Linux Group Permissions not being enforced correctly.

Question

I am running Ubuntu 10.04 server and am having some very counter-intuitive experiences with users/groups. For example:

sudo touch test_file                    # create empty file

sudo groupadd test                      # create 'test' group

sudo chown root:test test_file          # change group of file to 'test'

sudo chmod g+rwx test_file              # give write permissions to group

sudo usermod -a -G test {my-user}       # add my user to 'test' group

touch test_file                         # touch the file as my current user

The last line produces a permissions error.

I have ensured that my user is part of the 'test' group (groups {my-user} confirms this). The group of test_file is also definitely set to 'test' and the group permissions are set.

Why can't my user write to the file test file?

Solution 1

When adding a user to a new group, that won't be applied in any currently-running processes, only new ones. You need to log out and then log back in.

Solution 2

Both logging out and rebooting server methods didn't work for me.

This method however is working for me: (reference to this answer)

chmod g+rwxs <parent folder>

Comments

[Daenyth]

This also has the effect of replacing your current group. It's not always a good idea.

[matthias krull]

or hack su - username into your running console. you dont have to logout to login this way :)

[DrColossos]

There is no requirement that you must reboot the computer.

[tommy.carstensen]

What does the +s part do? Thanks.

[Julius]

It sets the setuid bit. This allows a file to be run as the owner of the file. Suppose you have a file you want to run as root, no matter who the person is running the file, you would set the setuid bit for that file.

[asac]

The key was x: looks like execution rights are needed for touch to work.

[Edward]

I would downvote this if i could. Overwriting groups could be very frustrating in the future... esp. with -R option!

[Kamil Maciorowski]

In the example touch test_file is a new process. It's not about currently-running vs new processes.