Skip to content

Linux Lab 5: Linux Kernel, Boot & Filetypes

This lab dives into the core components of the Linux operating system. We will explore how the system starts (the boot process), how services are managed (systemd), how to identify different types of files, and where important system files are located.

Table of Contents

Key Concepts

  • The Init Process (PID 1): The very first process started by the kernel during boot. It is responsible for starting all other processes and services on the system. Modern Linux systems use systemd.
  • Systemd Targets: These replace the old concept of "runlevels." A target is a state the system can be in.
    • multi-user.target: The standard non-graphical server mode (like Runlevel 3).
    • graphical.target: The mode with a GUI desktop (like Runlevel 5).
  • file Command: Linux doesn't rely on file extensions (like .exe or .txt) to know what a file is. The file command looks at the file's content (its "magic number") to determine its true type.
  • Filesystem Hierarchy Standard (FHS): The standard that defines the directory structure of Linux systems (e.g., /dev for devices, /opt for optional software).

Step-by-Step Walkthrough

1. The Init Process (systemd)

Question: What is the init process used by this system? Answer: systemd

Explanation: We checked this by looking at /sbin/init. 

sudo ls -l /sbin/init
# Output: ... /sbin/init -> /lib/systemd/systemd
This output shows that /sbin/init is a symbolic link pointing to /lib/systemd/systemd. This confirms that systemd is the program managing the boot process.

2. Systemd Targets (Runlevels)

Question: What is the default systemd target set in this system? Answer: graphical.target

Explanation: We used the systemctl command to query the current configuration: 

sudo systemctl get-default
# Output: graphical.target
This means that if you rebooted the machine right now, it would try to start a graphical user interface (GUI).

3. Changing the Default Target

Task: Change the target to multi-user.target. Command: 

sudo systemctl set-default multi-user.target
Why do this? Servers usually don't need a GUI. Running a GUI takes up valuable RAM and CPU. Setting the default to multi-user.target ensures the server boots into a lightweight, command-line-only mode, which is best practice for servers.

4. Identifying File Types (file command)

Question: What type of file is firefox.deb? Answer: Debian binary package Command: sudo file /root/firefox.deb

Question: What type of file is sample_script.sh? Answer: Bourne-Again shell script Command: sudo file /root/sample_script.sh

Explanation: The file command reads the header (the first few bytes) of the file to identify it. * It correctly identified .deb as a software package for Debian/Ubuntu systems. * It correctly identified .sh as a shell script (specifically a bash script, or "Bourne-Again shell").

5. System Directory Structure (/opt and /dev)

Question: Where should you install a new third-party IDE? Answer: /opt

Explanation: * /opt (Optional): This is the standard directory for installing add-on application software packages that are not part of the core operating system distribution (like Chrome, Slack, or a custom IDE). * /usr/bin: For package-manager installed software. * /bin: For essential system binaries.

Question: Which directory contains files related to block devices (from lsblk)? Answer: /dev

Explanation: * /dev (Devices): In Linux, "everything is a file." Hardware devices like hard drives (sda, nvme0n1), partitions, terminals, and USB inputs are represented as special files in this directory.

6. Hardware Information (lshw)

Question: What is the vendor of the Ethernet Controller? Answer: Red Hat, Inc.

Explanation: We used the lshw (List Hardware) command. 

sudo lshw -class network
This filters the output to show only network devices. The output showed: 
*-network
     description: Ethernet controller
     vendor: Red Hat, Inc.   <-- The Answer
     product: Virtio network device
This tells us the system is likely a Virtual Machine (VM) using the "Virtio" driver provided by Red Hat's virtualization technology.

Command Reference

Command Purpose Example
ls -l List files in long format (shows symlinks) ls -l /sbin/init
systemctl get-default Show the current default boot target systemctl get-default
systemctl set-default Change the default boot target sudo systemctl set-default multi-user.target
file Determine file type file myfile.tar.gz
lshw List Hardware sudo lshw -class network
lsblk List Block Devices lsblk