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Friday, July 31, 2009

Linux User Commands

Every user who has access to a Linux system needs a login and a password. Each user must belong to a primary group and for security or access purposes can belong to several secondary groups.

In order to create new logins, modify or delete users, you must already be logged in as root. The root login is the highest level and only certain individuals should have access to the root account.

useradd - Adding a new user
Options:
-d home directory
-s starting program (shell)
-p password
-g (primary group assigned to the users)
-G (Other groups the user belongs to)
-m (Create the user's home directory

Example: To add a new user with

  • a primary group of users
  • a second group mgmt
  • starting shell /bin/bash
  • password of xxxx
  • home directory of bubba
  • create home directory
  • a login name of bubba

useradd -gusers -Gmgmt -s/bin/shell -pxxxx -d/home/bubba -m bubba

usermod - Modifying existing user
Options:
-d home directory
-s starting program (shell)
-p password
-g (primary group assigned to the users)
-G (Other groups the user belongs to)

Example: To add the group 'others' to the user bubba

usermod -Gothers bubba

userdel - Deleting a user
Options:
-r (remove home directory)

Example: To remove the user 'bubba' and his home directory

userdel -r bubba

passwd - User's Password
Options:
user's name (Only required if you are root and want to change another user's password)

Example: To change the password for the account you are currently logged in as...
passwdEnter existing passwordEnter new passwordEnter new password again (to validate)

Example: To change the password for the user 'bubba' (only you are logged in as root)...
passwd bubbaEnter existing password (can be either bubba's password or root's password)Enter new passwordEnter new password again (to validate)

Where user and group information stored
User names and primary groups are stored in /etc/passwd. This file can be directly edited using the 'vi' editor, although this is not recommended. Format of the file is...
User (name normally all lower case)
Password (encrypted - only contains the letter 'x')
User ID (a unique number of each user)
Primary Group ID
Comment (Normally the person's full name)
Home directory (normally /home/
Default shell (normally /bin/bash)
Each field is separated by a colon.
Passwords for each user are stored in /etc/shadow. This file should only be changed using the passwd command.
Group information is stored in /etc/group. This file can be directly edited using the 'vi' editor. Format of the file is...
Group name
Group password (hardly ever used)
Group ID
User names (separated by commas)
Each field is separated by a colon.

Default files
When a new user is created, the default files and directories that are created are stored in /etc/skel.

This directory can be modified to fit your needs. Modifications only effect new users and does not change anything for existing users.

su - Switch User
To switch to another user, use the su command. This is most commonly used to switch to the root account.

Example: To switch to root account...suEnter root's passwd
Example: To switch to the user 'bubba'...su bubbaEnter bubba's or root's passwd

To return to original user, enter exit

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