Configuring-AFS-folder-permissions-with-FS.md

When working with a lab partner, you may find that you'd like to share files with them. There are multiple ways to do this, with varying effectiveness. You could use something like Google Drive, but that's kinda dumb. But wait, don't we all use the ECE machines? Can't we configure a way to share files on there? The answer is yes!

What is fs?

fs is a way we can control access to folders and files in AFS (Andrew File System), which is the file system we use at CMU. Using fs we can allow some users/groups access to certain folders, and deny the rest. Fun fact: your private directory has fs configured to only give you (and administrators) access!

If you're curious, a longer guide on how to use fs is found here.

How to use fs to share files with another student

The first thing you want to do is create a public folder to house all of your 240 lab files. The reason for this is because if you put your lab files in private, then your lab partner can't get to it! You can name it whatever you want, but just for example:

mkdir ~/18240_lab
cd ~/18240_lab

Now create a folder for the lab you're working on.

mkdir labX
cd labX

Once you're in that folder, then you can simply use the following commands to give your lab partner access.

fs sa . <labpartner-id> write
fs sa . <labpartner-id>@andrew.cmu.edu write
fs sa . system:anyuser none
fs sa . system:ece none
fs sa . system:authuser none

This will give your lab partner (replace <labpartner-id> with their Andrew ID) read and write access to that labX folder, and deny any other user access.

Note that the above will configure the permissions for the current folder. Do not run the commands in some place like your private folder! Failure to lock down your private files is an academic integrity violation!

To confirm that it works, run

fs la .

and you should get something like

Access list for . is
Normal rights:
    system:administrators rlidwka
    <your-id> rlidwka
    <partner-id> rlidwk
    <partner-id>@andrew.cmu.edu rlidwk

Now you can get the absolute path to your lab folder by typing

readlink -m ~/18240_lab/labX

which will give you an absolute path, which you can give your partner and they can cd into it to access the lab files.

Sharing files in an existing folder with subfolders

You may already have a directory that has many folders in it. Unfortunately the last section only configures the current working folder. Thankfully there is a way around this.

Make sure you cd into your lab folder first. Then run

find -type d -print0 | xargs -i --null fs sa {} <labpartner-id> rlidwk

to recursively configure permissions for the current folder and every subfolder within.