Linux User Commands

Linux, with its robust and flexible nature, provides a powerful set of commands for user administration and management. Whether you're a system administrator or an enthusiast, understanding these commands is essential for maintaining a secure and efficient Linux system. In this tutorial, we'll explore key user-related commands, including creating and deleting users, managing user groups, using the 'finger' command, and changing user permissions.

1. Creating a User:

To create a new user, you can use the `useradd` command. The following example demonstrates how to create a new user named 'John' to show 

sudo useradd john

You can also set a password for the new user using the `passwd` command:

sudo passwd john

2. Deleting and Disabling Accounts:

To delete a user account, you can use the `userdel` command:

sudo userdel john

To disable an account without removing it, you can use the `passwd` command with the `-l` option:

sudo passwd -l john

This locks the user account by replacing the encrypted password with a placeholder, preventing login.

3. Adding Users to User Groups:

You can view the existing groups on your Linux operating system by entering the following Linux user commands:

groupmod "Press Tab key twice"

Adding a user to a specific group is done using the `usermod` command. In this example, we add the user 'john' to the 'developers' group:

sudo usermod -a -G developers john

The `-aG` options stand for append and group, ensuring the user is added to the specified group without affecting existing group memberships.

4. Finger Command:

The `finger` command provides detailed information about users. To check information about the user 'john,' use the following command:

finger john

This will display information such as the user's login name, real name, terminal, login time, and more.

5. Changing User Permissions:

To modify user permissions, use the `chmod` command. 

chmod option permissions file

• options: Optional flags that modify how the command behaves.
• permissions: The new permissions to set (e.g., u+rwx for user read, write, and execute).
• file: The file or directory for which you want to change permissions.

Let's break down the permissions:

• `u` stands for user/owner.
• `g` stands for group.
• `o` stands for others.
• `a` stands for all (user, group, and others).

The symbols used for permissions are:

• `r` stands for read.
• `w` stands for write.
• `x` stands for execute.
• `+` to add a permission.
• `-` to remove a permission.

Here are some common examples of using `chmod`:

1. Granting Read and Write Permissions to the Owner:

chmod u+rw file.txt

This command grants the user (owner) of `file.txt` both read and write permissions.

2. Revoking Write Permission from the Group:

   

chmod g-w file.txt

This command removes write permission from the group of `file.txt`.

3. Adding Execute Permission for Others:

   

chmod o+x script.sh

This command adds execute permission for others (users who are not the owner or in the group) to `script.sh`.

4. Setting Read and Execute Permissions for All:

   

chmod a+rw directory

This command sets read and execute permissions for everyone (user, group, and others) on the specified directory.

5. Numeric Representation of Permissions:

You can also use a numeric representation to set permissions. Each permission has a numeric value:

•  `r` (read) is 4.
•  `w` (write) is 2.
•  `x` (execute) is 1.

For example, to give read and write permissions to the owner and read-only permissions to the group and others:

chmod 644 file.txt

Here, `6` corresponds to read (4) + write (2) for the owner, and `4` corresponds to read-only for the group and others.

Understanding and effectively using `chmod` is fundamental for managing file and directory permissions in a Linux system. It allows you to control who can read, write, and execute files, ensuring the security and integrity of your system.