Day 5 - Week 1 Comprehensive Review: Time to See Through the Concepts

📌 Key Takeaways

• Week 1 is all woven into one storyline: "what happens when you power on."
• Order: firmware (POST) → partitions/MBR·GPT → GRUB2 → kernel·initramfs → systemd (PID 1) → shell.
• The comparison tables are the exam points — kernel/user mode, GPL/BSD, MBR/GPT, SysV/systemd, hard/symbolic links.
• The 3 big pitfalls: dynamic linking with LGPL has no disclosure obligation, SUSE is an independent family but uses RPM, runlevel 0 = shutdown, 6 = reboot.
• Level 1 is a written exam (concepts, comparisons, history) + a practical exam (commands). Don't memorize commands with your eyes only; type them yourself.

Week 1 Revisited as a Flow

The best way to understand Linux is to follow "what happens when you power on." Every concept from this week is inside this one storyline.

• When you power on, the firmware (BIOS or UEFI) wakes up, checks the hardware (POST), and picks a boot device (Day 3).
• That disk is divided into partitions, and the partition information is held in an MBR or GPT table (Day 3).
• The firmware runs the GRUB2 bootloader, and GRUB2 finds the kernel (vmlinuz) in /boot and loads it into memory (Day 3).
• The kernel, with a monolithic structure holding all the core functions (Day 1), mounts the real root filesystem via initramfs and then runs the first process, systemd (PID 1) (Day 3).
• systemd starts services in parallel to make the system usable, and finally the shell greets the logged-in user (Day 1).
• The shell runs the user's commands with the fork-exec-wait model, and those commands live in /bin and /sbin (Day 1, Day 4).
• Everything the user handles is a file on the FHS tree extending from a single root (/) (Day 4), and the true identity of that file is not its name but its inode (Day 4).
• The reason all this software can be shared freely is the copyleft of the GPL license, and that freedom grew the distribution ecosystem of Ubuntu, Red Hat, SUSE, and others (Day 2).

💡 Concept: The Level 1 exam tends to test "connections between concepts" more than isolated facts. For example, "Why do we put /boot on a separate partition?" can only be answered by understanding partitions (Day 3) + the boot process (Day 3) + filesystems (Day 4) all at once. When reviewing, it is better to organize these links than to memorize individual commands.

The Key Comparison Table You Must Remember

We've gathered in one place the "which one is it?" comparisons that appeared this week. Even just looking at this table right before the exam helps a lot.

⚠️ Pitfall: There are three easy-to-confuse traps on the exam. ① A closed-source app that dynamically links an LGPL library has no obligation to disclose its source. ② SUSE is an independent family, but its package format is RPM. ③ Runlevel 0 is shutdown (poweroff), and 6 is reboot.

Frequently Used Commands at a Glance

We've grouped the commands that appeared this week by function. The Level 1 practical exam requires hands-on experience typing them.

# --- System information ---
uname -r                    # Kernel version
uname -a                    # Full system information
cat /etc/os-release         # Distribution information
echo $SHELL                 # Current shell
 
# --- Package management (Red Hat) ---
rpm -qa                     # All installed packages
dnf install <pkg>           # Install with automatic dependencies
dnf update                  # Update everything
 
# --- Package management (Debian) ---
dpkg -l                     # All installed packages
apt install <pkg>           # Install with automatic dependencies
apt update && apt upgrade   # Refresh the list, then upgrade
 
# --- Boot/services ---
systemctl status <svc>      # Service status
systemctl enable <svc>      # Start automatically at boot
systemctl get-default       # Check the default target
grub2-mkconfig -o /boot/grub2/grub.cfg   # Regenerate GRUB
 
# --- Disks/partitions ---
lsblk                       # Block device tree
fdisk -l                    # Partition table
blkid                       # UUID and filesystem type
 
# --- Files/links/inodes ---
ls -li                      # Listing including inode numbers
stat <file>                 # Detailed metadata
ln <src> <link>             # Hard link
ln -s <src> <link>          # Symbolic link
file <file>                 # Determine file type

When memorizing commands, learning "what the options mean" along the way makes them easier to apply. For example, if you know the i (install), v (verbose), and h (hash progress display) of rpm -ivh, you will naturally understand rpm -Uvh (Upgrade) too. If you know that the i in ls -li is inode, ls -a (all, hidden files) comes to mind as well. Most options are the first letter of an English word.

Exam Preparation Mindset

Linux Master Level 1 is split into a written exam and a practical exam. The written exam asks about concepts, comparisons, and history, and the practical exam checks whether you can actually type commands.

• Concept questions: Get definitions exactly right, as in "the four core functions of an operating system," "the Unix philosophy," or "the meaning of copyleft." The choices are made to look similar and confuse you, so memorize the keywords clearly.
• Comparison questions: The comparison table above is asked as is. You should be able to state the key difference between A and B in one line.
• Command questions: Get commands, options, and configuration file paths exactly right. Even paths like /etc/default/grub are asked.
• Order questions: For questions asking about order, such as the 4 boot stages or the fork-exec-wait order, remember the stages as a picture.

Linux Master is a nationally accredited private certification administered by the Korea Association for ICT Promotion (KAIT). Level 2 verifies beginner-level Linux skills, and Level 1 verifies practical server operation and administration skills. For Level 1 you must pass the 1st-round written exam and then take the 2nd-round practical exam, so rather than memorizing commands with your eyes only, installing Linux yourself in a virtual machine and typing them is the shortcut to passing.

Starting next week, we move into real hands-on skills: file and directory commands, permission and ownership management (chmod, chown), user and group management, and process control. The inodes and file types you learned today are the starting point.

📖 Glossary

• POST: The stage where the firmware self-tests the CPU, memory, and so on right after power comes on.
• initramfs: A temporary RAM filesystem holding the drivers needed to mount the real root.
• PID 1: The first user process the kernel launches (systemd). It is the ancestor of all processes.
• Copyleft: The core device of the GPL that forces derivatives to be released under the same free license.
• LGPL: Weak copyleft. If you only dynamically link a library, you don't have to disclose your app's source.
• RPM family / DEB family: The Red Hat camp that uses rpm and dnf, and the Debian camp that uses dpkg and apt.
• Target: The name of a system state that systemd uses in place of runlevels (such as multi-user.target).
• inode: The data structure that holds a file's metadata. The file name is not in it.
• FHS: The standard that decides what goes in which directory. It's why locations are the same even across different distributions.
• fork-exec-wait: The 3 steps by which the shell runs a command — clone, swap in the program, and wait for it to finish.