Minimum & Maximum UID’s
Here comes a very small but relevant information about user accounts. Often asked in interviews.
We all know that whenever we create user account on Linux machines during your RHCE Training and RHCSS Training using either useradd or adduser command a UID (User ID) get associated or attached with that user account.
You should understand that, for your server, UID, is the only way to identify a user. (Usernames are just a convenience because we, as humans, tend not to handle being identified by numbers well.)
In general, all users need a unique UID. Red Hat Enterprise Linux starts generating local UIDs at 500 and the highest UID available by default is 60000.
The minimum and maximum values for UID and GID are defined in /etc/login.defs, which can be changed if needed.
Normally, UIDs below 500 are reserved for system accounts that are needed to start services.
The UID 0 is also special—the user who has it has complete administrative permissions to the server. UID 0 is typically reserved for the user root.