Linux Academy Notes
101.1 Determine and configure Hardware settings
Pseudo (fake) File System:
Everything in Linux is seen as a file and it is stored in the RAM.
Once the session is powered off the and the RAM gets cleared, this file system disappears.
Once the machine is reopened it creates a new file System for the new session which is kept in RAM.
/proc - Contains information about the processes running on a system.
Processes are listed by PID with hardware and process data both in the same directory structure.
/sys - Contains information about the system’s hardware and kernel modules. No process information is listed here.
man proc - shows local documentation on the /proc pseudo file system.
Working with Kernel Modules:
Kernel is the core framework of the OS
GNU/Linux OS - Contains Bash commands, shells, what the system needs to function.
Linux kernel is monolithic: meaning that it handles all memory management and hardware device interactions
Has the ability to dynamically load and unload drivers.
Example: Kernel module Floppy
Investigating Hardware:
/dev- contains information on all of the connected hardware on a system
udev - the device manager for the Linux Kernel, links information on system hardware to /dev
D-Bus - Sends data messages between applications, a conduit of information about what’s going on the system
udev utilizes dbus to notify users and the system when new hardware is attached.
101.2 Boot the System
The Linux boot Sequence:
INIT: Short for initialization
When machine starts the Kernel looks for the program in /sbin/init. Once this starts init is in control.
First thing init does is it looks for the configuration tab in /etc/inittab
This also holds the Runlevels of the OS
Predefined configuration that the computer will operate within. Applies to system as a whole.
Traditional Scripts for Stop/Start for the system to function are in:
/etc/init.d - contains the scripts for the services on the system
/etc/init.d/rc. - scripts that orchestrate how the runlevel scripts run and what occurs when a runlevel changes.
Upstart: - Boot Sequence
Upstart allows for multiple processes to run simultaneously rather than the old init system where it has to wait for a sequence to finish before it can start.
Init is static; it does not natively respond to changes on a system (manual config)
Upstart is dynamic, it can respond to changes on the system.
Ex: Plugging in a monitor, init will not recognize this and will have to be set up manually, where upstart will do this automatically.
When something on a Linux system changes it is known as an event. When an event is triggered it will start and upstart job.
Jobs are split in 2 categories: Tasks & Services
Task: will do what it is required of it and then it will return to a waiting state once it is finished. (one time use)
Service: will not stop by itself and will only kill a service if an event calls for it, or if an admin does so.
Job flow:
Initial State: waiting (to do something)
Starting where the job is required.
Running: the job is actually processing what it is supposed to do If it fails it will enter respawn state, where the system tries to run the task up to 10 times before it drops it as failed.
Stopping: where the job has finished what it is supposed to do, but it is not stopped just yet.
Killed: request for the job to be terminated as it is finished
Post-stop: where the job is actually finished.
After that the status becomes waiting again.
systemd:
init and parts of upstart rely on Bash shell scripts which require the bash interpreter to run, systemd has removed this.
Systemd is backwards compatible, but not guaranteed 100% with all functions.
Unit File Locations:
Provided by package installation: /usr/lib/systemd/system
These should not be modified as they might be susceptible to change by an update.
Unit file location for system administrators: /etc/systemd/system
These take precedence over those in /usr
Runtime unit files: /run/systemd/system
View all unit files on system: systemctl list-unit-files
FULL DOC for the Unit File: man 5 systemd.unit
systemctl cat <something>.unit will print out content of unit file specified.
Systemd boot system:
Kernel still looks for /sbin/init
Systemd just took the place of /sbin/init
Developers have created a symbolic link for /sbin/init to redirect to /lib/systemd/systemd which actually boots systemd.
101.3 Change Runlevels/Boot Targets and Shutdown or Reboot the System:
runlevel - view current runlevel which applies on the system as a whole
telinit - change to another runlevel (alternative command: init )
In order to change runlevel you have to be the root user
Changing runlevels at boot:
Interrupt the GRUB boot process by pressing any key during startup, then at the GRUB selection menu highlight a kernel to modify.
Press the “a” key to add arguments to the end of a kernel line, enter in a runlevel number.
Change your Working Environment: targets:
A target unit is a unit that syncs up to other units when the computer boots or changes states.
Often used to bring system to a new state.
Types of Targets:
multi-user.target - multi-user system similar to what runlevel 3 provide
graphical.target - multi-user system with desktop environment, similar to what runlevel 5 provides
rescue.target - pulls in a basic system and file system mounts and provides a rescue shell
basic.target - basic system, used during the boot process before another target takes over
sysinit.target - system initialization
Target Units:
Reboot and Shutdown your System:
Reboot Commands:
Shutdown Commands:
Configuration file is under /etc/acpid
102.1 Design Hard Disk Layout:
Main File System Location:
Sizing: Older rule of thumb ranged from 1.5x to 2.0x the size of available RAM RAM is cheaper and it’s up to admin to set up SWAP space, but usually not less than 50% of RAM
Partitions and Mount Points:
/dev/sda - sda is full partition, usually that will divided in multiple sections assigned as /dev/sda1 ; /dev/sda2 ; /dev/sda3, etc.
Mount point: is when you take a partition or disk and mount it to a directory, for example /home - meaning it will house all the home directories.
Introduction to LVM: (Logical Volume Manager)
Allows the creation of “groups” of disks or partitions that can be assembled into a single (or multiple) filesystems:
Can be used for nearly any mount point EXCEPT /boot
Flexibility, allows for resizing of volumes
Snapshots - allows a point in time “photo” to be taken of the volume itself and is used as a backup
PV - Physical Volumes; VG - Volume Group; LV - Logical Files;
102.2 Install a Boot Manager:
Legacy Grub (Grand Unified Boot Loader)
BIOS → The first 512 bytes is a boot image searched → which searches for the core image file → which loads the /boot/grub partition and allows for the system to actually load.
Grub configuration file: grub.conf/menu.1st device.map
How to install GRUB:
Device can be /dev/sda or /dev/hd0 …. ETC
GRUB2
MBR (Master Boot Record)
Supported only 26 total partition (4 partitions with one partition extended to 23 partitions)
Partition size limited to 2TB
GPT (GUID Partition Table)
Supports 128 Partitions
Partition size up in the ZB range
Needs UEFI (Unified Extensible Firmware Interface) to boot:
Replacement for traditional BIOS cand act in legacy BIOS mode
Requires a 64bit OS
Prevents unauthorized OS from booting on the System
Interacting with the BOOT LOADER:
GRUB Legacy:
On SystemD you will have to use system.unit=rescue.target to boot in runlevel 1
102.3 Manage Shared Libraries:
Files that contain functionality that other applications can use.
These files end in a “.so” extension for shared object
Found under the following locations:
Two types of library files:
Dynamic (ends in .so) Statically Linked (ends in .a)
102.5 Use RPM and YUM Package Management
YUM: Yellowdog Updater modified
Handles RPM package Dependencies
Installs, upgrades and removes packages
Users on RHEL, CENTOS, Scientific Linux, older versions of Fedora
YUM setup:
Global yum configuration options are set in /etc/yum.conf
Reads repository information from /etc/yum.repos.d
Cache latest repository information in /var/cache/yum
Other RPM Package Managers:
Zypper:
Used on SUSE Linux Distributions
Ex: zypper repos; zypper install vim
DNF - Dandified yum:
Used on Fedora Linux distributions
Future replacement for yum in RHEL
Uses same command syntax as yum
RPM (Redhat Package Manager):
The .rpm package contains:
Application or utility
Default configuration file
How and where to install the files that come with the package
Listing of dependencies that the package requires.
The rpm database: located in /var/lib/rpm
Dependencies need to already be installed or installed with the package
yum handles dependencies for you, rpm does not.
ex: rpm2cpio some.rpm | cpio -idmv
102.6 Linux as a Virtualization Guest:
Virtualization and Containers:
Emulations of a specific computer type
Operate based on the architecture and functions of a real computer and its implementation, can involve specialized hardware, software or both.
Virtualization Software allows you to set up on OS within another.
They both share the same physical hardware
The virtual machine is isolated from that hardware and has to communicate with in through a Hypervisor
Examples: KVM; QEMU; VMWare; Xen; VritualBox
2 Main types of Virtualization:
Full Virtualization - guest system is not aware that it is a virtual machine
Paravirtualization - guest system is aware that it is a virtual machine, uses guest drivers Virtual machines typically perform better with guest drivers
Virtual machines can be “cloned” or turned into templates to rapidly deploy new systems
NOTE: When working with Linux VMs you may need to change the system’s D-Bus Machine ID - done with dbus-uuidgen - ensures that each running Kernel interacts with system that has a unique ID
Virtual servers can be provisioned from cloud providers:
If you are using a cloned virtual machine, cloud-init is typically used to insure that user data is completely new
Creates new SSH Keys
Sets the system’s default locale
Sets the system’s hostname
Sets up mount points
Example: AWS, Azure
What is container:
An entirely isolated set of packages, libraries, and/or applications that are completely independent from their surroundings
2 Types of Containers:
Machine container: Shares a kernel and file system with the host computer
Application Container: Shares everything but the application files and library files that the application needs
Why is the difference important?
Virtualization: Invented to allow the sharing yet segregation of server instances from each other.
Protect one operating system from anther on the same system
Prevention of letting spare CPU cycles, memory, or disk space go to waste.
They are based on emulating virtual hardware through a hypervisor; heavy in terms of a system requirements
Containers: Uses shared OS - Each application gets isolated from other applications
More efficient in system resource terms - Can run multiple versions of the same app
More granular management of system resources
103.1 Work on the Command Line:
Your Bash Shell Environment:
Linux Shells: Is the command line environment that you work in on a Linux System
bash (bourne again shell) - the default
csh - C programming style syntax
ksh - Korn Shell, based on the Bourne Shell, with some features of the cshell added
zsh - Z Shell included elements of the Bash Shell and Korn Shell
Environment Variable: Setting that dedicate common functionality and locations for various purposes
Syntax: VARIABLE=path,command,alias
Example: CWD=/home/user/Documents
Bash functions: Users can create their own custom functions within Bash
Example:
function yo() { echo “yo”
}
When using echo you can place $ followed by the variable name in order to see the exact value of the variable, all variable names are upper case
Example: echo $PWD
export - command used to export a variable to the current shell and any new shells started from the current shell
pwd - displays the full path to the current working directory
which - used to locate an application file that is located within the user’s PATH
type - Used to determine if something is a function, file, alias, built-in, or keyword
Typing bash within a bash shell, it opens a new shell for use.
Left Child Shell - Right new login shell
Bash Quoting:
“weak” quotes - weak quotes, or double quotes, will expand variable, but characters used for path substitution or for pattern matching will not expand
Example: echo “$PATH” will print the PATH
ls “*” will not work as the ls command would be looking for a file named *
‘strong’ quotes - inside strong or single quotes, nothing is interpreted
Example: echo ‘$PATH’ will print $PATH to the screen
Bash History and the Manual Pages:
history - Command that shows the most recently ran commands
.bash_history - File located in the user’s home Directory that contains the previously run commands
HISTFILESIZE - Environment variable that determines how many lines the .bash_history file will contain
Manual Pages:
Built-in manual pages for commands, configuration files, and system administrator tasks
Invoke with man
Man pages are broken out into ‘sections”
Section 1: Executable programs or shell commands
Section 2: System calls - functions provided by the Kernel
Section 3: Library calls - functions within program libraries
Section 4: Special files - typically those found in /dev
Section 5: File formats and conventions - for example /etc/passwd & other configuration files
Section 6: Games
Section 7: Miscellaneous items and conventions
Section 8: System administration commands, usually only for root
Section 9: Non-standard Kernel routines
103.2 Use Streams, Pipes and Redirects:
Basics of Viewing Text Files:
Message Digest:
To check for this run command and use “-c” Ex: sha512sum -c
Text Manipulation:
LPIC1 - search and replace only for sed - big command
This does not modify the file itself, in order to do that you will have to use the following command.
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