We often find that we need to add ourselves to a given group to access a resource.
Like, we need to access the cd reader and only people in the cdrom group can access it. Easy stuff, we just use sudo or su to get permissions and we do adduser my_user cdrom to add ourselves to the cdromgroup.

    #adduser my_user cdrom

But this is not end of story. In order to get effective group permissions we need to logout and login again in the system for our group permissions to be refereshed, which is literaly a pain in the ass sometimes.

There is a quick trick that will save our time here, just do:

    $newgrp cdrom

If we have been added to the group newgrp will give us instantly group permissions for the given group.
For mor info on the subject:

    $man newgrp

       usermod will not allow you to change the name of a user who is logged in. You must make certain that the named user is not executing any processes when this command is being executed
       if the user's numerical user ID is being changed. You must change the owner of any crontab files manually. You must change the owner of any at jobs manually. You must make any changes
       involving NIS on the NIS server.

mmarkk@imac:~$ echo $$
8244
mmarkk@imac:~$ newgrp
mmarkk@imac:~$ echo $$
17030
mmarkk@imac:~$ exit
exit
mmarkk@imac:~$ echo $$
8244