Learn Linux : User and Group Management Commands in Linux

Creating a user:

The syntax for creating a user in Linux is

Let’s create a user with default attributes.

When no option is used with useradd command the options like UID, GID, home dir and shell will be assigned default.
#useradd <username>
#useradd student

Observe that the uid, gid, home dir, and shell is assigned automatically.

The syntax is as follows to rename by a UID (user ID):

Now look at changes the user id , userid = student id = 1331 changed.

Set password to user :

As a root user we can assign any password to any user
The syntax for assigning a password is
#passwd to assign password to current user ( the one with which you have logged in, if it is root then root’s password will be changed
#passwd <user name> to assign a password to a specific user, only root can assign password to other user.

Modifying the user’s attribute

After creating a user if we need to modify the attributes of user like changing uid, changing secondary group id or adding a comment, locking or unlocking the user account, can be done by following command
Syntax. # usermod <options> <username>
all the options which are used with useradd command can be used and also the following,
-l to change login name
-L to LOCK account
-U to UNLOCK account
ex. # usermod -l newname oldname (changing the name of the user)
ex. # usermod -L newname to lock the user account
ex. # usermod -U newname to unlock the user account
Note: - when an account is locked it will show! (Exclamation mark) in /etc/shadow file.

Locking and unlocking a user account:

To lock a user a/c use the following
#usermod –L < user name>
#usermod –L student
Verify it in /etc/shadow file, it shows exclamation mark before user a/c or try login as student

Unlocking a user account:

Unlock the above a/c
#usermod –U < user name >
#usermod –U student
Verify it in /etc/shadow file, it shows exclamation mark before user a/c or try login as student

Observe in both pictures that once the account is unlocked the exclamation is gone.

Password parameters:

For any user we can set the parameters for the password, like min and max password age, password expiration warnings and a/c expiration date etc.
To view the advanced parameters of the user, use
#chage -l < user name>
#chage -l student

Last password change: When the password was change last time.
Password expires: Password expiry date
Password inactive: After password expiry grace period before the account gets locked.
Account expires: Date on which the account expires.
Minimum number of days b/w password change: once the password is changed, it cannot be changed until a min period of specified date. [0] means never.
Max number of days b/w password change: After changing the password how long it will be valid for.
Number of days of warning before password expires: start of warnings to change the password, no. of days before the password expires.

Changing the password parameters:

The second method is for, if you want to change a particular field of password aging policy
#chage <option> <value> <username>
The options which can be used are as follows
-m for Min password age
-M for Max password age
-d for last time the password is changed.
-W Password expiration warnings
-I Password inactive [-1 means inactive].
-E A/C expiration date

Deleting a User:

To delete a user the syntax used is
#userdel <username> it will only delete the user but home directory will be there. To delete the user with its home directory use the following command.
#userdel –r < user name >
#userdel –r student

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