User Account Management in Linux

Managing user accounts in Linux is a crucial task for administrators. This comprehensive guide covers handling user and group accounts, and managing file permissions using Access Control Lists (ACLs).

Creating a User

To create a new user:

useradd <username>

Identifying if a User is Created

To check if a user exists:

id <username>

Creating a User with Specific Options

To add a user to an existing group, specify the user shell type, add a comment, and define the user’s home directory:

useradd -g <existing group name> -s /bin/bash -c "comment" -m -d /home/username <username>

Example:

useradd -g QA -s /bin/bash -c "For testing only" -m -d /home/victor victor

This command adds the user victor to the existing group QA, sets the shell type to /bin/bash, adds a comment, and defines the home directory as /home/victor.

Deleting a User

To delete a user:

userdel <username>

To delete a user and their home directory:

userdel -r <username>

To force delete a user, even if they are logged in:

userdel -f <username>

Modifying a User

To add a user to a new group without changing the default group:

usermod -G <group name> <username>

To change the default group of a user:

usermod -g <group name> <username>

To move the content of the home folder to a new folder:

usermod -m -d /home/newfolder <username>

To lock or unlock a user:

usermod -L <username>    # Lock user
usermod -U <username>    # Unlock user

Group Management

Creating a Group

To create a new group:

groupadd <group name>

Deleting a Group

To delete a group:

groupdel <group name>

Checking Group Information

Check the /etc/group file for group details.

Understanding User and Group Info

User information is stored in the /etc/passwd file. Passwords and related details are stored in the /etc/shadow file.

File Permissions with ACL (Access Control List)

ACLs allow you to set more specific permissions for files or directories without changing the base ownership and permissions.

Commands

• setfacl: Set file ACL

• getfacl: Get file ACL

Adding Permissions for a User

To add permissions for a user:

setfacl -m u:<username>:rwx <target_file>

Example:

setfacl -m u:shan:rw- package-lock.json

Adding Permissions for a Group

To add permissions for a group:

setfacl -m g:<group_name>:rwx <target_file>

Removing Permissions

To remove a specific entry:

setfacl -x u:<username> <target_file>

To remove all entries:

setfacl -b <target_file>

Adding Permissions for a User in All Files Inside a Folder

To recursively add permissions for a user in all files within a folder:

setfacl -Rm u:<username>:rw <folder_name>

Viewing File Permissions

To see file permissions:

ls -l

File Permissions - Types and Levels

Permission Types

• r: Read

• w: Write

• x: Execute

Permission Levels

• u: User (owner)

• g: Group

• o: Others

• a: All

Changing Permissions

To add or remove permissions:

chmod u+r <file_name>    # Add read permission for the user
chmod u-r <file_name>    # Remove read permission for the user
chmod ugo+r <file_name>  # Add read permission for all
chmod a+rwx <file_name>  # Add read, write, and execute permissions for all

Changing Ownership

To change the owner:

chown <username> <file_name>

To change the group:

chown <username>:<group_name> <file_name>

Understanding Permission Scores

Each permission has a numerical value:

• r = 4

• w = 2

• x = 1

Example:

chmod 774 <file_name>

• rwx for the user (7 = 4+2+1)

• rwx for the group (7 = 4+2+1)

• r-- for others (4)

Common Permissions:

• 644: File Baseline

• 755: Directory Baseline

Switching Users

To switch users:

su - <username>    # Switch to user and move to their home directory
su <username>      # Switch to user but stay in the current directory

Note: su stands for Substitute User, not Switch User.

Understanding Sudo

sudo stands for "Super User Do". It temporarily grants a user administrative rights.

To grant sudo access to a user:

usermod -aG sudo <username>

Check the user’s groups with:

id <username>

You can also provide limited access to specific commands by editing the /etc/sudoers file using visudo.

Conclusion

By mastering the essential commands and techniques outlined in this guide, you can efficiently manage user accounts, groups, and file permissions in Linux. These skills are crucial for maintaining a secure and well-organized system. Whether you are creating new users, modifying existing ones, or setting precise file permissions with ACLs, these practices will help you ensure that your Linux environment is both functional and secure. Happy administering!

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