When you are using a linux system, it's useful to find out what groups you belong to, so you can understand whether you have access to files and directories. This is one of the simplest commands possible. I'm using Ubuntu linux, but this command should work on most varieties of linux.

groups <username>

If you don't enter a username, it defaults to your own username. For instance:

geek@ubuntuServ:$ groups
geek adm dialout cdrom floppy audio dip video plugdev lpadmin scanner admin fuse

You can also check the groups for any other user, including root:

geek@ubuntuServ:$ groups root
root : root fuse

I used this command recently to make sure that my user account was part of the "fuse" group, when I was getting sshfs set up. Very useful.