🌐 Linux Filesystem Overview for Custom ISO Creation¶

When creating a custom Ubuntu ISO using Cubic, it’s essential to understand the Linux filesystem hierarchy. Each folder has a specific role, and knowing where to place or edit files ensures your custom ISO works reliably and efficiently.

🏰 Root (/)¶

• Description: The root directory is the top-level directory of the Linux filesystem.

• Role in Cubic:

• All other directories branch from here.

• When you enter Cubic’s chroot, you are inside /, seeing the ISO as the complete system.
• Tip: Never add files directly in / unless necessary. Always use the appropriate subdirectory.

📂 /bin – Essential Binaries¶

• Contains: Core system programs used by all users (e.g., ls, cp, bash).
• Role in Cubic: Usually left untouched; only modify if adding fundamental system commands.

🛠 /etc – Configuration Files¶

• Contains: System-wide configuration files and settings.

• Important Files for Custom ISO:

• /etc/lsb-release → Defines OS name and version.

• /etc/os-release → System identification for applications and boot process.

• Role in Cubic:

• Edit these files to rebrand your ISO and set default system configurations.

🏠 /home – User Data¶

• Contains: Personal files and settings for each user.

• Role in Cubic:

• Preconfigure /etc/skel/ so that new users automatically inherit settings, themes, and extensions.

• Tip: Don’t store ISO-wide files here; use /usr or /opt instead.

📦 /usr – User Programs & Resources¶

• Contains: Applications, libraries, and shared resources.

• Key Subdirectories:

• /usr/bin → Programs and executables

• /usr/lib → Libraries required by programs
• /usr/share → Shared resources (icons, wallpapers, applications, GNOME extensions)

• Role in Cubic:

• Add custom apps, extensions, and themes here for system-wide availability.

• Tip: Use /usr/share/applications for .desktop files to make apps appear in menus.

🗄 /var – Variable Data¶

• Contains: Logs, caches, temporary files, and spool directories.

• Role in Cubic:

• Clean /var/cache/apt/archives and /var/log before building ISO to reduce image size.

🏛 /opt – Optional Software¶

• Contains: Third-party or optional applications.

• Role in Cubic:

• Ideal for bundling custom applications outside the package manager.

🖼 /boot – Boot Files¶

• Contains: Kernel, initial RAM disk, and bootloader configuration.

• Role in Cubic:

• Customize GRUB splash images and boot parameters.

• Ensure kernels match installed packages.

🧩 /lib and /lib64 – Libraries¶

• Contains: Shared libraries required by system programs.

• Role in Cubic:

• Usually left unchanged unless a specific program needs a custom library.

🔧 /sbin – System Binaries¶

• Contains: Administrative commands (e.g., fsck, ifconfig)
• Role in Cubic: Typically unchanged; important for live system utilities.

🏷 /tmp – Temporary Files¶

• Contains: Temporary runtime files.

• Role in Cubic:

• Clean before building ISO to keep image small.

🔌 /dev – Device Files¶

• Contains: Device nodes (hard drives, USBs, etc.)

• Role in Cubic:

• Automatically populated in live sessions. Usually not modified.

📊 /proc and /sys – Virtual Filesystems¶

• Contains: System and kernel information (dynamic, not real files).

• Role in Cubic:

• Provides runtime data. Not edited for customization.

🧩 Summary – Memory Aid¶

Think of Linux as a kingdom: