Linux Kernel

• Interface between hardware and software.

• The kernel is the core of a computer's operating system (OS). It manages the hardware, memory, and disk storage. It also acts as a bridge between the OS and the computer's hardware.

Introduction to Shell

What is Shell ?

• Is is like a Container

• Interface between users and Kernel/OS

• CLI is a shell

Find Your Shell

• echo $0 - Shows you are in Bash

  

• Available Shells “cat /etc/shells“

  

• Your Shell? cat/etc/passwd

  

Note:

1. Windows GUI is a shell

2. Linux KDE GUI is a shell

3. Linux sh, bash etc is a shell

Types of Linux Shells

1. Gnome - GNOME Shell is the graphical shell of the GNOME desktop environment starting with version 3, which was released on April 6, 2011. It provides basic functions like launching applications and switching between windows. GNOME Shell replaced GNOME Panel and some ancillary components of GNOME 2.

2. KDE - KDE stands for K Desktop Environment. It is a desktop environment for Linux based operation system. You can think KDE as a GUI for Linux OS. KDE has proved Linux users to make it use as easy as they use windows.

3. sh - The Bourne Shell (sh) actually refers to the original Unix shell developed by Stephen Bourne at Bell Labs. It is lightweight and is the base of many modern shells.

   Features:

   • Easy syntax for running commands.

   • Great scripting capabilities for automating tasks.

   • The ability to be used with most Unix-based systems.

Pros:

• Lightweight and fast.

• Widely supported across Unix/Linux systems.

Cons:

• Not enough interactivity and user-friendly.

• Similar command-line features, but no advanced programming features like modern shells

4. bash - Bash is an improved version of the Bourne Shell, and it is the default shell for many Linux distributions. It offers features such as editing the command line, improved scripting support, and more intuitive commands.

   Features:

   • History of commands and completion of the tab.

   • Support for more complex data structures, and advanced scripting functions.

   • Compatible with Bourne shell scripts.

Pros:

• User-friendly for beginners.

• Well-established community support and documentation.

Cons:

• A bit slower than minimalistic shells like sh.

• Not available by default in some systems (ex: macOS >= 10.15)

5. csh and tcsh - A shell that is more user-friendly to programmers and has a syntax similar to the C programming language. It targets familiar C users quite well.

   Features:

   • Built-in features for job control.

   • Scripting syntax similar to C.

   • History and aliasing support.

Pros:

• Perfect for C or similar developers.

• Provides job control for managing processes.

Cons:

• It’s not quite as user-friendly on a generic level.

• It’s not as powerful as Bash or KornShell for scripting.

6. ksh - KornShell (ksh) is a shell that was created by David Korn in the mid-1980s and features the best components of both the Bourne shell and the C shell, along with scripting and programming functionalities.

   Features:

   • In-built arithmetic operators.

   • Advanced scripting features.

   • Tight Bourne shell script compatibility.

Pros:

• Well-suited for complex scripting tasks.

• It offers enhanced debugging capabilities.

Cons:

• Learning curve for beginners.

• On some Linux distributions, the installer must be manually installed.

Shell Scripting

What is a Shell Script?

• A shell script is an executable file containing multiple shell commands that are executed sequentially. The file can contain:

  • Shell (#! /bash/bash)

  • Comments (# comments)

  • Commands (echo, cp, grep etc.)

  • Statements (if, while, for etc.)

• Shell script should have executable permissions (e.g., -rwx r-x r-x)

• Shell script has to be called from absolute path (e.g., /home/userdir/script.bash)

• If called from current location then ./script.bash

Shell Script - Basic Scripts

• Output to screen using “echo“

• Creating tasks

  • Telling your id, current location, your files/directories, system info

  • Creating files or directories

  • Output to a file “>“

• Filters/Text processors through scripts (cut, awk, grep, sort, uniq, wc)

Hands-On:

1. Create a script file with some echo message » then assign the executable permission to all » execute the file » also try to execute the script file from absolute path -

   

2. Create a script file with some basic tasks like- (whoami, pwd, hostname, ls -ltr) and then execute the file by giving the permission-

   

3. Create a script file by defining variables-

   

Input and Output of Script

• Create script to take input from the user

  • read

  • echo

If-Then Scripts

If then statements

• If this happens = do this

• Otherwise = do that

For Loop Scripts

• Keep running until specified number of variables

  

  

/Do-While Scripts

The while statement continually executes a block of statements while a particular condition is true or met

Case Statement Script

Check Other Servers Connectivity

• A script to check the status of remote hosts

  1. ping some reachable IP address and check for it

2. ping some unreachable IP and then check for it

   

3. Now, instead of showing on screen, print in a /dev/null file

   

4. Now, ping that IP as a variable

   

5. Ping multiple IP address using file method

   

Aliases

• Aliases is a very popular command that is used to cut down on lengthy and repetitive commands-

  1. alias ls=“ls -al“

  2. alias pl=”pwd; ls”

  3. alias tell=”whoami; hostname; pwd”

  4. alias dir=”ls -l | grep ^d”

  5. alias lmar=”la -l | grep Mar”

  6. alias wpa =”chmod a+w”

  7. alias d=”df -h | awk ‘{print $6}‘ | cut -c1-4”

User Aliases-

Applies only to a specific profile » /home/user/.bashrc

Global Aliases-

Applies to everyone who has account on the system » /etc/bashrc

Shell History CMD

• Command- “history“

  

• Command- “!985“

  

• The file where history of your shell commands saved= /home/yourname/.bash_history

• To view other users history of shell commands

  • Become root (su -)

  • cat /home/users-dir-name/.bash_history

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Thanks for going through this blog, Happy Learning !! 😁