Allow setting PATH variable for each user dynamically

[yuvipanda]

I have a conda environment per user in their home directory. so PATH needs to be something like PATH=/home/{username}/conda/bin:$PATH. However, currently there seems to be no way to do this, since username expansion isn't supported in environment files - plus PATH doesn't seem to be passed through to the spawned notebook (on Slurm at least)

Proposed change

1. Allow username expansion via {username} or similar in PATH
2. Add a separate traitlet just for PATH, similar to systemdspawner

Alternative options

Currently, I use the following to get what I want:

class CustomSlurmSpawner(SlurmSpawner):
    def start(self, *args, **kwargs):
        self.req_prologue = f'export PATH=/home/{self.user.name}/conda/bin:$PATH'
        return super().start(*args, **kwargs)

c.JupyterHub.spawner_class = CustomSlurmSpawner

Who would use this feature?

People who have per-user conda environments

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[rkdarst]

req_prologue is a configurable trait, so does c.BatchSpawner.req_prolog = "source $HOME/conda/bin/activate" or even the direct path things work? The script is a shell script so you should be able to do almost anything you need (I do many things like this, too), using $USER, $HOME, etc.

[rcthomas]

This seems like something that needs to be configured on a per-user basis, since conda environments (or whatever) could be anywhere.

[rkdarst]

I was going to add a {username} substitution because it seemed pretty useful, but it looks like it's already there. Did you try it?