How to use the command 'switch_root' (with examples)

The switch_root command is utilized in Linux systems to replace the current root filesystem with a new one, effectively changing the root of the file system tree. It is particularly important in scenarios such as system initialization after the Linux kernel has booted, where a temporary root filesystem (such as an initial RAM disk or initrd) is replaced by the real root filesystem located on disk. The switch_root command is often used in combination with chroot and mount to ensure the system continues to operate correctly by aligning system paths to the new root.

Use case 1: Move /proc, /dev, /sys, and /run to the specified filesystem, use this filesystem as the new root, and start the specified init process

Code:

switch_root new_root /sbin/init

Motivation:

This use case is particularly relevant during the boot process of Unix-like operating systems, where the system initially boots from a temporary root filesystem, often stored in RAM, before moving to the main root filesystem. During this transition, switch_root is used to unmount the temporary filesystems (/proc, /dev, /sys, and /run) from the initial environment, apply these directories to the new root environment, and initiate the main system process by starting the init system (specified here as /sbin/init). This is a critical step for systems that use an initial RAM disk (initramfs), allowing them to transition seamlessly to the primary filesystem.

Explanation:

• new_root: This parameter specifies the directory which will become the new root directory filesystem. It should be set-up as a mount point that has its own /proc, /dev, /sys, and /run directories.
• /sbin/init: This argument specifies the init system process to start once the switch is complete. In most Linux distributions, /sbin/init is the default init system program, responsible for bringing up the system to a usable state by launching all necessary services and user-space utilities.

Example output:

Though the command itself does not produce a textual output, a successful execution leads to a change in the root filesystem and the initiation of the system’s main process. In cases where the command fails, an error message, such as “switch_root: failed to mount new_root,” will be displayed, indicating issues with the new root filesystem setup.

Use case 2: Display help

Code:

switch_root -h

Motivation:

Invoking the help option with -h provides the user with a concise overview of the command’s usage, offering a list of all available options and arguments. This is especially beneficial for users who are new to switch_root or need a quick refresher on the correct syntax and capabilities of the command without scouring through extensive documentation.

Explanation:

• -h: This is a common command-line option used across many tools to display a help message. In this case, it provides a brief overview of the switch_root command, including acceptable arguments and their descriptions, helping clarify usage scenarios for the user.

Example output:

Usage: switch_root [-h|--help] newroot init [arg]...

This output informs the user of the basic syntax needed to utilize switch_root, including an explanation of the parameters newroot and init.

Conclusion:

The switch_root command is a powerful tool essential for transitioning from a temporary boot filesystem to the permanent root filesystem in Unix-like operating systems. It plays a critical role in the initialization and start-up processes, ensuring systems are able to boot efficiently and securely. Understanding how to utilize this command is vital for system administrators and developers dealing with OS setup and boot scripts. Through the specific use cases illustrated, we delve into the crucial scenarios where switch_root becomes an indispensable part of the system management toolkit.