/etc/group: why the username does not appear in its own primary group?

According with many tutorials in the /etc/group file is listed all the groups.

Through the cat /etc/group | grep omicron command (omicron is the default custom user created in the Linux installation) appears:

adm:x:4:syslog,omicron
cdrom:x:24:omicron
sudo:x:27:omicron
dip:x:30:omicron
plugdev:x:46:omicron
lpadmin:x:120:omicron
lxd:x:132:omicron
omicron:x:1000:
sambashare:x:133:omicron

According with the output I can understand that each line represents:

• The group name
• The X represents something about password or some protection
• The gid is the id of the group
• Finally what users are assigned to the group.

What is not clear for me:

• why appears omicron:x:1000: and not omicron:x:1000:omicron?

The omicron user has its primary group omicron too

Solution 1:

A user doesn't need to be listed as the member of a group in /etc/group if that group is their primary group. That's just how it is and has been for decades. Some user management tools do explicitly put the user in the /etc/group list, but not all.