Introduction

As a software engineer, I've always wanted to dive deeper into the world of Linux. From understanding the inner workings of the operating system to learning the basics of shell commands and beyond, Linux offers a wealth of knowledge that I want to explore.

In this article, I’ll take you on a journey through my learnings, covering essential commands, system concepts, and the fundamentals of working with Linux. Whether you’re just starting or looking to refine your knowledge, you’ll find some valuable insights here. So grab a cup of coffee, get comfortable, and let’s get started with Linux!

Working With Shell

Basic Commands

General

echo “hello”: Prints "hello" to the terminal.
uptime: Displays the time since the last reboot.
/ (root dir): Root directory.
~ (user dir): User's home directory.

Directory Navigation and Management

popd: Pops the last pushed directory off the stack and navigates to it.
cp -r: Recursively copies directories and their contents. Example: cp -r Europe/UK Europe/UnitedKingdom.

Viewing Files

more new_file.txt:
- [space]: Scrolls one screenful at a time.
- [enter]: Scrolls one line at a time.
- [b]: Scrolls backward.
- [/text]: Searches for "text".
- [q]: Exits.

Listing Files and Directories

ls: Lists files and directories.
- ls -l: Detailed listing (access mode, ownership, timestamps).
- ls -a: Includes hidden files (preceded by .).
- ls -t: Sorts by creation/modification time.
- ls -tr: Reverse order of ls -t.

Help Commands

whatis echo: Brief description of the command.
man touch: Displays the manual for touch.
h or -help: Displays help for most commands.
apropos [keyword]: Searches the manual pages for the keyword.

Examples:

➜  linux-training apropos modpr
modprobe (8)         - Add and remove modules from the Linux Kernel
modprobe.d (5)       - Configuration directory for modprobe

Bash Shell

Check Shell: echo $SHELL.
Alias: Create shortcuts for commands. Example: alias dt=date.
History: history lists previously run commands.

Linux Core Concepts

Introduction to the Linux Kernel

What is the Kernel?

The kernel is the core component of the operating system that interfaces between hardware and processes. It is responsible for:

Memory Management.
Process Management.
Device Drivers.
System Calls and Security.

Key Features:

Monolithic and Modular: Combines all core functionalities while allowing extensions via modules.

Checking Kernel Version:

Command: uname

Example:

➜  ~ uname
Linux
➜  ~ uname -r
5.15.146.1-generic

5: Kernel version.
15: Major version.
146: Minor version.
1: Patch release.
Generic: Distribution-specific info.

Kernel Source:

Kernel.org: Open-source repository for all kernel versions.

Kernel Space and User Space

Memory is divided into two regions:

Kernel Space:
- Runs kernel code, extensions, and device drivers.
User Space:
- Runs user applications written in languages like C, Java, Python, etc.

Example Workflow:

USB Driver interacts with kernel space (device driver) and user space via /dev/sdb1.

Working with Hardware

Commands:

dmesg: Displays kernel messages (ring buffer).
- Filter logs with grep:
```
$ dmesg | grep -i usb
```
udevadm monitor: Monitors details about newly attached or removed devices.
lspci: Lists PCI devices (e.g., Ethernet cards, video cards).

lsblk: Lists block devices.

Types:
- disk: Whole physical disk.
- partition: Partitioned disk space.

Example Output:

NAME    MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda       8:0    0   100G  0 disk
sda1      8:1    0    50G  0 part /
sda2      8:2    0    50G  0 part /home

lscpu: Displays CPU architecture information.
Types of CPU:
1. 64 Bits: Address & store 2^64 / 18EB maximum memory.
2. 32 Bits: Address & store 2^32 / 4GB maximum memory.
Register: A location in the CPU rapidly accessed to load data from memory and perform arithmetic operations.
lsmem --summary: Summarizes memory information.
free -m: Shows total and used memory.
lshw: Provides detailed hardware information.

Linux Boot Sequence

Overview:

BIOS/UEFI Initialization.
Bootloader Execution (e.g., GRUB).
Kernel Loading.
Init System Execution (e.g., systemd).
User Space Initialization.

System Targets (Runlevels)

Runlevels:

0: Halt.
1: Single-user mode.
3: Multi-user mode with networking.
5: Multi-user mode with GUI.
6: Reboot.

The term "runlevels" is used in SysV init systems. These have been replaced by systemd targets in modern systems.

Mapping of Runlevels to Targets:

runlevel 0 -> poweroff.target
runlevel 1 -> rescue.target
runlevel 2 -> multi-user.target
runlevel 3 -> multi-user.target
runlevel 5 -> graphical.target
runlevel 6 -> reboot.target

Filesystems and Hierarchy

File Types:

Command: file [filename] – Displays the file type.

Key Directories:

/: Root directory.
/home: User home directories.
/bin: Essential binaries.
/etc: Configuration files.
/var: Logs and variable data.

Commands:

df -h: Disk space usage.
du -sh [dir]: Directory size.

Package Management

Introduction to Package Management

What is a Software Package?

A software package is a compressed archive containing files required by software to run, including binaries, metadata, and configuration files.

Package Manager Functions:

Package integrity and authenticity.
Simplified package management.
Dependency resolution.
Grouping by functionality.

Types of Package Managers:

RPM-based: rpm, yum, dnf.
Debian-based: dpkg, apt, apt-get.

RPM and YUM

RPM

Low-level package manager used for installing, uninstalling, upgrading, querying, and verifying packages.

Examples:

rpm -ivh telnet.rpm   # Install
rpm -e telnet.rpm     # Uninstall
rpm -Uvh telnet.rpm   # Upgrade
rpm -q telnet.rpm     # Query
rpm -v <path>         # Verify

YUM

High-level package manager for RPM-based systems with automatic dependency resolution.

Commands:

yum repolist         # Show repositories
yum provides scp     # Find the package providing 'scp'