• Any process or action performed in the RHEL has to be associated with an user

id #Command used to get information of logged in user
ls -l #Command which lists the user associated with the files
ps -au #Command to list all processes along with user association

• User information is stored in /etc/passwd file in format as below: username:password:UID:GID:GECOS:/home/dir:shell

1. username: Value provided for every uid
2. password: Recent versions of RHEL has password saved in /etc/shadow
3. UID: user id assigned by OS
4. GID: group id associated
5. GECOS: For comments or description about user
6. /home/dir: Home directory of the user
7. shell: A program which is to be run when user logs in

• Local groups are defined in /etc/group and every user will have one primary group

Super user

• root is the super user in RHEL and has highest privilege among users

• su [-] command to change the user

su temporarily grants standard users root privileges and responsibilities but user should be aware of the root password

• sudo allows users to run commands as root or another user, based on settings in /etc/sudoers

• In RHEL, users in wheel group can use sudo

To Add Local Users

• useradd username sets reasonable defaults for all fields in /etc/passwd

• /etc/login.defs contains the default setting for new users when not explicitly mentioned while creating the user

• usermod can be used to modify the existing user properties, use 'usermod --help' to know different options available

userdel username #To delete user
userdel -r username #to remove user along with user's home directory and files
#Always make sure to use -r to avoid conflicts or issues

• If in case users were deleted without deleting directories associated with them then we could use below command:

#root can find unowned files and directories by running:
 find / -nouser -o -nogroup 2> /dev/null

• To list current password setting of a user chage -l username

• User needs account with password to authenticate to system, but does not need interactive shell, in this case Set user’s login shell to /sbin/nologin

nologin shell prevents interactive use of system, but does not prevent all access

Points to Remember:

• UID identifies the user at the most fundamental level with a numeric identifier

• Login shell provides the user's command line prompt

• /etc/group is location for local group information

• /etc/passwd is location of local user account information

• GID is a number that identifies the group at the most fundamental level

• su command allows you to become another user

• In Red Hat Enterprise Linux 7, by default, members of wheel group can run sudo to run any command as any user

• /etc/shadow stores local passwords in encrypted format

For Additional Reading: Red Hat Documentation