User and group management in Linux involves creating, modifying, and deleting users and groups, assigning them appropriate permissions, and controlling access to files and resources. This is typically achieved using command-line tools such as useradd, usermod, userdel, groupadd, groupmod, and groupdel. Here's an overview of these commands and their usage:

User and Group Management Tools:

• passwd: Change user passwords.

• id: Display user and group information.

• groups: Display groups a user belongs to.

• chown: Change file owner and group.

• chgrp: Change group ownership of files.

• su / sudo: Switch user or execute commands with superuser privileges.

User & Group Management:

User account management in Linux typically involves commands like useradd, usermod, groupadd, and others. I'll walk you through creating a user account, creating a group, adding users to groups, and modifying user account attributes.

1. Creating a User Account with useradd:

   To create a new user account, you can use the useradd command. For example, to create a user named "nahid," you would run:

     sudo useradd nahid
   

   This creates a new user account with the default settings.

   

2. Viewing User Information:

   To view user account details, you can use the id command. For example, to check the user that "nahid" belongs to:

    id nahid
    # getnet nahid
   

   

3. Setting a Password for the New User with passwd:

   After creating the user, set a password for the user with the passwd command:

     sudo passwd nahid
   

   You'll be prompted to enter and confirm the password for the new user.

   

4. Modifying Users (usermod):

    sudo usermod [OPTIONS] username
   

   • Modify attributes of an existing user (username).

   • Options can include changing the username, home directory, shell, etc.

Common Options:

Change Username (-l):

    sudo usermod -l new_username old_username

• Change the username from old_username to new_username.

  Here, we have created a new user named 'test-1.' Now, let's see what options are available to modify the 'test-1' user using the following command.

    sudo usermod test-1
  

  

  

  Now, we'll use a command to change the username. Initially, our current username was 'test-1,' and we'll modify it to 'test-2.'

    sudo usermod -l new_username old_username
  

    sudo usermod -l test-2 test-1
  

To check if the user has been modified, we executed the 'id test-1' command and found that the 'test-1' user doesn't exist anymore. Next, we ran the 'id test-2' command and noticed that the user's group ID and groups remained the same, only the username changed. Thus, our command has successfully executed.

1. Change User ID:

    sudo usermod -u 1002 username

1. Change Home Directory:

    sudo usermod -d /home/newhome username

1. Deleting Users (userdel):

    sudo userdel username
   

   • Delete a user (username) from the system.

   • -r option removes the user's home directory and mail spool.

1. Creating a Group with groupadd:

   To create a new group, use the groupadd command. For example, to create a group named "Devops, Dev,QA" you would run:

     sudo groupadd Devops
     sudo groupadd Dev
     sudo groupadd QA
   

   

   To view user group details, you can use the getnet group command. For example, to check the group list:

    getnet group
   

   

   

2. Modifying Groups (groupmod):

    sudo groupmod [OPTIONS] groupname
   

   • Modify attributes of an existing group (groupname).

   • Options can include changing the group name or group ID.

Now, we'll try to change the Dev group's name to Developer, so we need to execute the following command.

    sudo groupmod -n new_groupname old_groupname

    sudo groupmod -n Developer Dev

To confirm if the group name was modified, I executed the following commands. The group name changed from "Dev" to "Developer," and there's no longer any group named "Dev." Therefore, our command successfully executed the group name change.

Change Group ID (-g):

• Changes the group's GID (Group ID) to new_gid.

    sudo groupmod -g new_gid groupname

    sudo groupmod -g 1002 groupname

1. Deleting a Group:

    sudo groupdel groupname
   

    sudo groupdel Developer
   

   

   

   The group named "Developer" is not in the list of groups anymore, meaning the group has been successfully deleted.

2. Adding a User to a Group with usermod:

   You can add a user to a specific group using the usermod command. For instance, to add the user "john" to the "qa" group, run:

    sudo usermod -a -G groupname username
   

   The -a flag appends the user to the group without removing them from other groups.

   Let's add the "nahid" user to the "Devops" group using the following command:

 sudo usermod -a -G Devops nahid

Before executing our command, let's first check the complete information of the "nahid" user using the "id nahid" command. After execution, we notice that the "nahid" user account is only added to the "nahid" group and isn't part of any other groups.

Now, after executing our command, we'll verify if it has worked correctly. So, we'll run the "id nahid" command again. Upon execution, we observe that the "nahid" user is now added to two groups: one is the default group named "nahid," created automatically during user creation, and the other is the "Devops" group, to which we appended the "nahid" user. Hence, our command has successfully executed.

1. Adding a User to Multiple Groups:

   To add a user to multiple groups, you can list the groups separated by commas. For example, to add "niloy" to both the "Programmer", "Manager" and "QA" groups, run:

     sudo usermod -a -G Programmer,Manager,QA niloy
   

Before executing our command, let's first check the complete information of the "niloy" user using the "id niloy" command. After execution, we notice that the "niloy" user account is only added to the "niloy" group and isn't part of any other groups.

To view user account or group details, you can use the id or getent command. For example, to check the groups that "niloy" belongs to:

getnet group | grep niloy
id niloy

Now, after executing our command, we'll verify if it has worked correctly. So, we'll run the "id niloy" command again. Upon execution, we observe that the "niloy" user is now added to Four groups: one is the default group named "niloy," created automatically during user creation, and the other three is the "Programmer", "Manager" and "QA" group, to which we appended the "niloy" user. Hence, our command has successfully executed.

1. Viewing a Specific User's Default Group:

You can use a command like cat or less to view the content of the /etc/passwd file. Open a terminal and run one of the following commands:

cat /etc/passwd
#or
less /etc/passwd

1. Remove a user from a specific group

To remove a user from a specific group in Linux, you can use the gpasswd command to modify group memberships. To remove the user "john" from the group "john," you can use the following command:

sudo gpasswd -d username groupname

Before executing the command, we checked and found that the "niloy" user account was added within the "Programmer" group.

id niloy

sudo gpasswd -d niloy Programmer

In this command:

• gpasswd is the command to modify group passwords and memberships.

• -d specifies that you want to remove a user from a group.

• The first "niloy" after -d is the username you want to remove from the group.

• The second "Prograamer" is the name of the group from which you want to remove the user.

After running this command, the user "niloy" will be removed from the group "Prograamer"

After execution of our command, we noticed that there isn't any user named "niloy" in the "Programmer" group. Therefore, our command has been successfully executed.