CIS120 Book

CIS120 Linux Fundamentals by Scott Shaper

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.

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

Option	What It Does	When to Use It
`-n`	Numbers all lines	When you need to reference specific lines
`-v`	Shows non-printing characters	When you need to see special characters
`-b`	Numbers only non-blank lines	When you want to ignore empty lines
`-s`	Removes extra blank lines	When your file has too many empty lines
`-E`	Shows $ at the end of each line	When you need to see line endings
`-T`	Shows tabs as ^I	When you need to see tab characters
`-A`	Shows all special characters	When debugging file formatting

Practical Examples

Viewing a File

The most basic use of cat is to display a file's contents:

cat notes.txt

This shows everything in notes.txt on your screen.

Viewing Multiple Files

You can view several files at once:

cat file1.txt file2.txt

This shows the contents of both files, one after the other.

Creating a New File

To create a new file and type its contents:

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

This creates newfile.txt with whatever you type. Press Ctrl+D to save and exit.

Adding to a File

To add text to the end of an existing file:

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

This adds your new text to the end of existing.txt.

Viewing with Line Numbers

To see a file with line numbers:

cat -n notes.txt

This shows each line with its number, making it easier to reference specific parts.

Viewing Special Characters

To see all special characters (like tabs and line endings):

cat -A notes.txt

This shows:

$ at the end of each line
^I for tab characters
Other special characters

Viewing Non-Printing Characters

Sometimes text files contain characters that you can't see normally, like special formatting or control codes. The -v option makes these invisible characters visible:

cat -v notes.txt

For example, if your file contains:

A tab character, it will show as ^I
A Windows-style line ending, it will show as ^M

Note: The -A option is actually more comprehensive than -v. It's equivalent to using -vET (showing non-printing characters, line endings with $, and tabs as ^I). Use -A when you want to see everything, or -v when you only need to see non-printing characters without the line ending $ symbols.

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