Module 5: Command Line Skills

Module 5: Command Line Skills

• This module covers the basics of the command line interface, including the shell, commands, variables, command types, quoting, and control statements.

Exam Objective 2.1 Command Line Basics

• The objective is to understand the basics of using the Linux command line.

Introduction

• This module will cover the basics of the command line interface, the shell, commands, command types, quoting, and control statements.

Command Line Interface

• The Linux community promotes the CLI due to its power, speed, and ability to accomplish diverse tasks with a single command.
• The CLI provides precise control, speed, and automation through scripting.
• Learning the CLI allows for instant productivity on any Linux distribution.

The Shell

• The terminal accepts commands and passes them to a shell.
• The shell is a command line interpreter, translating user commands into actions for the operating system.
• The Linux environment supports many different shells.
• The Bash shell is the most commonly used shell for Linux distributions. It has features including command-line history, inline editing, scripting, aliases, and variables.

Commands

• A command is a software program that performs an action on the computer when executed on the CLI.
• To execute a command, type its name and press Enter.
• Some commands may require additional input like options and arguments.
• Options modify the command's behavior.
• Arguments provide additional information (filename, username, etc).
• The format is command [options] [arguments].

Arguments

• Arguments let you specify something for the command to act upon.
• If you give a directory as an argument to the ls command, it displays the directory's contents.
• Some commands accept multiple arguments.

Options

• Options extend or modify how a command behaves.
• The ls -l command, for example, produces a long listing of files.
• Typically, single-letter options are preceded by a single dash (-), while full-word options use two dashes (--).

uname

• The uname command displays information about the system.

whoami

• The whoami command displays the current user's name.

Command History

• The terminal saves executed commands in a history list, for easy re-execution.
• Pressing the Up Arrow key displays previous commands.
• The history command views the entire history list.
• You can execute a command from the history list by typing !number where 'number' corresponds to the command position in the history list.
• Typing !! will re-execute the previous command.
• You can execute commands using !command to re-run previous versions.

Command Types

• The type command helps determine a command's type.
• Internal commands are built directly into the shell.
• External commands are stored in files; the which command shows location.
• Aliases create shortcuts for longer commands.
• Functions are custom commands built from existing ones.

Variables

• Variables are named containers for storing data in the shell.
• Variables can be local (temporary) or environment (global).
• Local variables are specific to a terminal session.
• Environment variables are permanent.

Globbing/Wildcards

• Globbing uses special characters (*, ?, []) to match patterns of filenames.
• These characters can be combined for more complex matching.
• The * matches any sequence of characters (including none).
• The ? matches exactly one character.

Quoting

• Quoting alters metacharacters' meaning for glob, variables, etc.
• Double quotes keep them uninterpreted but allow variables and command substitution.
• Single quotes stop all interpretation of shell metacharacters.
• Backquotes are used for command substitution, executing a command and using its output in another.

Control Statements

• Control statements let you execute multiple commands at once or conditionally.
• The semicolon (;) separates commands, executing them sequentially.
• The double ampersand (&&) executes the second command only if the first one succeeds.
• The double pipe (||) executes the second command only if the first one fails.