Module 01 - Command Line Fundamentals

Lesson 03 - Viewing Files and Getting Help

Estimated time: 45–60 minutes

Difficulty: Beginner

Prerequisites: - Module 00 - Introduction to Linux - Lesson 01 - The Linux Shell and Navigation - Lesson 02 - Working with Files and Directories

Overview

Linux administrators spend a large amount of time reading configuration files, reviewing logs, and finding information about commands.

Unlike graphical operating systems, Linux provides powerful command-line tools for viewing and searching information quickly.

This lesson introduces the tools required to:

View file contents
Read large files
Monitor changing files
Find command documentation
Understand command options
Use command history

Learning Objectives

By the end of this lesson you will be able to:

Display the contents of files
Read large text files safely
View the beginning and end of files
Monitor log files
Use Linux manual pages
Find help for commands
Review previously executed commands

Creating Test Files

Before learning how to view files, create some example files.

Create a directory:

mkdir file-demo

Move into it:

cd file-demo

Create a file:

touch example.txt

Add some content:

echo "Linux command line training" > example.txt

Check the file exists:

ls

Viewing File Contents

The cat Command

The simplest way to display a file is using:

cat filename

Example:

cat example.txt

Output:

Linux command line training

cat is useful for small files.

Viewing Line Numbers

Use:

cat -n filename

Example:

cat -n example.txt

Output:

1  Linux command line training

Line numbers are useful when troubleshooting configuration files.

Reading Large Files

Large files can contain thousands of lines.

Examples:

Log files
Application configuration files
System reports

Using cat on very large files can make the terminal difficult to use.

For large files, use:

less

The less Command

Example:

less /var/log/syslog

less allows you to scroll through files.

Useful keys:

Key	Action
Space	Next page
b	Previous page
↑	Move up
↓	Move down
/	Search
q	Quit

Example search:

/error

finds occurrences of the word:

error

Viewing the Beginning of Files

The head command displays the first lines of a file.

Example:

head example.txt

By default it shows the first 10 lines.

To show a specific number:

head -20 example.txt

Displays the first 20 lines.

Viewing the End of Files

The tail command displays the last lines of a file.

Example:

tail example.txt

Useful for checking recent log entries.

Example:

tail /var/log/syslog

Monitoring Log Files

Linux systems generate many log files.

Examples:

/var/log/syslog
/var/log/auth.log

To watch a log file as it changes:

tail -f /var/log/syslog

The -f option means:

follow

The terminal will continue displaying new entries.

Exit with:

Ctrl + C

Getting Help in Linux

Linux has several built-in ways to find help.

The man Command

man displays the manual page for a command.

Example:

man ls

Output includes:

Command description
Available options
Examples
Related commands

Searching Manual Pages

Example:

man -k password

Searches available manual pages containing:

password

Command Help Options

Many commands provide their own help.

Example:

ls --help

Output:

Usage:
 ls [OPTION]... [FILE]...

Common help options:

--help
-h
-help

Finding Command Locations

The which command shows where a command is installed.

Example:

which ls

Output:

/usr/bin/ls

Understanding Commands

The whatis command provides a short description.

Example:

whatis ls

Output:

ls (1) - list directory contents

Command History

Linux stores previously executed commands.

View your history:

history

Example:

101 ls
102 cd /etc
103 systemctl status ssh

Re-running Previous Commands

Use the up arrow:

↑

to cycle through previous commands.

Searching History

Press:

Ctrl + R

Then type part of a previous command.

Example:

ssh

Linux will search your command history.

Combining Commands

Linux commands can be combined using pipes.

A pipe:

passes the output of one command into another.

Example:

ls /etc | less

This sends the output of ls into less.

Useful when commands produce a lot of output.

Practical Exercise

Task 1 - Create a Log File

Create:

touch application.log

Add some content:

echo "Application started" >> application.log
echo "Connection successful" >> application.log
echo "Error detected" >> application.log

Task 2 - View the File

Display the contents:

cat application.log

Task 3 - Use Different Viewing Commands

Try:

head application.log

tail application.log

less application.log

Task 4 - Find Help

Find information about:

grep

using:

man grep

and:

grep --help

Task 5 - Review History

Display your command history:

history

Search for a previous command:

Ctrl + R

Knowledge Check

Which command displays the contents of a small file?
Which command is better for reading large files?
What command displays the last lines of a file?
How do you monitor a log file as it changes?
What command displays documentation for Linux commands?
What does the pipe character (|) do?

Summary

In this lesson you learned:

✓ How to display file contents using cat
✓ How to read large files using less
✓ How to view file beginnings and endings using head and tail
✓ How to monitor logs using tail -f
✓ How to use Linux manual pages
✓ How to find command help
✓ How to use command history
✓ How to combine commands using pipes

Next lesson:

Lesson 04 - Practical Command Line Lab