GNU/Linux

• When autogrouping is enabled, the members of an autogroup are placed within the same task group.

• They are installed in /etc/systemd/system/

• It activates all dependencies of default.target. This is done in parallel.

  • default.target is a symlink to graphical.target or

  multi-user.target.

  • display-manager.service is also a symlink for the desktop

  managers

  • e.g. gdm.service, sddm.service.

                             cryptsetup-pre.target veritysetup-pre.target
                                                  |
(various low-level                                v
 API VFS mounts:             (various cryptsetup/veritysetup devices...)
 mqueue, configfs,                                |    |
 debugfs, ...)                                    v    |
 |                                  cryptsetup.target  |
 |  (various swap                                 |    |    remote-fs-pre.target
 |   devices...)                                  |    |     |        |
 |    |                                           |    |     |        v
 |    v                       local-fs-pre.target |    |     |  (network file systems)
 |  swap.target                       |           |    v     v                 |
 |    |                               v           |  remote-cryptsetup.target  |
 |    |  (various low-level  (various mounts and  |  remote-veritysetup.target |
 |    |   services: udevd,    fsck services...)   |             |              |
 |    |   tmpfiles, random            |           |             |    remote-fs.target
 |    |   seed, sysctl, ...)          v           |             |              |
 |    |      |                 local-fs.target    |             | _____________/
 |    |      |                        |           |             |/
 \____|______|_______________   ______|___________/             |
                             \ /                                |
                              v                                 |
                       sysinit.target                           |
                              |                                 |
       ______________________/|\_____________________           |
      /              |        |      |               \          |
      |              |        |      |               |          |
      v              v        |      v               |          |
 (various       (various      |  (various            |          |
  timers...)      paths...)   |   sockets...)        |          |
      |              |        |      |               |          |
      v              v        |      v               |          |
timers.target  paths.target   |  sockets.target      |          |
      |              |        |      |               v          |
      v              \_______ | _____/         rescue.service   |
                             \|/                     |          |
                              v                      v          |
                          basic.target         *rescue.target*  |
                              |                                 |
                      ________v____________________             |
                     /              |              \            |
                     |              |              |            |
                     v              v              v            |
                 display-    (various system   (various system  |
             manager.service     services        services)      |
                     |         required for        |            |
                     |        graphical UIs)       v            v
                     |              |            *multi-user.target*
emergency.service    |              |              |
        |            \_____________ | _____________/
        v                          \|/
*emergency.target*                  v
                              *graphical.target*

• The services are mainly stored in /etc/systemd/user/ and ~/.config/systemd/user/
• They are installed in
• Starts the unprivileged user@uid.service units. The user@.service file does not contain uid itself.
• It also activates units that default.target is depending on. The login manager will start the graphical-session.target when the user logs into a graphical session.

   (various           (various         (various
    timers...)         paths...)        sockets...)    (sound devices)
        |                  |                 |               |
        v                  v                 v               v
  timers.target      paths.target     sockets.target    sound.target
        |                  |                 |
        \______________   _|_________________/         (bluetooth devices)
                       \ /                                   |
                        V                                    v
                  basic.target                          bluetooth.target
                        |
             __________/ \_______                      (smartcard devices)
            /                    \                           |
            |                    |                           v
            |                    v                      smartcard.target
            v            graphical-session-pre.target
(various user services)          |                       (printers)
            |                    v                           |
            |       (services for the graphical session)     v
            |                    |                       printer.target
            v                    v
     *default.target*      graphical-session.target

• A service unit looks as follows:

  [Unit]
  Description=<description>
  
  [Service]
  Type=oneshot
  ExecStart=<shell command> # executed when the service starts
  ExecStartPre=<shell command> # it might be `sleep`
  

• Showing every timers. $ systemctl list-timers

• A timer unit looks as follows:

  [Unit]
  Description=<description>
  
  [Timer]
  OnActiveSec=10sec # 10 second after the timer activates
  OnBootSec=10m # 10 min after boot
  OnStartupSec=10m # 10 min after the systemd starts
  OnUnitActiveSec=300s # 300 sec from last activation of the unit the timer unit is activating
  OnUnitInactiveSec=1w # 1 week after the last deactivation of the unit the timer unit is activating
  RandomizeDelaySec=300s # add up to 300 sec to OnUnitActiveSec.
  
  [Install]
  WantedBy=timers.target # specify to create symlink under either
                         # /etc/systemd/system/timers.target.wants/
                         # ~/.config/systemd/user/timers.target.wants/
                         # when enabled.
  

• A timer unit runs a service unit with the same name. This service doesn't need Install section, as it is ran directly from timer.

• System Units

• User Units

• Reload the newly created daemons.
  • sh systemctl daemon-reload
• Create and edit a unit.
  • sh systemctl edit --force --full <name.unitname>
  • It's created under /etc/systemd/system/.
• Enable and start a daemon.
  • sh systemctl enable --now <daemon>
• Enable userspace daemon.
  • sh systemctl enable --user <daemon> # it enables <daemon>@<uid>
  • Creates symlink under ~/.config/systemd/user/*.target.wants/
    • default.target.wants
    • socket.target.wants
    • timer.target.wants

• The rules are based on the information of the device such as vender ID, product ID, and such.

• It is discouraged to mount a filesystem using udev rule directly. Use udisks as proxy, or systemd-mount.

  • The rules (with .rules extension) are in:
    • /usr/lib/udev/rules.d : system rules directory. It's mostly

  here.

  • /usr/local/lib/udev/rules.d : system rules directory
  • /run/udev/rules.d : volatile runtime directory
  • /etc/udev/rules.d : local administration directory

• CLI tool for udev

• hwdb(7)
• Hardware database for key-value pairs of modalias-like keys and udev-property-like values.
• Stored under /etc/udev/hwdb.d/, and compiled into /etc/udev/hwdb.bin by the systemd-hwdb

• The loader configuration is at esp/loader/loader.conf. esp is

for EFI system parition, either /boot or /efi.

• timeout=0 starts the default entry immediately.

• esp/loader/entries/*.conf are each a loader file, in which EFI programs can start.
• linux <path> is an alias for efi <path>, and initrd <path> is an alias for options initrd=<path>.
• options root must be a persistent block device naming, when multiple devices are present.
  • Label, UUID, disk id, GPT partition UUID, GPT partition lable

• bootctl install to install the EFI file.
• bootctl reboot-to-firmware [BOOL] query if supported, or set the reboot-to-firmware EFI flag to BOOL

• bundled with base.
• networkctl
  • list
  • status
  • up DEVICE down DEVICE

• Insert rules under /etc/systemd/netword/. The file must end in *.network.

• Match a device and configure it

  # /etc/systemd/network/20-wired.network
  
  [Match]
  Name=enp1s0
  
  [Network]
  DHCP=yes
  

• is used alongside.
• resolvectl
  • status
  • query HOSTNAME|ADDRESS
  • show-cache
  • show-server-state

• It controls both systemd-timedated and systemd-timesyncd
• show show systemd-timedated properties
• list-timezones
• set-time set-timezone
• set-local-rtc BOOL 0 or 1. default to 0
• set-ntp BOOL default to 0.
• timesync-status
• show-timesync

• manage system time and hardware time.
• The hardware or the real time clock(RTC) can be set to the localtime or the UTC. The system reads the hardware time into system time on bootup, synchronizes the system time during runtime, and writes the system time into hardware time on shutdown.
• The standard for RTC can be set by the operating system, to be either localtime or UTC. Windows uses localtime, and most UNIX-like system uses UTC.

• Network Time Protocol(NTP) client daemon, with Simple Network Time Protocol (SNTP)
• Configured in /etc/systemd/timesyncd.conf and /etc/systemd/timesyncd.conf.d/*.conf
• ntp and sntp is available as alternatives, which runs ntpd.
• faketime(1) from libfaketime can be used to set per-user or per-session datetime.
• The environment variable TZ can be used to change the timezone for a user or session.

• localectl

  • CLI tool to modify locale files such as /etc/locale.conf,

  /etc/vconsole.conf

  • status
  • set-locale LOCALE
  • list-locales
  • set-keymap MAP

• It manages files and directories that has systematic lifecyles.
• The config files for the system tmpfiles are in /usr/lib/tmpfiles.d/
• Make a config file that specifies the lifecycle of files, and use systemd-tmpfiles utility or the systemd timer.
  • The config file format is found int tmpfiles.d(5)

• It manages portable user accounts, and mounting of the home directory of the users.
• homectl
  • create

• Internally calls loadkeys and setfont

• journalctl
  • --system
  • --user
  • --boot -b
  • --dmesg -k

• userdbctl
• Manage user and group data
• Related to /etc/userdb, /run/userdb, /run/host/userdb, /usr/lib/userdb/*.user

• Matt Ahrens - Lecture on OpenZFS read and write code paths - YouTube

• Storage Pool Allocator
• Write to the physical disks and return block pointer to the DMU

Data Management Unit

• Translate into actual file location

ZFS POSIX Layer

• Handles the file interface from VFS.

ZFS Volume

Expose a plain block of storage.

• tar

• cpio
• It stores the file table in ASCII format, (in the new format)

• Nouveau: Kernel Driver and Userspace Driver
• NVK: Part of Mesa. Use Nouveau kernel driver.
• NVIDIA Linux Open GPU Kernel Module: Open Driver by NVIDIA
• NVIDIA Driver: Proprietary One.

• AMDGPU: Kernel Driver by AMD.
• RADEON SI, RADV: Part of Mesa. OpenGL and Vulkan repectively.
• AMDVLK: Copy of the proprietary driver.

• i915: Kernel Driver
• i965, ANV: Part of Mesa. Userspace Driver for OpenGL and Vulkan respectively.
• Xe: New Kernel Driver

• Stylized as Παν語. It is a text layout engine that includes HarfBuzz, the text shaping engine.

• Wayland compositor is a combination of Xorg window manager and compositor.
• Most of wayland compositors requires KMS enabled.
• It takes inputs from evdev via libinput, and deals with wayland clients, and displays the graphics using KMS or Mesa.

• The History of X11 - YouTube

• The config file is /etc/X11/xorg.conf or /etc/X11/xorg.conf.d/ or /etc/xorg.conf.
  • The config file can be generated automatically via # Xorg :0 -configure, and the new config file will be stored in /root/xorg.conf.new.
• The X sessions are stored in /usr/share/xsessions/

• https://wiki.archlinux.org/title/Xorg/Keyboard_configuration
• Xorg server uses X Keyboard Extension(XKB) to define keyboard layouts. ((669f0999-e2de-48bd-b186-c4ffddd75df8)) also defaults to use this.
• xmodmap directly access the internal keymap table.
• localectl can be used to define to the keyboard layout for both the Xorg server and the virtual console.

https://man.archlinux.org/man/Compose.5

• *a aa å, ss ß, ae æ, oe œ, o/ ø, !a ạ, ?a ả, c, ç
• -> →, ?? ¿, — —, oo °, .. …, fi fi, 0~ ⍬, <> ⋄, |- † (dagger), |= ‡(double dagger), PP ¶, So §, [] ⌷, :) ☺, ? ☭, .= •, .^ .- ·, !^ ¦, <” ", <' ', ," „, <3 ♥,
• ,
• ## ♯, #b ♭, #f ♮, #q ♩, #e ♪, #E ♫, #S ♬
• or ®, oc ©, L- £, S| $, W= ₩, Y= ¥, P= ₽, %o ‰, 12 ½, tm ™, sm ℠
• xx ×,:- ÷, {} ∅, +- ±, = ≠, =_ ≡, ~~ ≈, 88 ∞, RR ℝ, .: ∵, :. ∴, -, ¬, v √
• ^1 ¹, /1 ₁, ^( ⁽, /( ₍, ^_n ⁿ, ^_a ª, ^_o º, ^_i ⁱ, (1) ①, mu µ(micro sign)
• The full compose options are specified in /usr/share/X11/locale/en_US.UTF-8/Compose.

WM

• Window manager is a special X client that manages other clients' graphics.

• Files with .desktop extension name. It is a TOML file with specification.
• To be displayed in application menu, It needs to be in
  • /usr/share/applications/ or /usr/local/share/applications/ (system-wide)
  • ~/.local/share/applications/ (user-wide)
  • Entries in user-wide folder will override the system-wide ones.
  • The icons need to be either PNG or SVG format and stored in $XDG_DATA_DIRS/icons or /usr/share/pixmaps.

• Autostart desktop entries in
  • ~/.config/autostart/ (user-wide)
  • /etc/xdg/autostart (system-wide)
• The automatic placement is the job of the specific window manager.

XDG Desktop Portal - ArchWiki

• It is the standard for the D-Bus interface for various functionalities.
• It was designed to, but not limited to, support the sandbox environment of Flatpak.
• There is different implementation for the backend, such as xdg-desktop-portal-gtk, xdg-desktop-portal-kde, xdg-desktop-portal-wlr.
• The backends are chosen based on the XDG_CURRENT_DESKTOP environment variable, which is provided by the desktop environments themselves.
  • /usr/share/xdg-desktop-portal/$XDG_CURRENT_DESKTOP-portals.conf is used by xdg-desktop-portal daemon (systemd user service) on requests.
  • ~/.config/xdg-desktop-portal/portals.conf for any desktop environment, and ~/.config/xdg-desktop-portal/$DE-portals.conf for a specific desktop environment.
• On Wayland org.freedesktop.portal.GlobalShortcuts and org.freedesktop.portal.ScreenCast are used to compensate for the gap from the X11.

These variables are set between the login? and the profile.sh script.

• $XDG_DATA_DIRS
  • /usr/share/, /usr/local/share/ by default
  • I have added $HOME/.local/share as well.

• The user settings are stored in ~/.config/dconf/user which is a dconf database.
• dconf database is focused on fast reads which is good for settings database that is read frequently but not written as much.
• The settings are accessed with gsettings in command line and dconf-editor in GUI.

• GNOME Display Manager.
• It bootstraps GUI environment.

• GNOME developed its own UI library libadvaita. Advaita means nonduality in Sanskrit.

• Alt+F2=(run modal) =r to reload gnome-shell on Xorg.
• Super+LMB moves the window and Super+MMB resizes the window, and Super+RMB opens the window menu.

• <C-S-u>: Insert unicode by codepoint
• <C-A-S-u>: Insert unicode by name

• KDE expects the template files in ~/.local/share/templates/ as a .desktop files.

• nerd-fonts https://www.nerdfonts.com
  • Nerd Fonts - Icon Cheat Sheet
• Ligature Support
  • FiraCode Nerd Font firacode-nerd
  • Hasklug Nerd Font hasklig-nerd
  • Cascadia Mono Nerd Font cascadia-mono-nerd
  • Iosevka Nerd Font, IosevkaTerm Nerd Font, IosevkaTermSlab Nerd Font iosevka-nerd, iosevkaterm-nerd, iosevkatermslab-nerd
  • JetBrainsMono Nerd Font, JetBrainsMonoNL Nerd Font jetbrains-mono-nerd
  • Monoid Nerd Font monoid-nerd

• Japanese
  • adobe-source-han
• Indic fonts
  • ttf-indic-otf
• Arabic fonts
  • ttf-arabeyes-fonts

• STIX (installed)
• XITS: a fork of STIX

• Nerd Fonts (Group)
• noto-fonts-cjk
• Korean fonts: ttf-kopub (AUR), ttf-nanum (AUR)

• Fonts are saved under
  • /usr/local/share/fonts/ (system-wide)
  • /usr/share/fonts/ (package manager)
  • ~/.local/share/fonts/ (user)
  • ~/.fonts/ (deprecated).
• Run fc-cache after putting a font in one of the directories, to make it usable.

• fc: The infrastructure that manages fonts
• $ fc-list to view all installed fonts.
• Config files are in
  • /etc/fonts (System)
  • /usr/share/fontconfig (List)
  • ~/.config/fontconfig (User)
  • /etc/fonts/conf.d contains the list of paths to each fonts, which is managed by the fc.
    • It is globally accessible
  • /usr/share/fontconfig/conf.avail
    • The directory for the list of all fonts available.
    • TeX Live expose every fonts it contains here.
    • make a link to a file in this directory, in the /etc/fonts/conf.d makes it globally accessible.
      • # ln -s /usr/share/fontconfig/conf.avail/09-texlive-fonts.conf /etc/fonts/conf.d/09-texlive-fonts.conf
• Pango uses fontconfig library to access fonts, with the config files in /etc/fonts/ and ~/.config/fontconfig
• Older applications does not support fontconfig, so the index needs to be created
  • Such as, GTK 1.x and xfontsel
  • Use mkfontscale, mkfontdir

• pipewire session manager.
• It replaces and improve upon the default pipewire-session-manager.
• wpctl
  • status

• Qt based pipewire GUI patchbay.

• GTK based pipewire GUI patchbay.

• music_directory
• playlist_directory set to ~/.config/mpd/playlists/
• db_file set to ~/.config/mpd/database
• audio_input
• audio_output
  • type: pulse, pipewire, …
  • name: the alias for the output. It can be anything.

See Music Player Daemon — Music Player Daemon documentation, Music Player Daemon - ArchWiki

• Kernel Interface
• evdev is the generic input event interface. It passes the events generated in the kernel straight to the program, with timestamps. The event codes are the same on all architectures and are hardware independent.
• It handles the events generated by the various device drivers, and pass them via various interfaces, keystrokes to the kernel, mouse movements to the display server, etc.

• In-Kernel Input Handler
• Part of VT code, and it handles keyboard keystrokes and user inputs for VT consoles.

• Wrapper library for creating uinput devices and sending events.

• Input Method Engines: fcitx5-hangul, fcitx5-anthy, fcitx5-mozc, …
• Addons: clipboard, quickphrase, …
  • fcitx5-configtool includes the kcm-fcitx5 module for the Plasma settings, and fcitx5-config-qt is an independent GUI tool.

• Bash does not have types.
• Quotes are used to delimit the value

• # is for comment

• source .
  • Execute a script file
• exec fork a child process and execute, the script terminates right away after this.
• eval evaluate the string as a command.

• <C-c> sends the SIGTERM? to the process
• <C-d> inputs EOT(End of Transmission) 0x04
  • EOF(End of File)
    • It is implementation dependent and must be negative.
    • -1 is commonly used, such as in glibc.
  • cat closes the file when encountered EOT twice in a row.
• <C-s> suspend, <C-q> continue
  • It was used to control the terminal itself.

• shopt
  • Set shell options.
  • OPTIONS
    • globstar
      • If set, ** matches the entire filepath recursively.
      • It doesn't work when used with other patterns.

• The bash built-in commands are shown by help
• Advanced Bash-Scripting Guide
• Top Bash Reference Manual
  • Programmable Completion Bash Reference Manual

• useradd, usermod, userdel to manage users.
• The shell for a user is set by the -s flag.
• The user information is stored in /etc/passwd.

passwd <user> sets password for the specified user.

• Passwords are stored in /etc/shadow, encrypted.

groupadd, groupmod, groupdel to manage groups.

• The group information is stored in /etc/group.

• It modifies /etc/sudoers, the sudo configuration file, with syntax checking.
• It uses the editor specified by EDITOR shell variable. To use vim, export EDITOR=vim

• Edit a file with privilege.

• Register user sessions in the systemd login manager

• The default configuration file is /etc/security/pam_env.conf
• Source the /etc/environment
  • And then read the file specified by the user_envfile option, which is, by default, $HOME/.pam_environment

• AUR
• It must be in the beginning of the auth section.
• /etc/security/autologin.conf must be manually created, for it to store the password.
• Once the password is stored, the login happens automatically.

• Thin layer of software between hardware and operating systems
• Require kernel support on top
• Built into Arch Linux kernel.
• Provided by xen package

• Kernel-based Virtual Machine
• Kernel modules kvm and either kvm_intel or kvm_amd
• Provided by the Arch Linux kernel

• qemu-system-<arch> to run the virtual machine of a given architectrue
• -cdrom <file> to insert a cd. It could be the /dev/... or an ISO file.
• -m <size> to specify the RAM size. Default to few megabytes.
• -boot d to specify how to boot.
• -drive <file> to attach a virtual or physical drive.

• virt-manager
• Require dnsmasq

• Arch Linux.
• -S install
  • yu to update the core and upgrade all.
  • s search in the remote repository
• -R remove
  • d skip dependency check, dd skip all checks
  • n purge the config data (User configs are not purged)
• -Q query among what's installed
  • i show the detailed information, including dependencies.
  • m list foreign packages, which is not in the sync db
  • l files that are installed by the package
• -F find in which package a file or a binary is.

• The vendor specification for hooks are in /usr/share/libalpm/hooks/,
• and user specification can be stored in /etc/pacman.d/hooks/.

• Arch packages are generated by makepkg, run it in the directory.
• -i install on build completion
• -c clean build files
• Install via pacman -U <package>.pkg.tar.zst, or use -i option
  • It is installed as foreign package.

• It provides aur command that manages a separate repo for pacman.
• The repository is registered to pacman, in the /etc/pacman.conf. aur automatically use the first added repository.
• The repository directory is created with install and the database file with repo-add command.
  • A repository is a directory with REPO.db.tar.gz file.
  • See aur(1)
• aur
  • sync fetch and build
  • search

• Uplift your Linux systems programming skills with systemd and D-Bus Practical…
• libdbus is the base implementation used by dbus-daemon.
• systemd uses its own implementation sd-bus
• Processes communicates through a unix domain socket.

• The arch ISO includes archinstall - ArchWiki script.
• Jokingly prints Arch is the best - ArchWiki