knowledge-base

LPIC-1

About Linux

Components of Linux
- Boot Loader
  - Software manages boot process till the OS starts to load
- The Kernel
  - Core of OS, manages OS, CPU and peripherals
- Daemons
  - Processes lurking in the background that start during booth or after login (e.g. time)
- Shell
- Graphical Server
  - Subsystem to display graphic
  - AKA x-server
- Desktop Environment
  - Actual GUI for the user
- Applications
Why Linux ?
- Free
- Stable
- Secure
- Open Source
  - Free to run the program for any purpose.
  - Free to study how the program works
  - Free to change how the program works
  - Free to redistribute copies
Distributions
- What is a distribution ?
  - Collection of software
  - Package management system
  - Helps you install, upgrade and remove software
  - Keeps your server up to date
- Popular Examples
  - Red Hat
    - CentOS (based on red hat)
    - Fedora
  - Debian
    - Ubuntu
    - Mint
  - SuSE
  - Gentee
  - Arch
  - …
VirtualBox Bridged Networking
- If you do this, instead of NAT, you can SSH into the box instead of using the VirtualBox UI

System Architecture

Boot the system
- UEFI is the new version of BIOS
  - BIOS booting
    - -> BIOS boot from particular disk
    - -> This disk has a MBR (Master Boot Record), which stores the Boot Loader
    - -> The boot loader knows where the kernel is on disk and boot the OS
  - UEFI
    - UEFI Boot Loader (which lives on your disk) (is not in the MBR)
    - This boot loader calls the kernel, which boots the OS
- MBR : Information in the first sector of a disk that tells where and how the OS is.
- GRUB (Grand Unified Bootloader) : Boot loader package that supports multiple OS’s on a ps
  - You can modify the settings during bootup and make them persist by directly modifying the config files used by the GRUB
  - For Ubuntu in VirtualBox, tap esc during bootup
- Kernel is lowest level of replacable software to your hardware
- Once the Kernel has attached the root file system, it will run a program called init
- init is always the first process ran by a linux system, therefor, it gets always a PID of 1
  - There are a few different init programs that exist
    - sysvinit : Which based on sysv, oldest and first
    - systemd : Low memory boot process, mainstream, almost used everywhere now
    - upstart : Created by Ubuntu, but they switched to systemd in the end.
- quiet for kernel param is supress most boot messages. So less verbosity for bootup
Determine and configure hardware settings
- udev : Device manager for your kernel
  - Gives low level access to the linux device tree
  - Handles user space events (happens when hardware is removed or added to the system)
    - Eg. Loading firmware
  - Provided by temporary filesystem (tmpfs)
    - This is how udev provides access, which is mounted to /dev on startup
  - /etc/udev/rules.d : Folder for custom rules for the device manager udev.
    - You can create rules for what should happen/ran when something is plugged in or unplugged
- dbus
  - Inter-process communication mechanism
  - Framework that allows processes to talk to each other
  - Secure
  - Reliable
  - Provides high level OOP interface
- sysfs
  - Virtual filesystem
  - Presents information about various kernel subsystems
    - Hardware devices
    - Drivers
  - Mounted to /sys
- procfs
  - Similar to sysfs
  - Presents information about various processes
  - Presents information about system information
  - mounted to /proc
    - Can be used to interface with the kernel
    - Change parameters on the fly
  - Each running process will have a directory in /proc/<PID> + various other stuff
    - eg. /proc/cmdline -> Kernel name from bootup
    - eg. /proc/version -> Kernel version
    - eg. /proc/cpuinfo -> CPU info
- lsmod
  - List all Kernel modules in use and by which modules
- modprobe
  - Add or remove loadable kernel modules to/from the kernel
  - udev relies upon modprob to load drivers for automatically detected hardware
- rmmod
  - Remove kernel module (prob need root for that)
- lspci
  - Shows all PCI connected devices to the system
  - Show Device IRQ settings
Runlevels and boot targets
- Run Level : Number between 0 <-> 6 (max 9), determines which scripts/programs are run
  - Levels :
    - 0 - Halt or shut down system
    - 1 - Single user mode
    - 2 - Multi user mode without networking
    - 3 - Normal boot (multi user mode + networking)
    - 4 - Unused/customizable
    - 5 - Run level 3 + GUI display manager (if installed, so the graph env)
    - 6 - Reboot
  - Based on the run level, more or less scripts will be ran which are located in different places (e.g. Systemd or sysv)
    - inittab (came with sysv)
    - Ubuntu : /etc/rc0.d … /etc/rc6.d folders with scripts that are ran based on correlating run level. rcS.d us ran for any run level.
      - This is still there for compatibility reasons
    - systemd run levels :
      - Also has scripts
      - System scripts /etc/systemd/system
      - Package Scripts /usr/lib/systemd/system
      - /etc/systemd/system takes precedence over /usr/lib/systemd/systen
      - Uses targets (similar-ish to the other run level style)
        
        Run Level - Systemd Target
        
        0 - runlevel0.target, poweroff.target
        
        1 - runlevel1.target, rescue.target
        
        2,4 - runlevel2.target, runlevel4.target, multi-user.target
        
        3 - runlevel3.target, multi-user.target
        
        5 - runlevel5.target, graphical.target
        
        6 - runlevel6.target, reboot.target
        
        emergency - emergency.target
- with init/telinit you can tell what level to run

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systemctl
- How you drive/control systemd
- How you start, stop, restart applications (just like kubectl)
- E.g.
  - Status sound card : sudo systemctl status sound.target
  - stop sound card : sudo systemctl stop sound.target
  - Status sound card : sudo systemctl status sound.target
  - start sound card : sudo systemctl start sound.target
  - Status sound card : sudo systemctl status sound.target
Commands
- ps : List active processes
- ps aux | head : Shows the top processes where you will seee /sbin/init
- dmesg : Kernel messages logged from the last bootup
- head : take first n lines
- tail : take last n lines
- less : page through a long result
- man : manual
- sudo wall : Send a message to anyone logged in/has open shell (like announce a reboot)
- which : Where is an app running from?

Linux Installation and Package Management

Design hard disk layout
- / : root
- /usr : user binaries installed
- /home :
- /boot : All related to booting
- /var : Variable data (e.g. system logs …)
- /tmp : Everyone on the system can write to
Partitions
- Divide storage in multiple pieces
- Allows dual booting
- Separation of files
- Data organization
- System protection
- e.g. Separate partition for each user to separate and safeguard
- When you create a partition you need to mount them to directories.
- Every path in linux can be mounted to
  - We can mount to :
    - /home
    - /var
    - /tmp
    - /home/nick/blah/
  - Remember like with Docker. You can have a folder /home/ian/dbdata with data in it.
    - When you mount a partition to /home/ian/dbdata, the data in there will be hidden and overtaken with this mounted partition (like a volume with docker)
    - Once you unmoun the partition from that folder, the original data is again visible and accessible.
Swap
- Swap is a partition, used in case the RAM is full, Swap is used
- Unused pages fo the RAM will be saved in the SWAP partition
LVM
- Logical Volume Manager
- (like Disk Manager windows, on steroids)
- Allows to split disks into pools (Pools are also known as PE, Physical Extends)
- Create partitions from pools
- Can grow or shrink partitions

linux_02

Install a boot manager
- Boot Loader
  - Boots a linux system
  - Runs before the OS
  - Can be configured from the operating system
  - common boot loaders :
    - LILO
    - GRUB Legacy
    - GRUB2
- Configuring the /boot/grub/grub.cfg results in editing your boot loader logic
- Although the above is the auto generated result of /etc/grub.d and /etc/default/grub using grub-mkconfig
- grub-probe
- grub-install : To install grub to the MBR of the specified disk (in case that didn’t happen yet)
- Revise
  - update sudo vi /etc/default/grub file
  - run sudo update-grub
  - restart to see changes in action
Manage Shared Libraries
- Libraries : So typical libs/packages that bash scripts or actual code can use
- Key properties
  - Shared
  - Reusable
- Linking
  - When your application wants to use these libraries, they should be linked to them
    - Static Linking : Library is included in the application (each app has its own copy)
    - Dynamic Linking : Different applications using the exact same copy of the library
      - Update libraries in a single place
      - /etc/ld.so.conf lists the locations of the shared libraries on your system.
      - include statements means it was split into other linked files
  - ldd : ldd prints the shared objects (shared libraries) required by each program or shared object specified on the command line.
    - e.g. ldd /bin/ls
Use debian package manager
- dpkg : Debian Package Manager
  - install/upgrade/remove software
  - low level tool
  - (Does not automatically installes dependencies)
- apt
  - Advanced packaging tool
  - high level tool
  - install/upgrade/remove software
  - handles upgrading of entire system
  - Handle all package dependencies automatically
  - Uses online repositories
- /etc/apt/sources.list is the list for mirrors of apt and specifics on which repos per mirror to use
  - main : Officially supported software
  - restricted : supported software (not free under completely free license)
  - xenial : current version
  - universe : community maintained software
  - multiverse : Not free software
- apt-get update : update local package lists
- apt-get install <name> : Install software
- apt-get remove <name> : Uninstall software
  - ! does not remove remaining config files, use dpkg --purge <name> for cleanup
- apt-cache depends <name> : Show what a app depends on
- apt-cache search <name> : Search for packages with a particular name
- apt-get upgrade : Upgrade all current installed packages (respecting semver)
- apt-get dist-upgrade : upgrade everything and remove anything unused
wget Download a file to current location
Use RPM and YUM package manager
- RPM
  - RedHat Package Manager
  - rpm command
  - Low level tools
  - install/upgrade/remove software
- YUM
  - Yellowdog update/modifier
  - replaced YUP (yellowdog updater)
  - Utilites online repositories
  - manages dependencies
Quiz
- How do you temporarily add a directory to your shared library path? - LD_LIBRARY_PATH

knowledge-base

LPIC-1

About Linux

System Architecture

Linux Installation and Package Management

GNU and Unix Commands

Devices, filesystems and filesystem hierarchy

Shells, scripting and data management

User Interface and Desktop

Essential System Services

Networking fundamentals

Security

knowledge-base

LPIC-1

About Linux

System Architecture

Linux Installation and Package Management

GNU and Unix Commands

Devices, filesystems and filesystem hierarchy

Shells, scripting and data management

User Interface and Desktop

Manage user and group accounts and related system files

Essential System Services

Networking fundamentals

Security