Manage Group Permissions

Understanding and managing Unix group permissions is essential when working in a shared computing environment. Each file or directory has a user owner and a group owner, and permissions control how these resources can be accessed or modified.

This guide explains how group ownership and permissions work on ARCH systems and how to manage them to support collaboration within research groups.

Overview

Unix permissions are important for:

Sharing data with other group members
Maintaining correct group ownership for quota tracking
Ensuring project members can read/write shared files

Every user belongs to one or more Unix groups, typically based on project membership. When a user creates a file, the group ownership is usually set to their current group, which can be changed as needed.

Checking File Permissions and Group Ownership

To inspect ownership and permissions:

ls -l

Example output:

-rw-------  1 alice    projX     13511 Feb 14 10:00 data.txt
-rwxr--r--  1 bob      projY       215 Sep 19 09:21 run.sh

The third column shows the owner (user)
The fourth column shows the group owner
Permissions are shown in three sections: - User (owner) - Group - Others

If alice wants to share data.txt with other members of projX, she must ensure the group has the correct permissions:

chmod g+r data.txt

Changing Group Ownership

To change the group ownership of a file:

chgrp projX shared_results.txt

You can only change to groups you’re already a member of.

Viewing Your Group Membership

To list your current and supplementary groups:

groups

The first group listed is your current (active) group.

Setting the Active Group

To change your current group for a session:

newgrp projX

This is useful to ensure newly created files are assigned to the appropriate group.

You can also run a single command as a different group using:

sg projX <command>

Example:

sg projX mkdir new_folder

Summary of Useful Commands

Command	Purpose
ls -l	View file ownership and permissions
chmod g+r file	Add read access for the group
chgrp groupname file	Change file’s group owner
groups	Show your group memberships
newgrp groupname	Change your active group for the session
sg groupname <command>	Run one command using a different group context