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User Management

Attributes​

Each user registered in the system has a line entry in /etc/passwd which has all attributes of the user are listed. The attributes are:

  • Username.
  • UID
  • GID
  • Home directory
  • Default Shell
# beav - username
# 'x' means password is set and used in `/etc/shadow`.
# 1000 - UID
# 1000 - GID
# Theodore Cleaver - Comment
beav:x:1000:1000:Theodore Cleaver:/home/beav:/bin/bash

There are some system users (such as bin, daemon, sys) which are created for specific purposes and cannot be used to log in (this is why their default shell is /sbin/nologin).

Creating User Accounts​

The command will create a user with the default options:

sudo adduser $username

The following steps will occur when adding a new user (according to defaults set in /etc/defaults/useradd):

  1. The next available UID value (based on /etc/login.defs) will be assigned.

  2. A group called $username will be created and UID will be set to GID.

  3. A home directory /home/$username will be created.

  4. The contents of /etc/skel will be copied to /home/$username.

  5. An entry of either !! or ! is placed in the password field of the /etc/shadow file for $username's entry, thus requiring the administrator to assign a password for the account to be active.

Modifying User Accounts​

This command will remove the user from /etc/passwd, /etc/shadow and /etc/group.

sudo userdel $username

# Will also remove the home directory for this user
sudo userdel -r $username

This command will modify the user attributes.

sudo usermod $username

# Will lock account so the user cannot log in
sudo usermod -L $username
# OR
sudo chage -E 1970-01-01 $username

# Will unlock an account
sudo usermod -U $username

Password Management​

Use of /etc/shadow enables password aging on a per user basis. At the same time, it also allows for maintaining greater security of hashed passwords.

Only user can change its own password (unless root which can change for all users). To change passwords:

passwd

To set password aging (using root):

chage

# Force user to change password on next login
sudo chage -d 0 $username