cat: See What's in a File

You want to see what's in a file. You could open it in an editor. Or:

cat file.txt

The entire contents appear in your terminal. That's cat.

The name comes from "concatenate" - its original purpose was joining files together. But 99% of the time, you're just using it to read stuff.

Reading a File

cat readme.txt

The whole file, printed to your screen.

Reading Multiple Files

cat file1.txt file2.txt

Both files, one after the other. This is the "concatenate" part.

Starting a Pipeline

cat is often how you feed data into other commands:

cat log.txt | grep "error"

Read the file, then filter for errors.

cat data.txt | sort | uniq -c

Read the file, then run it through analysis.

Wildcards

cat .txt

All text files in the current directory, concatenated.

That's Really It

cat is dead simple. It reads files and outputs them. The power comes from what you pipe that output into.

Quick Reference

| What you want | Command | |---------------|---------| | Read a file | cat file.txt | | Read multiple files | cat file1.txt file2.txt | | All matching files | cat .log | | Start a pipeline | cat file \| grep pattern |

See Also

• head - just the beginning

• tail - just the end

• grep - search within

Practice

In CTF challenges, cat flag.txt is often how the challenge ends. Simple, but satisfying.

cat is the "hello world" of file reading. Everyone starts here.