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LPI Linux Essentials Exam 010-160 - Topic 5.1 - Switching or Running Commands as Other Users

A key aspect of system administration and management involves the ability to switch user contexts and execute commands with different user permissions. This capability is fundamental for maintaining system security, performing administrative tasks, and managing user access to resources. The mechanisms to switch users or run commands as other users in Linux are achieved primarily through two commands: su and sudo.

The su command, short for "substitute user" or "switch user", allows you to switch to another user account within your terminal session. By using this command, you can operate as if you were logged in as that user, with all the permissions and limitations of the target user account. There are nuances in using su, such as the difference between executing su and su - (or su -l), which impacts the environment variables and the shell behavior that you inherit in the switched user context.

On the other hand, sudo, which stands for "superuser do", is a powerful utility that allows a permitted user to execute a command as another user, as specified by the security policy. The most common use of sudo is to run commands with root privileges, which are necessary for performing various system-level tasks. sudo provides fine-grained control over user permissions, and it is configured through the /etc/sudoers file and the /etc/sudoers.d directory. Understanding how to use sudo effectively, including how to run individual commands or switch to a root shell using sudo -i, is essential for efficient and secure system management.

This guide will delve into the details of these commands, explaining their usage, differences, and configurations. By understanding these tools, you'll gain the ability to manage your Linux system more effectively, ensuring that tasks are performed securely and efficiently under the appropriate user permissions.

su Command

The su command in Linux is used to switch the current user context to another user within a terminal session. It's a fundamental tool for accessing different user accounts, including the root account, from the command line.

To use su, you simply type su followed by the username of the account you want to switch to. For example, su username will prompt you for the password of the specified user, and upon successful authentication, you will be switched to that user's shell.

A distinction in the usage of su lies between su and su - (also expressed as su -l). This difference revolves around the environment variables and the shell experience you inherit when you switch the user.

When you execute su username, you switch to the specified user, but you retain the environment variables from your original user. This means that aspects like the home directory, the shell type, and the PATH variable remain as they were in your original session, which might not correspond to the typical environment of the user you switched to. This can lead to unexpected behaviors, especially when running scripts or applications that depend on user-specific environment settings.

Using su - username (or su -l username) initiates a login shell. This means it simulates a full login as that user, almost as if you logged into the system as that user from the start. In this mode, all the environment variables are set as per the target user’s login configuration. This includes the home directory, the shell preferences, and the PATH variable. This mode is often used when you need to perform tasks that require the exact environment of the target user, especially for administrative or troubleshooting tasks.

One of the common uses of su is to switch to the root user. By simply executing su or su - without a username, you switch to the root user. Switching to the root user is a sensitive operation, as it gives you complete control over the system, and hence it should be done with caution.

In summary, understanding the subtle differences between su and su - is important for effective system administration. It ensures that you operate under the correct user environment, which is crucial for maintaining system security and proper execution of commands.

sudo Command

The sudo command in Linux is an incredibly versatile tool for managing system permissions. It allows users to run commands with the security privileges of another user, typically the root user. This command is crucial for system administration, providing a secure way of granting administrative permissions without sharing root passwords.

The most straightforward use of sudo is to prefix a command that requires elevated privileges with sudo. For example, sudo apt-get update will run the apt-get update command with root privileges. By default, sudo executes commands as the root user. However, you can specify a different user by using the -u option, like sudo -u username command.

The sudo -i command is used to switch to the root user's environment. Unlike sudo su which simply opens a root shell, sudo -i initiates a login shell. This means it loads the root user’s environment variables and settings. This is especially useful for performing a series of administrative tasks that require root privileges, as it avoids the need to prefix every command with sudo.

Some considerations of using sudo are:

  • Authorization: The ability to use sudo is governed by the /etc/sudoers file. This file specifies which users can run which commands and as which users. It is designed to provide granular control over permissions.
  • Accountability: When a command is executed using sudo, the details are typically logged. This provides an audit trail, which is important for security and accountability.
  • Password Prompt: By default, sudo requires the user's password (the one executing sudo, not the root password). This is an additional security measure to ensure that only authorized users can execute sensitive commands.
  • Convenience vs. Security: While sudo is convenient, it's important to use it judiciously. Unnecessary use of sudo, especially in scripts, can pose security risks.
  • Learning Curve: For new users, understanding the nuances of sudo can be challenging, but it is an essential skill for effective and secure system management.

In essence, sudo is a powerful tool in the Linux administrator's toolkit. It allows for controlled escalation of privileges, enabling users to perform tasks that require higher permissions securely and conveniently. Understanding how to use sudo effectively, including running individual commands and switching to a root shell, is essential for efficient and secure Linux system management.

sudo Configuration Files

The configuration of sudo in Linux hinges on two main elements: the /etc/sudoers file and the /etc/sudoers.d directory. These components are fundamental for defining user and group privileges for using sudo.

The /etc/sudoers File

The /etc/sudoers file is where the rules for sudo privileges are set. It dictates who can run what commands and on which machines. For instance, a typical entry like root ALL=(ALL:ALL) ALL implies that the root user can execute any command from any terminal. Another example could be username ALL=(ALL) NOPASSWD: ALL, which allows the user 'username' to execute all commands on all hosts without needing to enter a password. Similarly, a group-based entry like %admin ALL=(ALL) ALL grants all members of the 'admin' group the ability to run any command on any host.

Editing the /etc/sudoers file is best done using the visudo command. This command is designed to edit the file in a safe manner, ensuring that no syntax errors are introduced. visudo typically defaults to using Vi or Nano as the editor, depending on the system’s configuration.

The /etc/sudoers.d Directory

In addition to the main sudoers file, the /etc/sudoers.d directory allows for modular configuration files. This setup facilitates managing individual configurations for users or groups without modifying the main sudoers file. For example, creating a file named webadmin in this directory with content such as webadmin ALL=(ALL) NOPASSWD: /usr/bin/systemctl restart apache2 would allow the 'webadmin' user to restart the Apache service without a password prompt. Similarly, a file for a developers' group, named developers, might contain %developers ALL=(ALL) ALL, offering full sudo access to all users in that group.

Security and Best Practices

When managing these configuration files, it’s crucial to maintain strict file permissions, such as 440, to prevent unauthorized modifications. Following the principle of least privilege is also key; privileges should be granted conservatively to minimize security risks. Regular audits of the sudo configurations are recommended to ensure they align with current needs and security standards.

In essence, understanding and properly setting up the /etc/sudoers file and the /etc/sudoers.d directory is critical for Linux system administration. These configurations play a vital role in controlling access to elevated privileges and defining what commands users and groups can execute, striking a balance between necessary access and maintaining system security.


In this guide, we've explored the critical aspects of switching user contexts and executing commands with different user permissions in Linux, focusing on the su and sudo commands.

The su command allows for user switching within a session, with nuances like su - impacting the environment settings inherited.

The sudo command, essential for executing commands with elevated privileges, is configured through the /etc/sudoers file and the /etc/sudoers.d directory, with sudo -i enabling a switch to the root user's environment. Emphasizing best practices like the principle of least privilege, secure file editing using visudo, and regular configuration audits, the guide underscores the importance of these tools in effective and secure Linux system administration. Understanding and properly utilizing these functionalities ensures that Linux systems are managed with both efficiency and a high standard of security.

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