A root user can have all the privileges. But a normal user can gain access like a root with su or sudo command and their own password.
So what's the difference?
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su (and sudo) command traditionally require root's password. However, you can setup
sudo so that ordinary users can achieve root privileges with their own password by modifying
/etc/sudoers (as root, preferably with
Modern Linux distributions preconfigure the first user to be able to sudo with her own password. This prevents accidental system malconfiguration by the user, and enables them to gain full control without the need of a separate root password.
su command it to temporary change an identity to any user on a system and execute many programs with his/her/its permissions. It doesn't have to be the root. If the user executing
su isn't the root, he have to enter the password of the user he want get identity.
sudo command is to execute one command with permissions of any user. It doesn't have to be the root too. The command is very configurable and provide some kind of precise access control. The entering own password is optional and configurable. Some distribution let the first user of the system to execute with
To add to the above answers,
su user1 with the user's password shall switch your credential to user1 till you type exit
su shall assume root by default.
sudo as mentioned in other answers, can be granted to trusted users and a ristricted set of commands. moreover,
sudo can be configured to log commands executed. This is a good way to track misuse of privilege.
Not all normal users can use sudo, they have to be in the sudoers file and you can control which commands or types of commands the user can execute. Also, only certain users can use su to switch to the root user. Normally you would only have sudo permissions for a limited set of commands and full su permissions for a limited period of time.
Only users with super-user privileges can sudo or su, normal users cannot. This is configured in /etc/sudoers, which should always be edited with visudo.
The benefits of this system are:
rootwould be the first login name to guess).