Linux - User (Uid)


Every user who has access to a Linux system needs a login and a password.

Root User

The root login is the super admin user.

In order to create new logins, and modify or delete users, you must already be logged in as root.

The term root may refer to:

  • the root account (the superuser, who has permission to do anything),
  • the root account's home directory (/root)
  • and the root directory for the entire file system (/).


Each user:

  • must belong to a primary group
  • may belong to several secondary groups for authorization purposes.

How to display user and group information

User id

The User Id can seen with:

  • The id command:
uid=0(root) gid=0(root) groups=0(root)

  • With the environment variable $UID, you get the User ID number
if [ "${UID}" -eq 0 ]; then
  echo "I'm root"

Default directories

  • /home — Default location for users' home directories. For example, a user with the username foo has the home directory /home/foo
  • /tmp — The reserved directory for all users to store temporary files. Files stored here are not permanent. A system process removes old files from this directory on a periodic basis. Do not write any files or directories that you want to keep here.

Default file - The skeleton directory

The /etc/skel/ directory is for “skeleton” user files, which are used to populate a home directory when a user is first created. This directory can be modified to fit your needs. Modifications only effect new users and does not change anything for existing users.


Tools and GUI

The easiest way to manage users and groups is through the graphical applications:

  • Red Hat Linux:
    • RHEL4 and higher: system-config-users
    • User Manager: redhat-config-users.
  • SUSE Linux: yast or yast2

The following table lists the available commands line for managing users and groups:

Task Command
Creating groups Linux - Group (Gid)
Modifying groups Linux - Group (Gid)
Deleting groups Linux - Group (Gid)
Creating users useradd
Modifying users usermod
Deleting users userdel
Change/set a password. passwd
Switch to another user su
Verification of the password, group, and associated shadow files pwck: verify integrity of password files
Conversion to shadow passwords and back to standard passwords pwconv, pwunconv


User names and primary groups are stored in /etc/passwd


  • all users
cut -d: -f1 /etc/passwd
  • All users who contains the letters “vis” in their names (use the pipe symbol followed by the grep executable which has a pattern as input: '.*vis')
cat /etc/passwd | grep -i '.*vis'



the useradd command add a new user.


  • -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 oinstall
  • a second group dba
  • starting shell /bin/bash
  • password of xxxx
  • home directory of gerardnico
  • create home directory
  • a login name of gerardnico
useradd -g oinstall -G dba -s /bin/shell -p xxxx -d /home/gerardnico -m gerardnico


user module

- name: "Sqlline - Create the group"
    name: '{{ sqlline_group }}'
    state: present

- name: "Sqlline - Create the user"
    name: '{{ sqlline_owner }}'
    comment: Sqlline Installation user
    shell: /bin/bash
    state: present
    group: '{{ sqlline_group }}'
    password: {{ upassword | password_hash('sha512') }}



This command usermod modifies an existing user. You must use all the options in the same way as you create it.


  • -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 gerardnico

usermod -G others gerardnico

To suppress a group for a user using the command line, you will have to list all the groups that you want the user in. For example if the user currently in group1,group2,group3,group4 and you want him out of group3 then

usermod -G group1,group2,group4 loginName



This command delete a a user,


  • -r (remove home directory)

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

[root@ebs121 /]# userdel -r gerardnico
bash: userdel: command not found
[root@ebs121 /]# /usr/sbin/userdel -r gerardnico


See Linux - Password (User)

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