Linux cat Command: Syntax, Options, Real Examples

The linux cat command is one of those tools you use every single day without thinking about it. Then one day you realize you only know 20% of what it does. Here’s the other 80%.

What Is the Linux cat Command?

cat stands for concatenate. Its original purpose was to join files together, but it’s become the go-to command for quickly viewing file contents. It reads one or more files and writes them to standard output, which you can view in the terminal or redirect elsewhere.

Where cat Fits in Daily Linux Work

You’ll reach for cat when you need to quickly read a config file, check a log snippet, or verify a file was written correctly. For longer files, less or more are better. cat is for short files where you want the whole thing at once without paging.

cat Command Syntax

cat [OPTIONS] [FILENAME...]

Without a filename, cat reads from standard input until you press Ctrl+D. With one or more filenames, it reads and prints them in order.

How to Use the cat Command

Display File Contents

The most common use case:

cat /etc/hostname

cat /var/log/syslog

For multiple files, cat displays them sequentially in the order provided.

Concatenate Multiple Files

This is what cat was actually designed for. Combine files into one:

cat file1.txt file2.txt > combined.txt

The > redirects the combined output to a new file. Without it, the combined content prints to the terminal.

Add Line Numbers with -n

The -n flag numbers every output line, which is useful when referencing specific lines in config files or scripts:

cat -n /etc/nginx/nginx.conf

The -b flag numbers only non-blank lines if you want to skip counting empty ones.

Suppress Blank Lines with -s

The -s flag squeezes multiple consecutive blank lines into a single blank line. Useful with verbose config files or log outputs that have excessive whitespace:

cat -s verbose_config.txt

Show Non-Printing Characters with -A

The -A flag shows tabs as ^I and line endings as $ characters. This is invaluable when debugging Windows-formatted files uploaded to a Linux server, where hidden carriage returns (^M) break scripts:

cat -A script.sh

If you see ^M at the end of every line, the file has Windows line endings. Fix it with dos2unix.

Create and Append to Files with cat

Create a New File

Use cat with redirection and a heredoc to create a file interactively:

cat > newfile.txt

Type your content, then press Ctrl+D to save and exit. The file gets created with whatever you typed.

Append to an Existing File

Use >> instead of > to append without overwriting:

cat >> existing.txt

Type new content, Ctrl+D to finish. This adds to the end of the file.

cat with Pipes and Redirects

cat works well as the start of a pipeline when you want to feed a file into another command:

cat access.log | grep "404" | wc -l

That said, many commands accept filenames directly (grep “404” access.log), making the cat unnecessary. It’s a style choice for readability versus efficiency.

When NOT to Use cat: Common Misuse

The ‘useless use of cat’ is a real thing in Linux scripting circles. Piping cat into commands that can read files directly adds a pointless process fork:

• cat file.txt | grep pattern  →  use grep pattern file.txt instead
• cat file.txt | head -20  →  use head -20 file.txt instead
• cat file.txt | wc -l  →  use wc -l file.txt instead

These aren’t wrong, but they’re unnecessary overhead. On large files or in tight loops, it adds up.

FAQ: Linux cat Command