Linux Shell Scripting Explained | TryHackme Linux Shells

This post is part of a Cybersecurity 101 series and focuses on Linux shells, their types, features, and how to use Bash scripting for automation.

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Introduction to Linux Shells

Linux shells provide an efficient and resource-friendly way to interact with the OS via the command-line interface (CLI).

Different types of Linux shells include Bash (/bin/bash), Dash (/usr/bin/dash), Zsh, and Fish.

Each shell offers unique features such as auto-completion, spell correction, syntax highlighting, and user-friendliness.

To check the current shell:

echo $SHELL

To switch shells, use:

sudo chsh -s /usr/bin/dash

Popular Linux Shells and Their Features

Bash, SH, and Zsh are widely used due to their robust scripting capabilities.

Zsh offers advanced features like syntax highlighting and user-friendly customization.

Fish Shell is the most user-friendly shell and is available for Windows, macOS, and Linux.

Basic Linux Commands in Shell

PWD: Displays the current directory.

LS: Lists directory contents.

Nano: Opens a file for editing.

Introduction to Bash Scripting

Bash scripts automate repetitive tasks and have the .sh extension.

Shebang (#!) specifies the interpreter (#!/bin/bash).

Comments use #.

Variables are declared as variable=value. For strings, values are enclosed in quotes.

Use echo to print output to the terminal.

Script Example: Searching for a Flag in Log Files

Purpose: Automates searching for a specific string in .log files.Script Components:

• Variables: Set directory and flag string.
• Echo: Prints progress messages.
• For Loop: Iterates over .log files.
• If Statement: Uses grep -q to search for the flag.
• Output: Prints the filename if the flag is found.
• Permission Command:

chmod +x script.sh  # Grants execute permission 
sudo ./script.sh    # Runs the script with elevated permissions

Key Bash Commands Explained

History: Displays previously executed commands

history

chmod +x: Grants execute permission to a script.

For Loops:

• Iterate through numbers:

for i in {1..10}; do echo $i; done

• Iterate through files:

for file in /var/log/*.log; do echo $file; done

Advanced Example: Locker Script

Purpose: Verifies user identity with multiple factors before granting access.Key Features:

• Username verification.
• Custom logic for identity validation.

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