User management in Linux

User accounts in Linux works differently from the system in Windows.
ACLs are configured based on the system of UIDs. These are unique for each accounts. root account is UID 0 .UID 0-999 are typically reserved for system accounts.

Although there is only one UID 0 for administrator login, it is possible to “create” another administrator account by pointing said ID to UID 0. What we are doing is to create a replica of root ID.
useradd –o –u 0 userid
where –0 allow the creation of a user account with a duplicate (non-unique) UID ,    and –u link the ID to root UID.

Having another root level account on a system may not be desirable for user security governance and tracking. The use of sudo is recommended instead of having multiple administrator logins.

In Centos,
# visudo
Search for the commands section to grant the ID specific access rights. For example, to grant ID acme with root access with sudo, add a line underneath root.

## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
acme    ALL=(ALL)       ALL

Set a password
# passwd user
Set  password expiry date
# chage -m 0 user  where -m is the minimum no of days