Users & Groups

Each user in UNIX has the following parameters:

• User name
• Encrypted password (or ‘x’ if hash is stored)
• User Identifier (UID)
• Group Identifier (GID)
• Full name or description
• User’s home directory
• User’s shell
• Expiration date

Each group in UNIX has the following parameters:

• Group name
• Encrypted password (or ‘x’ if hash is stored)
• Group Identifier (GID)

root is super user. It has unlimited rights in the system.

Do not use without necessity!

User:

• su
• sudo
• useradd
• userdel
• usermod
• passwd
• finger

Groups:

• groupadd
• groupdel
• groupmod
• groups

• to create a new user - useradd <name>

  • NOTE: this just creates a user, no home directory and other stuff like shell etc..
  • to create a complete user run adduser <username>
    • Then whats the use of useradd?
      • it is used heavily in scripting where we dont want to give inputs to adduser but do everything from script

• to set password to created user - passwd <user_name>

• to switch to that user su - <user> or su <user>

  • su - - switch to root by default

• check all users in /etc/passwd

• to check all passwords /etc/shadow

• to delete user userdel <user_name>

  • this won’t delete home dir… to delete user along with home dir use -r like.. userdel -r username

Key files involved in user management:

• /etc/passwd – Stores user account details.
• /etc/shadow – Stores encrypted user passwords.
• /etc/group – Stores group information.
• /etc/gshadow – Stores secure group details

useradd -u <CUSTOM_USER_ID> -G <GROUP> -d <HOME_DIR> -m -c "<COMMENT>" -s /usr/bin/bash <USER_NAME>

Modifying Users

Modify an existing user with usermod:

• add to a group

  • usermod -aG <GROUP> <USER>

• Change the username:

  usermod -l new_username old_username

• Change the default shell:

  usermod -s /bin/zsh username

Groups

Creating Groups

groupadd groupname

Adding Users to Groups

usermod -aG groupname username

NOTE: usermod -G group username here user is removed from his previous group and added to current group, bcoz no a flag in command

Adding a User to Sudo Group

On Debian-based systems:

usermod -aG sudo username

On RHEL-based systems:

usermod -aG wheel username

Viewing Group Memberships

groups username

Changing Primary Group

usermod -g new_primary_group username

what is su - username ?

• if we run su with - then , .bashrc_profile file will be executed after login, without - i.e su username it is not loaded…