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LPI Linux Essentials Exam 010-160 - Topic 3.2 - Viewing Data with cat, head & tail
Introduction
When working with text files in Linux, a few commands almost always come into play. Among them, cat, head, and tail are frequently used for a variety of text manipulation tasks directly from the terminal. This guide aims to give you a comprehensive understanding of these commands, supplemented with a shared practice file for hands-on exercises.
Preparing a Practice File
First, let's create a practice file that you'll use throughout this guide for hands-on exercises. Execute the following command in your terminal:
echo -e "Line 1\nLine 2\nLine 3\nLine 4\nLine 5\n\n\n\n\nLine 6\nLine 7\nLine 8\nLine 9\nLine 10\nLine 11\nLine 12\nLine 13\nLine 14\nLine 15\nLine 16\nLine 17\nLine 18\nLine 19\nLine 20" > practice_file.txt
This command will generate a text file named practice_file.txt containing 24 lines, 20 of which state a line number and 4 lines in a row that are blank. This file will used in the later cat, head, and tail exercises.
cat Command
The cat command is short for concatenate. It is predominantly used for displaying file contents, but its functionality extends beyond that. It can also append text to existing ones, and combine multiple files together.
Use Cases:
- Displaying the content of files
- Concatenating multiple files together
Common Options:
| Option | Description |
|---|---|
-n |
Numbers all output lines |
-s |
Suppresses repeated empty output lines |
Exercises:
Use cat to display the contents of practice_file.txt:
cat practice_file.txt
Concatenate practice_file.txt with another file, /etc/hosts:
cat practice_file.txt /etc/hosts > combined_file.txt
View the contents of the new file combined_file.txt.
cat combined_file.txt
Add line numbers to the cat command output:
cat -n practice_file.txt
Suppress repeated empty blank lines:
cat -s practice_file.txt
head Command
The head command is utilized to read the beginning portions of text files. By default, it displays the first 10 lines, but this can be customized. It is especially useful when you wish to inspect large files without the need for a text editor.
Use Cases:
- Quickly viewing the initial part of large files
- Extracting the starting lines of files
- In pipelines for text manipulations
Common Options:
| Option | Description |
|---|---|
-c [num] |
Shows the first [num] bytes of a file |
-n [num] |
Shows the first [num] lines of a file |
Exercises:
Use head to display the first 10 lines of practice_file.txt:
head practice_file.txt
Display only the first 5 lines of practice_file.txt:
head -n 5 practice_file.txt
Show the first 15 bytes of practice_file.txt:
head -c 15 practice_file.txt
tail Command
The tail command essentially serves as the counterpart to head, displaying the end lines of a file. By default, the command shows the last 10 lines, but you can specify a different amount. The tail command can also monitor a file for new content in real-time, which is particularly helpful for keeping an eye on log files.
Use Cases:
- Real-time monitoring of log files
- Viewing the end lines of files
- Text manipulation in pipelines
Common Options:
| Option | Description |
|---|---|
-c [num] |
Displays the last [num] bytes of a file |
-f |
Follows the file and displays new lines as they appear |
-n [num] |
Displays the last [num] lines of a file |
Exercises:
Use tail to display the last 10 lines of practice_file.txt:
tail practice_file.txt
Show only the last 5 lines of practice_file.txt:
tail -n 5 practice_file.txt
Show the last 15 bytes of practice_file.txt:
tail -c 15 practice_file.txt
Follow practice_file.txt in real-time, observing new lines as they are added:
Create a process that will update practice_file.txt with new lines periodically.
count=1
until [ $count -eq 100 ]; do
echo count = $count >> practice_file.txt
sleep 1
count=$((count + 1))
done &
Watch as the file is updated every second:
tail -f practice_file.txt
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