How does the bootloader pick up the command after a “restarting system with command”?

Understanding “Restarting System With Command”

The Concept

The phrase “Restarting System With Command” typically implies a scenario where a system is being restarted with an additional instruction, often a command, to be executed upon rebooting. This command is not directly executed during the restart process but is rather passed to the system’s bootloader, which is responsible for loading the operating system.

Bootloader’s Role

Bootloaders, such as GRUB, are programs that execute before the operating system loads. They are responsible for:

• Initializing the system’s hardware.
• Locating and loading the operating system kernel.
• Passing control to the kernel.

In the case of a “Restarting System With Command,” the bootloader will receive the command from the previous system session and store it for execution upon restart.

How the Bootloader Handles the Command

1. Command Storage

Different bootloaders use various mechanisms to store the command:

• Configuration Files: Some bootloaders write the command to a configuration file that is read during the boot process. This file can be a text file or a binary file.
• Boot Parameters: Bootloaders can use boot parameters (e.g., kernel command-line arguments) to pass the command to the kernel.
• Environment Variables: Bootloaders might use environment variables to store the command for the kernel to access later.

2. Command Execution

After the bootloader loads the kernel, it will:

• Retrieve the Command: The bootloader reads the command from the storage location (configuration file, boot parameters, or environment variable).
• Execute the Command: Depending on the command, the bootloader might directly execute it (if it’s a simple command) or pass it to the kernel for execution.

Example: Using a Configuration File in GRUB

GRUB Configuration

Here’s a simple example of using a configuration file (/boot/grub/grub.cfg) in GRUB to pass a command upon restart.

menuentry 'Restart with Command' {
    linux /boot/vmlinuz-5.15.0-41-generic root=UUID=f0e4e023-5f12-4846-8c90-a118d62a7085
    initrd /boot/initrd.img-5.15.0-41-generic
    # This line passes the command 'echo "Restarting with command!"' to the kernel
    linuxefi /boot/vmlinuz-5.15.0-41-generic root=UUID=f0e4e023-5f12-4846-8c90-a118d62a7085  
           initrd=/boot/initrd.img-5.15.0-41-generic  
           "echo 'Restarting with command!'"
}

Restarting with command!

Explanation

• The menuentry block defines a menu entry called “Restart with Command”.
• The linux line loads the kernel and specifies boot parameters.
• The line starting with "echo 'Restarting with command!'" is the command that will be passed to the kernel upon rebooting.

Conclusion

When a system is restarted with a command, the bootloader plays a crucial role in receiving and executing that command. Bootloaders utilize various storage methods (configuration files, boot parameters, environment variables) to preserve the command and ensure its execution upon restart. This mechanism allows for seamless integration of post-restart operations, enhancing system functionality and automation.