How to Use the Command 'runuser' (with Examples)

The runuser command in Linux allows the execution of commands with an alternate user identity without requiring the user to input a password. This capability can be particularly useful for system administrators who need to manage tasks running under different user privileges. Primarily, it is used in scripts and automated processes where interactive password entry is not feasible. However, note that root privileges are necessary to utilize runuser.

Use Case 1: Run Command as a Different User

Code:

runuser user -c 'command'

Motivation:

The ability to run a command as a different user is crucial for environments where permission segregation is a necessity. For example, if a script is configured to perform system updates but needs to be executed under a user account that doesn’t have superuser privileges, runuser enables this without exposing passwords or credentials.

Explanation:

• user: This specifies the username under which the command should be executed. Here the command will be run with the same privileges as this user.
• -c 'command': This option indicates the command to be executed. The command should be enclosed in quotes to ensure that it is passed as a single parameter to the shell.

Example Output:

Suppose you want to list the contents of a directory as the ‘guest’ user:

$ runuser guest -c 'ls /var/www'
index.html
style.css
script.js

Use Case 2: Run Command as a Different User and Group

Code:

runuser user -g group -c 'command'

Motivation:

Certain applications or scripts might require specific group permissions to access particular files or directories. Running commands with both different user and group privileges allows administrators to handle files or execute commands securely while respecting group-level permissions.

Explanation:

• user: Specifies the user identity under which the command will run.
• -g group: This option declares the group under which the command will be executed. It ensures that the command also inherits the group privileges.
• -c 'command': The command to be executed, crafted to run with the specified user and group privileges.

Example Output:

Running a file access operation as the ‘webadmin’ user and group:

$ runuser webadmin -g webadmin -c 'ls /var/www/html'
home.html
about.html
contact.html

Use Case 3: Start a Login Shell as a Specific User

Code:

runuser user -l

Motivation:

Starting a login shell as a specific user is particularly beneficial when needing to perform a series of tasks or explore environments specific to that user without physically logging out and back into the system as a different user. This use case is ideal for administrators needing to troubleshoot user-specific issues directly.

Explanation:

• user: Denotes the objective user whose shell you want to access. It will initiate a login shell that mimics an interactive user session.
• -l: This option specifies that the shell should be a login shell, sourcing the user’s shell initialization files.

Example Output:

If you start a login shell for user ‘developer’, you will see:

$ runuser developer -l
developer@hostname:~$

Use Case 4: Specify a Shell for Running Instead of the Default Shell

Code:

runuser user -s /bin/sh

Motivation:

Specifying a shell different from the default is useful in scenarios where a special shell environment (like a minimal Bourne shell for scripting) is required instead of what’s typically configured for the user. It helps in maintaining compatibility and ensuring script execution in a predictable shell environment.

Explanation:

• user: The user account under which the command is executed.
• -s /bin/sh: Assigns the specific shell you wish to use for the command execution.

Example Output:

Suppose we want to run a shell session using the Bourne shell:

$ runuser developer -s /bin/sh
$

Use Case 5: Preserve the Entire Environment of Root

Code:

runuser user --preserve-environment -c 'command'

Motivation:

Preserving the root environment is essential when you need user-specific tasks to function within the full context of root’s environment variables, paths, and settings, without wiping parts of the environment. This use case is particularly helpful for running maintenance scripts that depend on the root environment.

Explanation:

• user: Indicates the user context under which the command is executed.
• --preserve-environment: Keeps the root user’s environment intact while switching to the target user’s privileges for the command.

Example Output:

Preserving the entire environment while running a diagnostic script:

$ runuser sysadmin --preserve-environment -c 'env | grep PATH'
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin

Conclusion:

The runuser command is a powerful tool primarily leveraged in automated systems and scripts where task execution under different user privileges is a necessity. From handling commands in diverse user and group contexts to maintaining specific environments, runuser addresses the complexity of managing user permissions without manual intervention, thus enhancing efficiency and security.