cat Command in Linux: Syntax, Examples & Cheat Sheet (Beginner Friendly)

Tested on Ubuntu 22.04 / 24.04 and Debian 12

The cat command in Linux and Unix is one of the most basic yet powerful utilities for working with text files. The name cat comes from concatenate, which explains its primary role: reading, combining, and displaying file contents.

At its simplest, cat prints the contents of one or more files to standard output. With options, it can also show line numbers, highlight tabs and trailing spaces, and even help create or append files.

This article is written as a beginner-friendly cat command cheat sheet, optimized for quick learning and real-world usage.

How to use cat command

The cat command is commonly used to:

• View small text files quickly
• Combine multiple files into one
• Create or append files using standard input
• Inspect invisible characters such as tabs or trailing spaces

Although cat is not a full text editor, it is often used with redirection operators (> and >>) for simple file creation and updates.

When you should NOT use cat

Although the cat command is widely used, it is not always the best choice:

• Avoid cat largefile | grep ... — use grep file directly
• Avoid cat file | while read — prefer while read < file
• Avoid using cat for paging — use less or more

cat is best suited for small files, debugging, concatenation, and inspection, not for processing very large files.

cat Command Syntax and Overview

cat command syntax

cat [OPTION] [FILE]...

You can run cat:

• Without options
• With one or more options
• With one or multiple files
• With standard input (-)

Common cat Command Options

cat Command – Quick Cheat Sheet

cat vs less vs more (When to Use What)

These commands serve different purposes for viewing file content.

Key differences:

• cat prints everything at once and exits
• less supports scrolling, searching, and jumping
• more only allows forward navigation

Recommendation:

• Use cat for simple output and pipelines
• Use less for almost all interactive viewing
• Use more only on minimal systems where less is unavailable

Basic File Viewing with cat

Display File Contents Using cat

cat file.txt

This prints the entire contents of file.txt to the terminal.

Show Line Numbers with cat

We can use --number or -n argument to print the line numbers of the file:

But to get the number of lines excluding and blank or empty lines we use -b or --number-nonblank:

Completely empty lines are skipped, so line numbers may jump when blank lines exist in the file.

NOTE: -b takes priority over -n when both are used.

Number Non-Blank Lines Using cat

Use the -b option to number only non-empty lines. Blank lines are not counted.

Command:

cat -b file.txt

Explanation:

• -b numbers only lines that contain content
• Blank lines are printed without line numbers
• Useful when blank lines should not affect numbering

Difference from -n:

• -n → numbers all lines (including empty ones)
• -b → numbers only non-blank lines

Display TAB Characters Using cat

To reveal tab characters inside a file:

cat -T file.txt

Tabs will appear as ^I, which is useful for debugging formatting issues.

Sample output:

here I have used^ITAB
here I added multiple TAB^I^I

Show End-of-Line Characters (Trailing Spaces)

Trailing whitespaces can break scripts and configs. We can use -E or --show-ends arguments with cat command to add $ (dollar sign) at the end of the line which can show us trailing whitespace (if any).

cat -E file1 | grep '[[:blank:]]\$'

Each line will end with a $ symbol.

Sample output:

here I added extra space at the end of line $
here I added multiple extra whitespaces at the end of line  $

Creating and Modifying Files Using cat

Create a New File Using cat Command

You can create a new file and add content interactively:

cat > newfile
first line
second line
Ctrl+D

Append Data to a File Using cat

Use >> to append content instead of overwriting:

cat >> fruits.txt
apple
mango
Ctrl+D

Append more data later:

cat >> fruits.txt
guava
grapes
Ctrl+D

Replace Multiple Empty Lines with a Single Empty Line

Use -s or --squeeze-blank with cat command to suppress repeated empty output lines.

For example here I have added multiple empty lines in my /etc/hosts file:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
192.168.43.48   controller     controller.example.com
192.168.43.49   worker-1       worker-1.example.com
<-- Empty Line 1-->
<-- Empty Line 2-->
<-- Empty Line 3-->
192.168.43.50   worker-2       worker-2.example.com
192.168.43.50   sample.example.com

Let’s replace repeated empty lines with single empty line:

Merge Multiple Empty Lines

To squeeze repeated blank lines into a single line:

cat -s file.txt

This improves readability when viewing logs or large text files.

Suppress Repeated Empty Lines Using -s

The -s option collapses multiple consecutive empty lines into a single empty line.

Command:

cat -s file.txt

Explanation:

• Replaces multiple blank lines with one
• Helps clean poorly formatted files
• Commonly used when preprocessing text output

Working with Multiple Files

Concatenate Multiple Files Using cat

You can combine multiple files:

cat file1 file2 file3

To save the combined output:

cat file1 file2 > merged.txt

Insert Input Between Files Using -

You can insert custom input while merging files:

cat file1 - file2 > output.txt

⚠️ The terminal waits for user input at -. Press Ctrl+D to continue.

This technique is powerful but requires careful tracking of input order.

Standard Input and Pipelines

Read from Standard Input with cat

Use - to read from standard input:

cat -

Everything you type is echoed back until you press Ctrl+D.

Using cat with Pipes (stdin → stdout)

cat is often used in pipelines to pass input from one command to another.

Read from standard input:

cat

Pipe input into cat:

echo "Hello World" | cat

Combine cat with other commands:

cat file.txt | grep "error"
cat file.txt | awk '{ print $1 }'

Use cat to merge streams:

cat file1.txt file2.txt | sort

Explanation:

• cat reads from stdin and writes to stdout
• Acts as a bridge between commands in pipelines
• Useful for combining, redirecting, or inspecting streams

View Binary Files Safely with cat

Using cat directly on binary files can corrupt the terminal or display unreadable characters. Use safe alternatives when inspection is needed.

Avoid:

cat binaryfile

Safer options:

cat binaryfile | less
cat binaryfile | hexdump -C
cat binaryfile | strings

Explanation:

• less prevents terminal corruption
• hexdump shows a structured hex view
• strings extracts readable text from binaries

Tip: If the terminal becomes unreadable, reset it using:

reset

cat Command in Real-World Usage

Using cat command in loops

In production environments, cat is often used with loops to process file contents line by line.

Example: Extract usernames and UIDs from /etc/passwd:

for i in $(cat /etc/passwd); do
  USERNAME=$(echo "$i" | cut -d: -f1)
  UID=$(echo "$i" | cut -d: -f3)
  echo "User: $USERNAME | UID: $UID"
done

Tip: For large files, prefer while read loops for better performance.

Using cat in Shell Scripts

The cat command is commonly used in shell scripts to read files, combine content, or feed input into other commands.

Read a file inside a script:

cat config.txt

Assign file content to a variable (small files only):

content=$(cat file.txt)

Concatenate multiple files into one:

cat part1.txt part2.txt > combined.txt

Use cat with here-documents:

cat <<EOF > message.txt
Hello User,
This is a generated file.
EOF

When NOT to Use cat (Use less, head, tail Instead)

While cat is useful, it is not always the right tool.

Avoid using cat when:

• Viewing very large files
• You need interactive scrolling or searching
• You only need the start or end of a file

Better alternatives:

• Use less for interactive viewing less largefile.log
• Use head to view the first lines head file.txt
• Use tail to view the last lines tail file.txt
• Use tail -f to follow logs in real time tail -f app.log

Rule of thumb:

• Small files → cat
• Large or interactive viewing → less
• Partial content → head or tail

[[AQ]]

Further Reading

• man page for cat command

What’s Next

• tar command examples
• grep command examples