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.

TryHackMe Linux Shells | Room Answers

Who is the facilitator between the user and the OS?
Shell

What is the default shell in most Linux distributions?

Bash

Which command utility is used to list down the contents of a directory?

ls

Which command utility can help you search for anything in a file?

grep

Which shell comes with syntax highlighting as an out-of-the-box feature?

Fish

Which shell does not have auto spell correction?

Bash

Which command displays all the previously executed commands of the current session?

history

What is the shebang used in a Bash script?

!/bin/bash

Which command gives executable permissions to a script?

chmod +x

Which scripting functionality helps us configure iterative tasks?

loops

What would be the correct PIN to authenticate in the locker script?

7385

Which file has the keyword?

authentication.log

Where is the cat sleeping?

under the table

Conclusion:

This post thoroughly explains Linux shells, how to interact with them, and how to write and execute Bash scripts to automate tasks. It highlights the importance of understanding different shells and leveraging Bash for scripting in cybersecurity and system administration.

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