cat Command in Linux: View and Join Files

Quick take: The cat command prints file contents to the terminal. Use it to view files, join them together, or create small files with redirection. Add -n to number lines and -A to reveal hidden characters.

Introduction

The cat command (short for concatenate) reads files and writes their contents to standard output. It is the quickest way to dump a file to the screen, and combined with redirection it can join files or create new ones.

This guide covers viewing files, numbering lines, revealing non-printing characters, and using cat with redirection — plus when to reach for less instead.

Syntax

The basic syntax of the cat command is:

cat [OPTIONS] [FILE...]

Common Options and Parameters

The most useful options and parameters for the cat command:

Practical Examples

Real cat commands you can run today:

# Print a file to the screen
cat notes.txt
# View a file with line numbers
cat -n script.sh
# Join two files into one
cat part1.txt part2.txt > whole.txt
# Append one file to another
cat extra.txt >> log.txt
# Create a small file from the terminal (Ctrl+D to finish)
cat > todo.txt
# Reveal hidden characters and line ends
cat -A config.conf

cat vs less, head, and tail

cat prints an entire file at once, which is perfect for small files but wrong for large ones — you lose the top before you can read it. Knowing the alternatives keeps you efficient:

• less file — scroll a large file page by page, search with /, and quit with q.
• head -n 20 file — see just the first 20 lines.
• tail -n 20 file — see the last 20 lines; tail -f follows a growing log live.

Reach for cat when you want the whole (small) file or need to concatenate; reach for less, head, or tail when the file is large or you only need part of it.

Common Mistakes and the Useless Use of cat

A frequent habit worth dropping is the “useless use of cat” — piping cat into another command that already accepts a filename. Writing cat file | grep error spawns an extra process for no benefit; grep error file is cleaner and faster. The same applies to wc, sort, and most text tools.

The other pitfall is using cat on a binary file, which floods the terminal with control characters and can leave it in a broken state. If that happens, run reset to restore it. To inspect a binary safely, use file to identify it or xxd to view a hex dump instead.

Tips and Best Practices

• For large files use less filename instead — cat dumps everything at once and you lose the top of the file.
• cat -A is invaluable for spotting stray tabs, trailing spaces, or Windows line endings that break scripts and config files.
• Avoid the “useless use of cat”: instead of cat file | grep x, write grep x file.

Final Thoughts

cat is the fastest way to see what is in a file and a handy tool for joining files together. Remember -n for line numbers and -A for debugging invisible characters, and switch to less when a file is too big to scroll past. It works hand in hand with grep, redirection, and the shell pipeline.