GNU/Linux

Data Structure for describing hardware.

Used by OpenFirmware, OpenPOWER Abstraction Layer (OPAL), Power Architecture Platform Requirements (PAPR) and in the standalone Flattened Device Tree (FDT) form

Unified Extensible Firmware Interface.

A firmware architecture spec. Contains ACPI. Originally developed by Intel under the name of EFI.

UEFI or BIOS can be entered by tapping F1,2,7,9,10,12 or DEL on bootup.

See .

Generate the ramdisk as specified in *.preset---linux.preset for linux package.

cpio is used

• -P generate all presets
• -p <preset> generate the preset specified in <preset>.preset

• ELF Header
  • Architecture
  • Endianness
  • Offset and number of Program Headers and Section Headers
• Program Headers: How to load an executable or shared library into a memory?
  • The memory layout
  • Thread Local Storage(TLS)
  • Dynamic: Dynamic Linking(Load)
• Data
• Section Headers
  • Init Array
  • Symbol Table
  • Dynamic Symbol Table
  • Relocation
  • The request for the external parts to fill in the addresses for certain functions.

• sched(7)
  • The scheduling is done by the CFS(completely fair scheduler), since Linux 2.6.23.
• The interrupt to the kernel is called after a set amount of period set by the kernel, preventing a process to loop forever.

• 4..1..+

cgroup

• Hierarchical group structure that can control a group of processes.
  • Features
  • Resource Limiting
  • Prioritization
  • Accounting
  • Crontrol
  • /sys/fs/cgroup/ contains the cgroup informations.

• ptrace(2) allows to look into other processes. The default policy only allows the root to access it.
  • In Python, use ctypes.CDLL for including external C libraries or, just use the python_ptrace package.
• ftrace

ip(8) is used to interact with the kernel network stack.

• $ ip
  • link active connection between two physical-layer interfaces.
    • add dev <device> type <type>
    • delete dev <device>
  • interface physical or virtual interface can be used to connect. IP addresses are assigned to interfaces.
  • device physical, such as hosts and routers, or virtual devices can have multiple interfaces.
  • address manage IP addresses of interfaces.
    • add <CIDR> dev <device>
    • delete <CIDR> dev <device>
    • show [dev <device>] [scope {host|link|global}]
  • tc traffic control
    • It configures the Linux kernel packet scheduler.
  • route, rule See routing.

Lower-layer devices like switches and repeaters don't use IP addresses for their basic function.

The simplified packet flow diagram:

                               XXXXXXXXXXXXXXXXXX
                             XXX     Network    XXX
                               XXXXXXXXXXXXXXXXXX
                                       +
                                       |
                                       v
 +-------------+              +------------------+
 |table: filter| <---+        | table: nat       |
 |chain: INPUT |     |        | chain: PREROUTING|
 +-----+-------+     |        +--------+---------+
       |             |                 |
       v             |                 v
 [local process]     |           ****************          +--------------+
       |             +---------+ Routing decision +------> |table: filter |
       v                         ****************          |chain: FORWARD|
\****************                                           +------+-------+
Routing decision                                                  |
\****************                                                  |
       |                                                          |
       v                        ****************                  |
+-------------+       +------>  Routing decision  <---------------+
|table: nat   |       |         ****************
|chain: OUTPUT|       |               +
+-----+-------+       |               |
      |               |               v
      v               |      +-------------------+
+--------------+      |      | table: nat        |
|table: filter | +----+      | chain: POSTROUTING|
|chain: OUTPUT |             +--------+----------+
+--------------+                      |
                                      v
                               XXXXXXXXXXXXXXXXXX
                             XXX    Network     XXX
                               XXXXXXXXXXXXXXXXXX

Userspace utility to interact with the kernel-level firewall.

• $ iptables
  • -t [filter|nat|mangle|raw] table. filter by default
  • --list, -L [<chain>] list
    • -vL verbose: include packet and data count and interface names.
    • -nL numeric: print IP addresses and port numbers in numeric format.
  • --append, -A <chain> <rule spec> append at the end
  • --insert, -I <chain> <rule spec> insert at the start
  • --replace, -R <chain> N <rule spec> replace
  • --delete, -D <chain> N

<rule spec> include any of the following matches:

• -p <proto> protocol
• -s <source CIDR> source IP
• --dport <port num> destination port
• --sport <port num>
• -i, -o in or out interface
• -m <match> use extension modules
  • tcp
  • state

and the target:

• -j <action>.

The successor of the iptables.

part of Netfilter project, that combines {ip,ip6,arp,eb}tables.

Routing is performed in the kernel.

• $ ip route
  • add <destination CIDR> [via <gateway ip>] dev <device> [table <table id>]
    • <table id> is main by default. Different tables are looked up depending on the context.
  • del ...

$ routel or $ ip route list [table main|local|global|all] lists the routing table that looks like <dest> via <nexthop ip> dev <device> proto <proto> scope <scope> src <source ip> metric <num>, each meaning the following:

• protocol specifies the protocol that installed this route.
  • boot during bootup
  • kernel during kernel autoconfiguration
  • static by the administrator
  • dhcp by DHCP
  • redirect due to ICMP redirect
  • ra by Router Discovery protocol
• scope host for local routing within the system, and link for direct unicast and broadcast.

• $ ip rule set the rules for which table to look up.
  • add <selector> <action> action can include looking up specific table, doing NAT.
    • Selector can be fwmark (firewall mark) of the packet, which allows different routing rule for a specific process.
  • del <selector <action>
  • show [<selector>]

Modalias - ArchWiki

Module Alias

• A device provides hardware information and it is exposed in /sys/devices/.../modalias
  • pci:v00008086d000024DBsv0000103Csd0000006Abc01sc01i8A
  • v: Vender ID, d: Device ID
    • Specify the device. Rendered as xxxx:xxxx - sv: Subsystem Vender ID, sd: Subsystem Device ID
  • bc: Base Calss, sc: Subclass
    • Specify the functionality of the device. Rendered as xxxx
  • i: Programming Interface
• depmod collects the modalias from each device drivers and make a list in /lib/modules/$(uname -r)/, in

particular modules.alias, so that depmod can matched the modalias of the hardware to when modprobe is executed.

A driver kernel module exposes implementations of systemcalls for specific devices to the kernel.

• How Do Linux Kernel Drivers Work? - Learning Resource - YouTube
• Linux Device Drivers, Third Edition {LWN.net}

Dynamic Kernel Module Support.

• Framwork for incorporating modules outside of kernel. Automatically

recompiles DKMs when kernel recomplies, to keep things working.

• linux-headers is required to build the module against.
• dkms
• status
• install
• remove

• systemd-analyze command can be used to track the bootup time.

• .service, .socket, .device, .mount, .automount, .swap, .target, .path, .timer, .slice, .scope.
• Special units are provided by the systemd. Many of them cannot be renamed.

• systemd-logind

Manages sessions that are registered by getty, display manager, and terminal emulator.

• Responsibility
  • Keeping track of users and sessions, their processes and their idle state. This is implemented by allocating a systemd slice unit for each user below user.slice, and a scope unit below it for each concurrent session of a user. Also, a per-user service manager is started as system service instance of user@.service for each logged in user.
  • Generating and managing session IDs. If auditing is available and an audit session ID is already set for a session, then this ID is reused as the session ID. Otherwise, an independent session counter is used.
  • Providing polkit[1]-based access for users for operations such as system shutdown or sleep
  • Implementing a shutdown/sleep inhibition logic for applications
  • Handling of power/sleep hardware keys
  • Multi-seat management
  • Session switch management
  • Device access management for users
  • Automatic spawning of text logins (gettys) on virtual console activation and user runtime directory management
  • Scheduled shutdown
  • Sending "wall" messages
• It uses pam_systemd.so
• /etc/systemd/logind.conf customize the session behavior
  • IdleAction=ignore|suspend|hibernate|poweroff|...
  • IdleActionSec=30min|30s|...

• lvm Logical Volume Manager
  • It creates volume groups in which a logical volume can be allocated.
    • The root file system can be on logical volume in Linux.

• g GPT partition table
• n new partition
• t set type for the partition

• It uses extents in order to allocate pages to a file. ext3 allocated each pages

individually, making the file tree larger.

• Copy on Write (COW) (Implicit Sharing, Shadowing). A change of a file is saved in a separate

location on write, with journal tracking the history.

• It may add an extra load, but it keeps the integrity of the system.

CLI tool

• gzip compress the file in place .gz
  • -k keep the original file
  • -c --stdout
  • -d decompress
  • -r recursively compresses the files in the directory
  • -f force
• xz
  • .xz
• bzip2
  • .bzip
• ziptool zipcmp
  • .zip

• The GUI puts them in ~/.local/share/Trash

Dot files came around unintentionally. When the Unix file system were being developed, either Ken or Dennis decided to check for the dot in the beginning of the filename in order to hide the . and .. file. This functionality then became adopted to create hidden files.

Linux Dot Files Were Never Meant To Exist - YouTube

• See systemd

A monolith network controller. It takes care of DNS, DHCP, Wi-Fi.

• $ nmcli
  • connection
    • show
    • modify CONNECTION_NAME (OPTION_NAME OPTION_VALUE)...
      • The CNNECTION_NAME can be found in the first column of nmcli connection show.
      • ipv4.method (manual), ipv4.address, ipv4.gateway, ipv4.dns
• nmtui

• It modifies the filter iptable in the kernel firewall, so that it can

directly manages the packets?

• allow <start_port>:<end_port>/<protocol>
• reload

Examples

• KDEConnect - KDE UserBase Wiki

sudo ufw allow 1714:1764/udp
sudo ufw allow 1714:1764/tcp
sudo ufw reload

KDE setting can configure it.

Authentication is done via password by default. Set

PasswordAuthentication no # modify, not add

The public key authentication can be set up with following tools.

• ssh-keygen generate key pair. They are stored in ~/.ssh/ by default.
• ssh-copy-id copy the public key to the ssh server to register the client.
• ssh-agent the authentication agent
  • ssh-add add private key identites to the agent. Run eval $(ssh-agent -s) beforehand.

When connecting to a server, ssh tests the following identities by default:

• ~/.ssh/id_rsa
• ~/.ssh/id_dsa
• ~/.ssh/id_ecdsa
• ~/.ssh/id_ed25519

Additionally, ssh -i <key file> <user>@<host> to use external identity file not registered to the agent. All registered identity files are tested, unless -o IdentitiesOnly=yes option is given.

Stored in ~/.ssh/config

The format looks like:

<Var> <val> # global option

Host <alias>
    Hostname <domain/ip address>
    User <username>
    IdentityFile <path to private key>
    IdentitiesOnly yes  # test only the files given in IdentityFile

A driver is a set of software that "drives", in other words "makes it work", specific hardwares. It can be firmware, kernel BLOB, Mesa-like translation layer, and everything in between.

• Linux Graphics Drivers explained: AMD, NVIDIA, INTEL, Open Source and Proprie…

• Accelerated video encoding/decoding.
• Provided by the libva and used alongside with a driver.
• AMD: libva-mesa-driver (VA-API), mesa-vdpau (VDPAU).
  • libva-utils provides vainfo that inspects the current setup.

• Cross-platform GUI widget toolkit.

SDDM, Simple Desktop Display Manager

• The configuration file is in /etc/sddm.conf and /etc/sddm.conf.d/, and the default setting is in /usr/lib/sddm/sddm.conf.d/default.conf
• The scripts and the themes are stored under /usr/share/sddm/.

• It is a successor of Xorg for modern systems. It is minimal by design, delegating much of the desktop specific stuff to compositors. wlroots library is developed to prevent fragmentation of effort.
• The entries for sessions are stored in /usr/share/wayland-sessions/ as .desktop entries.

• First released in 2004.
• Implementation of the X11 protocol, the 11th version of X.

X Desktop Group, Freedesktop

• XDG is a standard for desktop environments.

• GNU Network Object Model Environment.
• Based on GTK and XDG-compatible.

• A Wayland desktop environment developed independent of GNOME or KDE by Pop!_OS team.

• .ttf
• .otf
• .woff2
• metafont

• Advanced Linux Sound Architecture
• It is a kernel components that provides sound devices in a form of sound cards, both physical and virtual, to the usermode programs.
• Although it is possible for a program to connect directly to a sound card via ALSA, It only connect them one to one. If multiplexing is needed, a sound server is required.

• It takes multiple sources and multiplexes it, but no more than that.

• Packages: pulseaudio, pulseaudio-alsa, pulseaudio-bluetooth
• pactl
  • stat
  • info
  • list
  • {upload|play|remove}-sample
  • {load|unload}-module
  • {set|get}-{sink|source}-{port|volume|mute}
  • @DEFAULT_SINK@ @DEFAULT_SOURCE@ @DEFAULT_MONITOR@ can be used.

• It manages each source and sink in separate sessions, which gets rid of complicated workarounds. It is fully compatible with pulseaudio, and provides flexibility of jack.
• It also multiplexes a stream of video.
• Packages
  • pipewire It provides the user service pipewire. It is often triggered by socket.
  • pipewire-pulse Drop-in replacement for pulseaudio
  • pipewire-jack
  • pipewire-alsa
  • pipewire-roc ROC is the framework for remote audio streaming. This provides the ROC sink and source.

Music Player Daemon

• The user daemon is provided.
• It communicates through the port 6600 by default.
• Database -> Playlists -> Current Playlist (Queue) -> Play
• The frontend includes mpc (CLI), ario (GTK)

mdpris2 or mpd-mpris provides MPRIS support.

Utility to send commands to MPRIS clients.

• Kernel modules that generate events.
• usbkbd, usbmouse, atkbd=(PS/2), =psmouse

7. uinput module — The Linux Kernel documentation

• Kernel module for emulating evdev devices exposed through /dev/input/uinput in the userspace.

• GNOME focused
• ibus

GTK_IM_MODULE=ibus
QT_IM_MODULE=ibus
XMODIFIERS=@im=ibus

• ibus-hangul

• KDE focused
• fcitx5-im group includes fcitx5, fcitx5-gtk, fcitx5-qt, fcitx5-configtool
  • fcitx is no more being on development.
• fcitx5 provide the basic framework only giving English support. It is an interface in the place of the keyboard, for the input method engine (IME) to input texts.
• Input method modules support is provided by fcitx5-qt, fcitx5-gtk.
  • It is not necessary for Wayland native protocol.
  • KWin handle input methods with a "virtual keyboard", and it requires both. fcitx5-gtk is needed for chromium and vscode.
• C-M-h toggles the completion
  • It clashes with Evil mode

• keyd
• interception-tools
  • It grabs from an input stream and release it after modifying it.

• OpenRazer
  • The openrazer-daemon provides the dymanic kernel module and the daemon that can interact with it.
  • User must be added to the plugdev group via gpasswd -a $USER plugdev.
  • The daemon can be accessed through D-Bus.
  • RazerGenie is the Qt frontend that is available on FlatHub (limitedly, on AUR).

• VIA is available on the web. It interacts with the hardware directly through /dev/hidrawX.
• The file permission to read and write needs to be granted.

• VT
• /dev/ttyN

Console managed by the kernel.

Switch between them by C-M-<fN>.

• PTY
• /dev/pts/ptmx, /dev/pts/N

/dev/pts/ptmx is the multiplexer (manager) for pseudo-terminals. A pseudo-terminal consists of master and slave, where the master is accessed via file descriptor returned from /dev/ptmx and the slave is accessed via /dev/pts/N.

GUI program that creates a new pseudo-terminal, displays it, and interact with it

It register keystrokes and send conrol characters to the master.

getty (get tty) program initialize virtual terminal and prompt the user for login. After that, login is invoked to set up the environment.

login uses PAM to authenticate as well.

• CTTY

The kernel receives the key stroke, and send to terminal emulator or virtual terminal.

If the terminal emulator receives a key combination (e.g. C-c), it sends the control characters (e.g. ^C) to the pseudo terminal

The pseudo-terminal (or virtual terminal) then send signals (e.g. SIGINT) to the processes in the foreground process group it controls.

• C-c (SIGINT) terminate the process by default
• 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.

• The default shell from base.

See Bash.

Zsh - ArchWiki

Z Shell

• Configured in ~/.zshrc
• It does not use ((66bc32ae-22af-4fdf-9ad4-5a2eb51532c6)), it uses Zsh Line Editor (ZLE) instead.
• It is configured with bindkey builtin command.
• -v vi, -e Emacs
• Completion
  • zsh-completions provides extra completion functions.
  • autoload -Uz compinit; compinit;
• Plugins
  • Stored in /usr/share/zsh/plugins/
  • zsh-autosuggestions source .../zsh-autosuggestions.zsh
  • zsh-syntax-highlighting source .../zsh-syntax-highlighting.zsh
• The zsh syntax is compatible with Bash
• Shell Built-ins
  • where similar to which
  • r N redo last command (or the N th command in this session)

Package in base

• chown to change it.
• chmod change file mode
  • {u|g|o|a}{+|-}{r|w|x|X|s|t|u|g|o}=
  • = add and remove other
  • X execute/search if directory, or it has execute permission for some user.
  • s set user or group ID on execution
  • t restricted deletion flag or sticky bit
  • u g o set it equal to this.
  • [NN]NNN the octal code for the premission
  • = rwx

• The command that begin a session on the system

• It gives users in sudo group, privilege to execute as root.

• /etc/pam.conf
• /etc/pam.d: Linux-PAM configuration
• /usr/lib/pam.d: Linux-PAM vendor configuration
• system-auth: The authentication
• system-login: Perform authentication with system-auth, and setup sessions.
• system-local-login, system-remote-login: Wrapper of system-login to enable specific operations.

PAM-aware applications install their policy in this folder, for them to execute it on demand.

• PAM modules are installed under /usr/lib/security exclusively.
• The configuration files for individual modules are in /etc/security/.

• Agent program for each graphical environment that handles the on-demand authentication.

• Allowed actions or authorization rules can be defined.
• They are stored in
  • /usr/share/polkit-1/actions/
  • /usr/share/polkit-1/rules.d/ (package), /etc/polkit-1/rules.d/ (user)

• keyctl CLI tool
• library and set of utilities for accessing the kernel keyrings facility.

gnome-keyring keyrings frontend for GNOME

• /etc/environment: Global environment variables, set by the pam_env.so
• /etc/profile: The initial variables for the login shell only.
  • ~/.profile, ~/.bash_profile, …: Per user startup script
  • $ZDOTDIR/.zshenv, $ZDOTDIR/.zprofile, contains user profile and environment.
  • $ZDOTDIR/.zlogin, $ZDOTDIR/.zlogout contains terminal login shell configurations.
• ~/.bashrc, ~/.zshrc, … : Shell specific configurations which is sourced by non-login interactive shells.

• Create a .desktop file to execute a shell script within ~/.config/autostart/.
• Add .sh file under $HOME/.config/plasma-workspace/env/

• Native or baremetal hypervisor

• Hosted Hypervisor

• Provided by qemu-full package
• Does not provide any GUI or persistancy.
• Add -enable-kvm flag to use KVM.
• Default to the User-mode networking.
  • See Archwiki for more.

• Daemon for libvirt server.

• Command line libvirt client provided by libvirt.
• virsh -c qemu+unix:///session to start on arch

• The libvirtd need to be running.

• wayland prop set persist.waydroid.multi_windows the floating windows within the waydroid.
• wayland prop set persist.waydroid.fake_wifi APP_ID it appears connected to wifi for the app.
• See Setting up a shared folder | Waydroid

The Wine server starts automatically when the fisrt wine instance is created. It functions as the kernel for .exe programs, handling the scheduling, interprocess communication and so on.

• Part of GNOME project which includes GNOME desktop environment.
• Main repository is Flathub, the official repository of Flatpak.
• Apps are stored in /var/lib/flatpak/app/ and the desktop entries are in <appID>/current/stable/export/share/applications/.
• App data are under ~/.var/app/.
• It has its own set of dependencies that are independent of distros.
• The list are accessible from flatpak list
• flatpak
  • install
  • run
  • override
  • --env=ENVIRONMENT_VARIABLE=VALUE
  • --filesystem=FILEPATH
    • Flatseal can be used instead.
  • build

• AUR
• base-devel is required.
• Download the package in the aur. Possibly through git clone

• type/[tree.]subsype[+suffix]*[; parameter]
  • type: application, audio, image, message, multipart, text, video, font, example or model
  • tree: vnd. vender, prs. personal, x. unregistered (originally x- in 1993)
  • suffix: Augmentation of media type for the structure
• The default application for a file type is determined by looking up the MIME type in mimeinfo.cache which located in /usr/share/applications/ (and /var/lib/flatpak/exports/share/applications in the case of flatpak applications)
• mime.cache is also present in ~/.local/share/mime/, /var/lib/flatpak/exports/share/mime/, /usr/share/mime/

Message bus system that provides inter-process communication

• It consists of system-wide or user-wise daemon, and and libraries to make use of that.
  • The user-wise daemon is started in a session created by pam_systemd(8) and systemd-logind. The session bus is started alongside with the systemd --user.

For the system to use the locale data, binary files needs to be in /usr/lib/locale/. They contains various locale data such as datetime format (LC_TIME), currenty symbols (LC_MOENTARY) , and character classifications (LC_CTYPE).

The locale data is written as plain text in /usr/share/i18n/locales/, and they are compiled on demand by locale-gen command.

The system locale is determined by the default environment variables, such as LANG, LC_CTYPE. It is stored in /etc/locale.conf, and the file can be directly modified or indirectly (and more robustly) by localectl command.

Locale can be changed locally in a session or a program by overriding the environment variable.

• -t --troff Use groff -mandoc to format the man page. Implied when -T -H are used.
• -T[device] change the groff output suitable for device, which includes dvi, latin1, pdf, ps, utf8, X75, X100
• -H Let groff produce HTML output.

• (1): User Command
  • (2): System Call
  • (3): Library Function
  • (4): Special File — Device Files
  • (5): File Format and Filesystem
  • (6): Game and Funny Things
  • (7): Overview and Miscellaneous
  • (8): Administration and Privileged Command

• .pod is used.

• Arch 0.1: Where Did Arch Linux Come From? - YouTube