cat Command in Linux: Complete Guide to Display and Concatenate Files

The cat command is one of the most fundamental and frequently used commands in Linux and Unix-like operating systems. Short for “concatenate,” this versatile command allows you to display file contents, combine multiple files, and perform various text manipulation tasks directly from the terminal.

What is the cat Command?

The cat command reads files sequentially and writes their content to standard output (usually your terminal screen). Originally designed to concatenate files, it has evolved into a multi-purpose tool for file viewing and manipulation.

Basic Syntax

cat [OPTION]... [FILE]...

Basic Usage Examples

Displaying File Contents

The most common use of cat is to display the contents of a file:

cat filename.txt

Example Output:

$ cat welcome.txt
Welcome to CodeLucky.com
Learn Linux commands with practical examples
Happy coding!

Displaying Multiple Files

You can display multiple files consecutively by listing them as arguments:

cat file1.txt file2.txt file3.txt

Example Output:

$ cat header.txt content.txt footer.txt
=== HEADER ===
This is the main content of our document.
It contains important information.
=== FOOTER ===

File Concatenation

Combining Files into a New File

To concatenate multiple files and save the result to a new file:

cat file1.txt file2.txt > combined.txt

Example:

$ cat part1.txt part2.txt > complete_document.txt
$ cat complete_document.txt
Chapter 1: Introduction
This is the first part of our document.

Chapter 2: Implementation
This is the second part with detailed steps.

Appending Files

Use the append operator >> to add content to an existing file:

cat additional.txt >> existing_file.txt

Essential cat Command Options

-n (Number Lines)

Display line numbers for all lines:

cat -n filename.txt

Example Output:

$ cat -n sample.txt
     1	First line of the file
     2	Second line with content
     3	Third line here
     4	Last line of the file

-b (Number Non-Empty Lines)

Number only non-empty lines:

cat -b filename.txt

Example Output:

$ cat -b sample.txt
     1	First line of the file
     
     2	Third line after empty line
     3	Last line of the file

-s (Squeeze Blank Lines)

Suppress multiple consecutive empty lines:

cat -s filename.txt

Example Output:

$ cat -s document.txt
Line 1

Line 2 (multiple empty lines above squeezed to one)

Final line

-A (Show All Characters)

Display all characters including non-printing characters:

cat -A filename.txt

Example Output:

$ cat -A sample.txt
Line with tab^Icharacter$
Line with spaces    $
Line with newline$

-T (Show Tabs)

Display tab characters as ^I:

cat -T filename.txt

-E (Show Line Ends)

Display $ at the end of each line:

cat -E filename.txt

Advanced Use Cases

Creating Files with cat

Create a new file using cat with input redirection:

cat > newfile.txt
Type your content here
Press Ctrl+D to save and exit

Example:

$ cat > config.txt
server=localhost
port=8080
debug=true
^D
$ cat config.txt
server=localhost
port=8080
debug=true

Using cat with Here Documents

Create multi-line content using here documents:

cat << EOF > script.sh
#!/bin/bash
echo "Hello, World!"
date
EOF

Reading from Standard Input

Use cat without arguments to read from standard input:

$ cat
Hello, this is input
Hello, this is input
Press Ctrl+D to exit
Press Ctrl+D to exit

Practical Examples and Workflows

Log File Analysis

Combine multiple log files for analysis:

cat /var/log/app1.log /var/log/app2.log > combined_logs.txt

Configuration File Backup

Create backups by concatenating with timestamp:

cat config.conf > "config.conf.backup.$(date +%Y%m%d)"

Quick File Comparison

View files side by side for comparison:

cat file1.txt && echo "--- SEPARATOR ---" && cat file2.txt

Performance Considerations

Large File Handling

For large files, consider using alternatives:

• less or more for pagination
• head to view first few lines
• tail to view last few lines

# View first 20 lines
head -20 largefile.txt

# View last 20 lines  
tail -20 largefile.txt

# Interactive viewing
less largefile.txt

Memory Usage

The cat command loads the entire file content into memory, which may not be suitable for very large files (several GBs).

Common Pitfalls and Solutions

Binary Files

Avoid using cat on binary files as it can corrupt your terminal display:

# Check file type first
file suspicious_file.bin

# Use hexdump for binary files
hexdump -C binary_file.bin | head

Overwriting Files

Be careful with output redirection to avoid accidentally overwriting files:

# Wrong: This will create an empty file
cat important_file.txt > important_file.txt

# Correct: Use a different output file
cat important_file.txt > important_file_copy.txt

Combining cat with Other Commands

Using Pipes

Combine cat with other commands using pipes:

# Count lines in multiple files
cat file1.txt file2.txt | wc -l

# Search in multiple files
cat *.log | grep "ERROR"

# Sort combined content
cat data1.txt data2.txt | sort > sorted_data.txt

Text Processing Pipeline

# Complex text processing
cat access.log | grep "404" | awk '{print $1}' | sort | uniq -c | sort -nr

Alternative Commands

While cat is versatile, other commands might be more appropriate for specific tasks:

Security Considerations

File Permissions

Ensure you have read permissions for files you want to display:

ls -la filename.txt
-rw-r--r-- 1 user group 1024 Aug 24 23:30 filename.txt

Sensitive Data

Be cautious when using cat on files containing sensitive information, especially in shared environments or when your terminal session might be logged.

Conclusion

The cat command is an essential tool in any Linux user’s toolkit. From simple file viewing to complex file concatenation tasks, mastering cat and its various options will significantly improve your command-line productivity. Remember to choose the appropriate tool for each task and consider performance implications when working with large files.

Practice these examples in your own Linux environment to become proficient with the cat command. As you become more comfortable, you’ll find countless creative ways to integrate cat into your daily Linux workflows.