Users, groups, and others

Under Linux, every file is owned by exactly one user and one group. Every user has an identifying number, the User ID (UID). The same applies for a group: it is resolved by a Group ID (GID). Every user has exactly one UID and one primary GID; however, users can be members of multiple groups. In that case, the user will have one or more supplementary GIDs. You can see this for yourself by running the id command on your Ubuntu machine:

reader@ubuntu:~$ id
uid=1000(reader) gid=1004(reader) groups=1004(reader),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lxd),1000(lpadmin),1001(sambashare),1002(debian-tor),1003(libvirtd)
reader@ubuntu:~$

In the preceding output, we can see the following things:

• The uid for the reader user is 1000; Linux typically starts numbering normal users at 1000
• The gid is 1004, which corresponds to the reader group; by default, Linux creates a group with the same name as the user (unless told specifically not to)
• Other groups include adm, sudo...