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Understanding the Shell (Bash Basics)

The shell is a command-line interface that allows users to interact with the operating system by typing commands. In Linux, the most common shell is Bash (Bourne Again Shell). In this submodule, we’ll explore the basics of Bash and how to use it to communicate with the Linux system.

What is Bash?​

Bash is an open-source command-line interface and scripting language that serves as a shell for Unix-based operating systems, such as Linux and macOS. It is the most widely used shell in Linux distributions.

Bash provides users with a text-based interface to interact with the operating system, allowing you to run commands, scripts, and automate tasks.

Basic Bash Terminology​

Before we start with commands, let's look at some key terminology related to Bash:

  • Shell: The interface that allows users to interact with the operating system.
  • Terminal: A program that opens the command-line interface (CLI) and runs the shell.
  • Prompt: The line that appears in the terminal where the user types commands.
  • Command: A specific instruction given to the operating system to perform a task (e.g., ls, pwd, cd).
  • Script: A text file containing a sequence of Bash commands that can be executed together.

The Bash Prompt​

When you open a terminal, you'll be greeted with the Bash prompt. The prompt typically consists of the following parts:

  • Username: The name of the user currently logged into the system.
  • Hostname: The name of the computer or server.
  • Current directory: The location of the current working directory.
  • Prompt symbol: A symbol indicating that the system is ready to accept a command.

Example of a basic prompt:

user@hostname:~$

In this example:

  • user: The username of the current user.
  • hostname: The name of the computer.
  • ~: The current directory (in this case, the home directory).
  • $: The prompt symbol, indicating that Bash is waiting for input.

Note: If you're logged in as a root user, the prompt symbol will usually be # instead of $.

Running Basic Bash Commands​

In Bash, commands are typed after the prompt and executed by pressing Enter. Here are some basic commands you will frequently use:

1. pwd - Print Working Directory​

The pwd (print working directory) command displays the absolute path of the directory you are currently in.

Example:

$ pwd
/home/user

2. echo - Print Text to the Screen​

The echo command is used to print text to the terminal.

Example:

$ echo "Hello, World!"
Hello, World!

3. clear - Clear the Terminal Screen​

The clear command clears the terminal screen, giving you a fresh prompt.

Example:

$ clear

Command Syntax​

Bash commands follow a specific syntax:

  • Command: The program or action you want to execute (e.g., ls, cd).
  • Options/Flags (optional): Modify the behavior of the command (e.g., -l for long listing with ls).
  • Arguments (optional): Specify the target for the command (e.g., file or directory names).

Example Syntax:​

$ command [options] [arguments]

Example with ls:​

$ ls -l /home/user
  • ls: The command to list files.
  • -l: The option to display the listing in long format.
  • /home/user: The argument specifying the directory.

Using Wildcards​

Wildcards allow you to match patterns in filenames, making it easier to select multiple files at once. There are a few common wildcards:

  • *: Matches any number of characters (including none).
  • ?: Matches a single character.
  • []: Matches any single character within the brackets.

Example:​

  • *.txt: Matches all files ending with .txt.
  • file?.txt: Matches files like file1.txt, file2.txt, but not file10.txt.
  • file[1-3].txt: Matches file1.txt, file2.txt, and file3.txt.

Running Multiple Commands​

You can run multiple commands in a single line by separating them with semicolons ;. For example:

$ cd /home/user; ls -l; echo "Done!"

This will:

  • Change the directory to /home/user.
  • List the files in that directory.
  • Print "Done!" to the screen.

You can also use logical operators to run commands conditionally:

  • &&: Executes the second command only if the first one succeeds.
  • ||: Executes the second command only if the first one fails.

Example:​

$ mkdir new_directory && cd new_directory

This creates a new directory and then enters it, but only if the directory creation is successful.

Tab Completion​

Bash has a helpful feature called tab completion, which automatically completes file and directory names when you press Tab. For example:

  • Type the beginning of a file or directory name and press Tab to autocomplete.
  • If there are multiple matches, press Tab twice to see a list of possible completions.

Command History​

Bash keeps a history of commands you’ve previously typed. You can:

  • Up arrow: Scroll backward through your command history.
  • Down arrow: Scroll forward to more recent commands.
  • !!: Execute the last command again.
  • !n: Execute the nth command from the history list.

Example:​

$ !!

This will re-execute the last command you typed.

Conclusion​

Bash is a powerful tool for interacting with Linux. By understanding the basic commands and concepts outlined in this section, you’ll be able to effectively navigate the system and execute common tasks. As you progress in this course, you will dive deeper into more advanced Bash functionality and scripting.

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