ubuntu - Linux Group Permissions not being enforced correctly.

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?

Answer

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.