Linux Operating Systems Fundamentals Study Guide: Lessons 5–6

How to use this guide

This guide combines the Lessons 5–6 chapter material. Lesson 5 follows a Linux system from power-on to a running set of services and back down again. Lesson 6 covers what users see on top of that running system: the graphical desktop, accessibility, localization, time, and printing.

Study in this order:

1. Learn the vocabulary, commands, options, and file paths in the lesson summaries.
2. Explain the key comparisons and decision rules in your own words.
3. Practice the commands in a lab VM, then revisit the high-yield distinctions across both lessons.

Many questions hinge on one exact subcommand, option, or path. Learn the real name and be ready to reject look-alikes that don’t exist, such as systemd set-default, systemctl set-def, /etc/init-d, or XFree86 --query.

The ideas that connect Lessons 5 and 6

• Lesson 5 — getting the system running: firmware → boot loader (GRUB) → kernel → init system (systemd or SysVinit) → services and targets. It also covers stopping the system, warning users, and running Linux as a virtual guest.
• Lesson 6 — making the system usable: the graphical stack (display server, window manager, desktop environment), accessibility, remote desktops, locales and character sets, time zones and clocks, and printing with CUPS.

The connection is the graphical target. systemd’s graphical.target (runlevel 5) is what starts the display manager, and switching to multi-user.target (runlevel 3) is how you stop the GUI to fix X configuration.

Lesson 5 — Booting, Initializing, and Virtualizing Linux

The four boot stages

1. Firmware (BIOS or UEFI) runs the Power-On Self-Test (POST) and finds a boot loader.
2. Boot loader (GRUB) chooses the kernel and its options.
3. Kernel loads, mounts the root filesystem (with help from the initrd/initramfs), and starts the init program.
4. Initialization (systemd or SysVinit, PID 1) starts the services.

Reading boot messages

• The kernel stores its messages in the kernel ring buffer, a fixed-size circular buffer in memory.
• dmesg displays the kernel ring buffer and is the command to use when troubleshooting the boot process. Many systems also save it to /var/log/dmesg.
• On systemd systems, journalctl -b shows messages from the current boot (-k for kernel messages only).
• Some distributions also write /var/log/boot or /var/log/boot.log.

dmesg | less
dmesg | grep -i sda
sudo journalctl -b

Firmware: BIOS and UEFI

Feature	BIOS	UEFI
Where the boot loader lives	Master boot record (MBR), the first disk sector	.efi files on the FAT-formatted EFI System Partition (ESP)
Size limit	One sector	None in practice
Linux mount point	None	/boot/efi

• Chainloading lets one boot loader hand off to another, which is how one menu offers several operating systems.
• Secure Boot and shim: on UEFI systems with Secure Boot, the firmware loads the signed shim.efi first (the default), and shim then loads grubx64.efi, the GRUB boot loader.
• ls /sys/firmware/efi succeeds only on a UEFI-booted system.

GRUB Legacy

• Configuration file: /boot/grub/menu.lst (or /boot/grub/grub.conf on Red Hat systems).
• Counts disks and partitions from 0: (hd0,0) is the first partition on the first disk.
• Install it once with grub-install /dev/sda; it rereads its configuration file on every boot.

Command	Purpose
default, timeout	Default entry (from 0) and seconds to wait
title	Menu text; starts each OS section
root	Partition holding /boot
kernel	Kernel image plus kernel parameters
initrd	Initial RAM disk with boot-time drivers
rootnoverify	Marks a partition holding a non-Linux kernel, such as Windows. GRUB Legacy only; GRUB 2 doesn’t use it.
chainloader +1	Hand off to the boot loader in that partition’s first sector

title Windows
    rootnoverify (hd0,0)
    chainloader +1

GRUB 2

• The generated file is grub.cfg (/boot/grub/, /boot/grub2/, or /boot/efi/EFI/<distro>/). Don’t edit it.
• Edit /etc/default/grub (settings such as GRUB_TIMEOUT and GRUB_CMDLINE_LINUX) or the scripts in /etc/grub.d/, then regenerate the file.
• After changing /etc/default/grub, run grub-mkconfig (Red Hat: grub2-mkconfig). It writes to STDOUT unless you add -o to name the output file. Ubuntu’s update-grub does the same thing.
• Partitions count from 1: (hd0,1) is the first partition of the first disk.
• Entries use menuentry "Name" { ... }, set root=, linux, and initrd (or linuxefi / initrdefi on UEFI).

sudo grub-mkconfig -o /boot/grub/grub.cfg
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Using the menu: hold Shift as GRUB takes control to display a hidden menu (tap Esc on many UEFI systems). Press E to edit an entry for one boot, C for the GRUB command line, and Ctrl+X (GRUB 2) or B (Legacy) to boot the edited entry.

Kernel boot parameters

Add these to the end of the kernel (Legacy) or linux (GRUB 2) line for one boot:

Parameter	Effect
ro	Initially mount the root partition read-only (the default); it’s remounted read-write later in the boot
rw	Mount root read-write from the start
root=	Choose the root filesystem
single (or S, 1)	Boot into single-user mode, without launching the usual daemons
init=/bin/bash	Run a different program instead of /sbin/init
systemd.unit=rescue.target	Boot a systemd system into a specific target
selinux=0	Disable SELinux for this boot

systemd units and unit files

• A unit is anything systemd manages, named name.type, such as sshd.service or multi-user.target.
• systemctl list-units lists the units systemd has loaded; add --type=service for services only. systemctl list-unit-files lists installed unit files and whether they’re enabled.
• Unit file priority, highest first: /etc/systemd/system/ → /run/systemd/system/ → /usr/lib/systemd/system/.

Section	Key directives
[Unit]	Description, After/Before (order only), Requires, Wants, Conflicts
[Service]	ExecStart (the program to run when the service starts), ExecStop, ExecReload, Type, Restart
[Install]	WantedBy (the target that pulls the unit in when it’s enabled)

systemd watches the process ID (PID) of the ExecStart program to tell whether the service has crashed.

[Service]
ExecStart=/usr/sbin/sshd -D $OPTIONS
Restart=on-failure

[Install]
WantedBy=multi-user.target

After editing a unit file, run systemctl daemon-reload, then restart the service.

Managing services with systemctl

Command	What it does
start / stop / restart	Change the service now
enable / disable	Start (or don’t start) the service at the next boot; --now also starts or stops it immediately
reload	The service rereads its own configuration file
daemon-reload	systemd rereads its unit files
mask / unmask	Block, or unblock, a unit from starting at all
status, is-active, is-enabled	Check state

The command is always systemctl, not systemd.

Targets

• default.target is the unit a systemd system boots into; it’s a link to the real target, which then starts other units based on dependencies.

Target	Meaning	Runlevel
poweroff.target	Shut down	0
rescue.target	Single-user maintenance, local filesystems, no network	1
multi-user.target	Multi-user, networking, text only	3
graphical.target	Multi-user with a GUI	5
reboot.target	Reboot	6
emergency.target	Root filesystem read-only, root only	—

systemctl get-default                          # display the default target
sudo systemctl set-default multi-user.target   # set the default target (next boot)
sudo systemctl isolate multi-user.target       # switch targets (runlevels) now

• get-default displays the boot target, set-default changes it, and isolate switches to another target immediately.

SysVinit and runlevels

• /etc/init.d/ stores the startup and shutdown scripts for each service (sometimes linked from /etc/rc.d/init.d/).
• /etc/rc.d/rcN.d/ holds S##name (start) and K##name (kill) links for runlevel N.
• /etc/inittab sets the default runlevel (id:5:initdefault:).
• telinit tells init to change runlevels, and telinit q makes it reload /etc/inittab without a reboot. init N and telinit N are equivalent.
• runlevel prints the previous and current runlevels.
• service NAME start|stop|status runs a script in /etc/init.d/.

Stopping the system

Command	Effect
halt	Stops all processes and the CPU immediately; the power may stay on
poweroff	Stops everything and turns the power off
reboot	Stops everything and restarts
shutdown	Scheduled and flexible: -h/-H halt, -P power off, -r reboot, -c cancels a pending shutdown

sudo shutdown -r +10 "Rebooting for updates. Save your work."
sudo shutdown -h now
sudo shutdown -c

• Shutdown sends SIGTERM to processes, including the X server, so they can exit cleanly and close their files. SIGKILL (kill -9) can’t be caught and gives no chance to clean up.

Notifying users

Tool or file	When users see it
wall	Sends a message to all logged-in users’ terminals (type the message, then Ctrl+D)
mesg n / mesg y	Refuse or accept wall messages
/etc/issue / /etc/issue.net	Before a local / remote login
/etc/motd	After login (message of the day)
notify-send	Desktop pop-up for GUI users

Virtualization, containers, and cloud

• Type 1 hypervisors (KVM, Xen, Hyper-V, ESXi) run on the hardware; Type 2 (VirtualBox, VMware Workstation Player) run on a host OS.
• VMs can be created from scratch, as a clone, from a template, or imported as OVF/OVA.
• After cloning, regenerate unique identifiers such as the host name, MAC address, and /etc/machine-id.
• CPU flags vmx (Intel) or svm (AMD) show hardware virtualization support; the hypervisor flag means you’re already inside a VM.
• Containers share the host kernel; VMs run a full guest OS. Docker (application) and LXC (OS) are common container engines.
• IaaS, PaaS, SaaS go from most to least customer control. cloud-init configures cloud instances at first boot.

Lesson 5 terms to know

POST, BIOS, UEFI, MBR, ESP, /boot/efi, shim.efi, grubx64.efi, chainloading, dmesg, kernel ring buffer, journalctl -b, GRUB Legacy, menu.lst, rootnoverify, GRUB 2, grub.cfg, /etc/default/grub, grub-mkconfig -o, update-grub, Shift, ro, single, initrd, initramfs, systemd, unit, ExecStart, WantedBy, systemctl list-units, enable, daemon-reload, default.target, get-default, set-default, isolate, multi-user.target, graphical.target, rescue.target, SysVinit, /etc/inittab, /etc/init.d, telinit, runlevel, service, halt, shutdown -c, SIGTERM, SIGKILL, wall, mesg, /etc/motd, hypervisor, clone, template, OVF, machine ID, container, IaaS, and cloud-init.

Lesson 6 — Configuring the GUI, Localization, and Printing

What the GUI is for

• A desktop computer is used by one person or a small number of people for everyday productivity tasks, such as working with a spreadsheet or browsing the web.
• The GNU toolset and the X Window System are core Unix tools that the normal Linux system relies on. Startup scripts and Apple’s Aqua interface are not.
• Not every Linux program is graphical. Some web browsers are text-mode, such as Lynx, which runs in a terminal.
• You can launch a GUI program from the desktop’s menus or by typing its name in a terminal emulator.

The layers of a Linux GUI

• Display server carries requests between programs and the hardware (X.Org or Wayland).
• Compositor combines all windows into one screen image.
• Window manager places, sizes, and decorates windows (Mutter, KWin, Muffin, Marco, Xfwm).
• Desktop environment provides the overall look and feel (GNOME, KDE Plasma, Cinnamon, MATE, Xfce).

X11: XFree86 and X.Org

• Linux first used XFree86; most distributions moved to X.Org (X version 11) in 2004.
• Modern X.Org auto-detects hardware, so a configuration file often doesn’t exist.
• To create a configuration file you can edit, stop the GUI and run Xorg -configure as root (or XFree86 -configure with XFree86). The server queries the hardware and writes a new file.
• ~/.xsession lists commands for starting X clients. It replaces the system-wide default session or window manager, so it must launch the window manager or desktop you want.
• ~/.xsession-errors logs problems with the session.

xorg.conf section	Purpose
InputDevice	Keyboard and mouse
Monitor	Monitor settings
Device	Video card
Screen	Binds one video card to one monitor, with resolution and color depth
Module	Modules to load; Disable keeps a default module from loading, and Load for the same module takes precedence
Files	Paths to fonts, modules, and keyboard layouts
ServerLayout	Ties input and output devices into one session

Keyboard options in X:

Option	Sets
XkbModel	The keyboard model, such as pc105 for a 105-key keyboard
XkbLayout	The keyboard layout (language), such as us or gb

Wayland

• Wayland is the modern replacement for X. Its compositor is the display server, and it isolates programs from one another for better security.
• XWayland runs older X11 programs in a Wayland session.
• echo $XDG_SESSION_TYPE prints wayland or x11.

Desktop environments

	GNOME Shell	KDE Plasma	Cinnamon	MATE	Xfce
Display manager	GDM	SDDM	LightDM	LightDM	LightDM
File manager	Files (Nautilus)	Dolphin	Nemo	Caja	Thunar
Window manager	Mutter	KWin	Muffin	Marco	Xfwm

• KDE Plasma is built on the Qt widget set (GUI library). GNOME uses GTK.
• Linux file managers include Dolphin and Thunar. Thunderbird is an email client and Apache is a web server.
• A display manager provides the graphical login. XDMCP lets a display manager offer logins to remote X terminals, but it uses no compression and no encryption, so it’s normally disabled.

Accessibility

Need	Tool or setting
Screen reader	Orca (GNOME)
Magnifier	KMag (kmag) in KDE; Zoom in other desktops
High-contrast theme	GNOME control center Appearance section (or Universal Access)
Braille display	brltty; native kernel support requires kernel 2.6.26 or later
Hearing	Visual alerts flash the screen instead of beeping
Motor	Sticky keys, slow keys, bounce keys, mouse keys, on-screen keyboard

Remote GUI access

Method	Port	Encrypted?
X11 forwarding (ssh -X)	22	Yes, through SSH
Direct X11 (xhost + DISPLAY)	6000+	No
XDMCP	177	No (and no compression)
VNC	5900 + display	No by default; tunnel through SSH
Xrdp (RDP)	3389	Yes, TLS
NX / X2Go	22	Yes, SSH
SPICE	—	Optional TLS; built for VM consoles

Localization: character sets

Character set	Key facts
ASCII	7 bits, 128 characters, English only
ISO-8859	8-bit sets for groups of languages
Unicode	One code point for every character in every alphabet, stored with single-byte or multiple-byte encodings
UTF-8	Unicode in 1–4 bytes; the Linux standard
UTF-16	Unicode in 2 or 4 bytes

Locales

• A locale is written language_COUNTRY.charset, such as en_US.UTF-8.
• Precedence: LC_ALL overrides every LC_* variable, which override LANG.
• locale shows the current settings; locale -a lists installed locales.
• export LANG=en_GB.UTF-8 changes the locale for the current session only.
• ~/.bashrc runs with every interactive Bash shell, so put personal export lines there to make them permanent.
• localectl set-locale changes the system-wide locale on systemd systems.

Time zones and clocks

• tzselect walks through a menu to find a time zone and prints the zone name, which can be used in scripts to set the zone.
• /etc/localtime is a link to a binary file in /usr/share/zoneinfo/. /etc/timezone (Debian) is a text file with the zone name.
• TZ=Europe/London date shows the time in another zone for one command.
• hwclock --systohc / --hctosys copy the time between the system clock and the hardware clock.
• timedatectl set-timezone America/Chicago sets the zone on systemd systems.

Printing with CUPS

• CUPS queues and filters print jobs. Its web interface is http://localhost:631/.
• lpr submits a job, lpq shows a queue, lprm or cancel removes a job.
• cupsdisable stops printing but keeps queuing jobs; cupsreject stops the queue from accepting new jobs.

Lesson 6 terms to know

desktop, GNU toolset, X Window System, Lynx, display server, compositor, window manager, desktop environment, XFree86, X.Org, Xorg -configure, XFree86 -configure, xorg.conf, Screen, Module, Disable, XkbModel, XkbLayout, ~/.xsession, ~/.xsession-errors, Wayland, XWayland, GNOME, KDE Plasma, Qt, Dolphin, Thunar, display manager, XDMCP, Orca, kmag, Appearance, high contrast, brltty, 2.6.26, ssh -X, VNC, Xrdp, SPICE, ASCII, Unicode, UTF-8, locale, LANG, LC_ALL, ~/.bashrc, localectl, tzselect, /etc/localtime, hwclock, timedatectl, CUPS, lpr, lpq, lprm, cupsdisable, and cupsreject.