The cat Command

Think of the cat command as your Swiss Army knife for working with text files in Linux. It's short for "concatenate" (which means to join things together), but it can do much more than just combine files. You can use it to view file contents, create new files, and even add text to existing files.

The examples below use the course setup. In ~/playground/chapter2 run cd ~/playground/chapter2 before trying the practice exercises at the bottom of the page.

Why Learn the cat Command?

The cat command is essential because:

• It's one of the most basic and frequently used commands
• It helps you quickly view file contents
• It's useful for creating and modifying text files
• It's often used in scripts and automation

Basic Syntax

cat [options] [file1] [file2] ...

Common Options

Practical Examples

cat notes.txt

cat file1.txt file2.txt

cat > newfile.txt
Type your text here
Press Ctrl+D when done

cat >> existing.txt
This text will be added to the end
Press Ctrl+D when done

You can also create mutli-line files with the << 'EOF' syntax. This is a here document. It is a way to create a file with multiple lines of text.

cat > newfile.txt << 'EOF'
This is the first line
This is the second line
This is the third line
EOF

This creates newfile.txt with the three lines. NOTE: When using the << 'EOF' syntax, you do not need to use the Ctrl+D command to save and exit. The EOF is the end of the file just press the enter key to save and exit.

cat -n notes.txt

cat -A notes.txt

cat -v notes.txt

Practice Exercises

These exercises use practice files that are created by the course setup script. Make sure you are in the ~/playground/chapter2 directory before you start the exercises.

Practice these cat commands:

1. Basic file viewing
   • cat hello.txt - View the contents of hello.txt
   • cat file1.txt - View file1.txt
   • cat file2.txt - View file2.txt
2. View files with line numbers
   • cat -n hello.txt - View hello.txt with line numbers
   • cat -n file1.txt - View file1.txt with line numbers
   • What line number is "Hello, World!" on?
3. View multiple files together
   • cat file1.txt file2.txt - View both files at once
   • cat hello.txt file1.txt file2.txt - View all three files
   • Notice how the contents are displayed one after the other
4. Practice with line numbering options
   • cat -n mixed.txt - See all lines numbered (including blank lines)
   • cat -b mixed.txt - See only non-blank lines numbered
   • Compare the output of both commands. What's the difference?
5. View special characters
   • cat special.txt - View it normally
   • cat -A special.txt - See the tab character displayed as ^I
   • cat -T special.txt - See only tabs as ^I
   • cat -E special.txt - See only line endings with $
   • Notice the differences between these options
6. Combine files into a new file
   • cat file1.txt file2.txt > combined.txt - Combine the two files into a new file
   • cat combined.txt - Verify the contents
   • cat -n combined.txt - See the combined file with line numbers
7. Practice with the -s option
   • cat spaces.txt - See it normally with multiple blank lines
   • cat -s spaces.txt - See how multiple blank lines are reduced to single blank lines
   • Compare the output - notice the difference?
8. Debug formatting issues
   • cat debug.txt - View the file normally
   • cat -A debug.txt - View all special characters
   • Identify where tabs (^I) and line endings ($) appear
   • cat -v debug.txt - View non-printing characters
   • Compare -A and -v outputs
9. Add content to an existing file
   • cat >> hello.txt - Then type "This line was added later." and press Enter, then Ctrl+D
   • cat hello.txt - Verify the new line was added to the end
   • cat -n hello.txt - View with line numbers to see the new line
10. Challenge: Create a formatted report
    • cat header.txt body.txt footer.txt > report.txt - Combine all three into report.txt
    • cat report.txt - View the complete report
    • cat -n report.txt - View with line numbers
    • cat >> report.txt - Then type "Generated: $(date)" and press Enter, then Ctrl+D (or just type a timestamp manually)
    • cat report.txt - Verify the timestamp was added

Tips for completing these exercises:

• All commands assume you are in ~/playground/chapter2; use cd ~/playground/chapter2 if needed.
• If you need to reset the practice files (for example after modifying or deleting them), run setup.sh again.
• Use ls to see all the files in the directory
• Remember: > overwrites, >> appends
• When typing into a file with cat > or cat >>, press Ctrl+D when you're done

Tips for Success

• Use -n for large files: Line numbers help you keep track of where you are
• Be careful with >: It overwrites existing files
• Use >> to add safely: It adds to the end without deleting
• Press Ctrl+D: This is how you finish typing when creating or adding to files

Common Mistakes to Avoid

• Using > instead of >> when you want to add to a file
• Forgetting to press Ctrl+D when creating or adding to files
• Not checking file contents before overwriting
• Using cat on very large files (use less instead)

Best Practices

• Use line numbers (-n) when working with code or configuration files
• Use -A when debugging formatting issues
• Always double-check before using > to overwrite files
• For large files, consider using less or more instead