Next, we take a look at the /etc/group file, which defines all the groups on a Linux system. Here's a sample line:

drobbins:x:1000:

The /etc/group field format is as follows. The first field defines the name of the group; the second field is a vestigial password field that now simply holds an x, and the third field defines the numeric group id of this particular group. The fourth field (empty in the above example) defines any users that are members of this group.

You'll recall that our sample /etc/passwd line referenced a group id of 1000. This has the effect of placing the drobbins user in the drobbins group, even though the drobbins username isn't listed in the fourth field of /etc/group.