grumpy-dragon:~$ exit
logout
Cubic picks the name and prints it, so there is nothing to invent. A given name
still works, as in cubic run --rm scratch --image ubuntu.
The VM instance is gone once the shell closes. cubic instances no longer
lists it:
$ cubic instances
Name OS Arch vCPUs Memory Disk Running
Cubic deletes the VM instance whenever the session ends, so a dropped connection or a crash of the guest cleans up as well.
Fast and Small
The disk of a throwaway VM instance is a thin overlay on the cached image rather
than a full copy, so it is created at once and stores only what the guest
writes. This makes --rm cheap to run again and again.
The overlay refers back to the image in the cache, so a throwaway VM instance is
not self-contained. cubic prune clears the whole image cache, so avoid it
while a throwaway VM instance still runs, or the overlay loses the base image it
reads from. A VM instance created without --rm holds a full copy and is not
affected.
Combine with Isolation
--isolate keeps the guest off the network, which pairs well with a throwaway
VM instance for code you do not trust. It runs in a clean guest, reaches nothing,
and leaves nothing behind:
$ cubic run --rm --isolate --image ubuntu
See Isolate a VM Instance for what isolation covers.
Only cubic run
--rm belongs to cubic run, which owns the shell session from start to
finish. cubic create, cubic start and cubic ssh have no --rm,
because they do not know when you are done with the VM instance. Delete a VM
instance you started that way with cubic delete.