Shell

🧠 What is a Shell?

A Shell is a user interface to the Operating System — it allows users to interact with the OS kernel, typically by typing commands.

Think of the shell as the middleman between you and the kernel.

It takes your commands → translates them → and passes them to the kernel to execute.

💻 Types of Shells

There are two main types of shells:

Type	Description	Example
Command-Line Interface (CLI)	Text-based interaction	`bash`, `zsh`, `sh`, `cmd.exe`, `PowerShell`
Graphical User Interface (GUI)	Visual windows/buttons	GNOME, KDE, Windows Explorer

When we say “shell” in system interviews, we’re usually talking about CLI shells like Bash.

🛠️ What Does a Shell Do?

Function	Description
🧾 Takes user input	Reads commands from the keyboard or a script
🔄 Parses commands	Breaks input into program name, args, options
🔁 Executes commands	Calls system calls (e.g., `fork()`, `exec()`) to run programs
🧠 Handles I/O redirection	Supports `>`, `<`, `>>`, `
🔁 Handles variables & scripting	Supports loops, conditionals, variables
💡 Provides environment	Sets PATH, HOME, PS1, etc.

🔍 Shell Workflow (CLI)

You type:   ls -la /home/gaurav
    ↓
Shell parses: command = ls, args = -la /home/gaurav
    ↓
Shell uses fork() to create a new process
    ↓
Child process uses exec() to run `/bin/ls`
    ↓
Kernel executes ls, returns output
    ↓
Shell prints output to your terminal

🔧 Common Shells in Linux

Shell	Features
sh (Bourne Shell)	Original UNIX shell
bash (Bourne Again SHell)	Most common, scripting support
zsh	Interactive shell, themes (Oh My Zsh)
fish	User-friendly, autocomplete
csh/tcsh	C-like syntax

You can check your shell with:

echo $SHELL

🧪 Example Shell Commands

ls -l               # List files in long format
cd /var/log         # Change directory
cat file.txt        # Print file contents
echo "Hello" > a.txt  # Output to a file

These are interpreted by the shell, which then uses system calls under the hood to execute them.

📜 Shell Script Example

#!/bin/bash
echo "Welcome, $USER"
date

This script runs a sequence of commands via the shell.

🧠 Interview-Ready Definition:

A Shell is a program that provides a user interface to the operating system, typically via a command-line interface. It interprets user commands, interacts with the kernel using system calls, and supports features like scripting, I/O redirection, and environment management.

✅ Shell vs Kernel vs Terminal

Component	Role
Kernel	Core OS — manages hardware & resources
Shell	Interface to talk to kernel
Terminal	Tool to display shell input/output

Gaurav’s Notes

Explorer