cat (display file contents) in Linux CLI - Commands & Configuration

Introduction

Sometimes you want to quickly see what is inside a file without opening a text editor. The cat command shows the contents of a file right in the terminal window.

When you want to read a short text file quickly without opening an editor.

When you want to combine multiple files into one by displaying them together.

When you want to check the contents of a configuration file to verify settings.

When you want to pipe the contents of a file into another command for processing.

When you want to create a new file by typing text directly into the terminal.

Commands

Create a file named greetings.txt with the text 'Hello, world!' inside. This sets up a file to display with cat.

Terminal

echo "Hello, world!" > greetings.txt

Expected Output

No output (command runs silently)

Display the contents of greetings.txt in the terminal so you can read the text inside.

Terminal

cat greetings.txt

Expected Output

Hello, world!

Create another file named welcome.txt with some text to demonstrate showing multiple files.

Terminal

echo "Welcome to Linux." > welcome.txt

Expected Output

No output (command runs silently)

Display the contents of both greetings.txt and welcome.txt one after the other in the terminal.

Terminal

cat greetings.txt welcome.txt

Expected Output

Hello, world!

Welcome to Linux.

Create a new file named newfile.txt by typing text directly into the terminal. Press Ctrl+D to save and exit.

Terminal

cat > newfile.txt

Expected Output

No output (command runs silently)

Display the contents of the newly created file to verify the text was saved correctly.

Terminal

cat newfile.txt

Expected Output

This is a new file created by cat.

Key Concept

If you remember nothing else from cat, remember: it shows file contents quickly and can combine or create files using simple commands.

Common Mistakes

Trying to use cat on a file that does not exist.

cat will show an error because it cannot find the file to display.

Check the file name and path carefully before using cat.

Pressing Enter instead of Ctrl+D when creating a file with 'cat > filename'.

The file will not save and exit until Ctrl+D is pressed, so it seems stuck.

Press Ctrl+D to signal the end of input and save the file.

Using cat on very large files without paging.

The terminal will flood with text making it hard to read or control.

Use commands like 'less' or 'more' to view large files page by page.

Summary

Use 'cat filename' to quickly display the contents of a file in the terminal.

You can show multiple files at once by listing them after cat.

You can create a new file by typing 'cat > filename' and entering text, then pressing Ctrl+D.

Practice

1. What does the cat command do in Linux?

easy

A. Deletes a file permanently

B. Displays the contents of a file on the terminal

C. Changes file permissions

D. Creates a new directory

Solution

1. Step 1: Understand the purpose of cat

   The cat command is used to read and display the contents of files in the terminal.

2. Step 2: Compare with other options

   Deleting files, changing permissions, or creating directories are done by other commands like rm, chmod, and mkdir.

3. Final Answer:

   Displays the contents of a file on the terminal -> Option B

4. Quick Check:

   cat shows file contents [OK]

Hint: Remember: cat = concatenate and show file content [OK]

Common Mistakes:

• Confusing cat with rm or mkdir
• Thinking cat edits files
• Assuming cat deletes files

2. Which of the following is the correct syntax to display the contents of a file named notes.txt using cat?

easy

A. cat notes.txt

B. cat -f notes.txt

C. cat > notes.txt

D. cat -r notes.txt

Solution

1. Step 1: Identify the basic syntax of cat

   The basic command to display a file is cat filename, so cat notes.txt is correct.

2. Step 2: Understand incorrect options

   -f and -r are not valid options for cat. The cat > filename syntax is used to create or overwrite a file, not display it.

3. Final Answer:

   cat notes.txt -> Option A

4. Quick Check:

   Basic syntax is cat filename [OK]

Hint: Use just 'cat filename' to show file content [OK]

Common Mistakes:

• Adding invalid options like -f or -r
• Using > which overwrites files
• Confusing cat with other commands

3. What will be the output of the command cat -n file.txt if file.txt contains:

Hello
World

medium

A. Hello
World

B. Hello 1
World 2

C. Error: invalid option

D. 1 Hello
2 World

Solution

1. Step 1: Understand the -n option

   The -n option numbers each line of the output when displaying file contents.

2. Step 2: Apply numbering to each line

   Given the file has two lines: "Hello" and "World", the output will prefix each line with its line number: "1 Hello" and "2 World".

3. Final Answer:

   1 Hello 2 World -> Option D

4. Quick Check:

   cat -n adds line numbers [OK]

Hint: Use -n to add line numbers to output [OK]

Common Mistakes:

• Ignoring line numbers
• Placing numbers after text
• Thinking -n is invalid

4. You run cat file1.txt file2.txt but get an error: cat: file2.txt: No such file or directory. What is the most likely cause?

medium

A. You used the wrong command syntax

B. file1.txt is empty

C. file2.txt does not exist in the current directory

D. You do not have permission to read file1.txt

Solution

1. Step 1: Analyze the error message

   The error says file2.txt is missing, so the system cannot find it in the current directory.

2. Step 2: Check other options

   Empty file1.txt or permission issues on file1.txt would not cause this error. The syntax is correct for displaying multiple files.

3. Final Answer:

   file2.txt does not exist in the current directory -> Option C

4. Quick Check:

   Missing file causes 'No such file' error [OK]

Hint: Check file existence if 'No such file' error appears [OK]

Common Mistakes:

• Assuming syntax error
• Ignoring missing file
• Blaming permissions wrongly

5. You want to combine the contents of chapter1.txt and chapter2.txt into a new file book.txt. Which command correctly does this using cat?

hard

A. cat chapter1.txt chapter2.txt > book.txt

B. cat > chapter1.txt chapter2.txt > book.txt

C. cat chapter1.txt > chapter2.txt > book.txt

D. cat book.txt > chapter1.txt chapter2.txt

Solution

1. Step 1: Understand output redirection with cat

   The command cat file1 file2 > newfile concatenates file1 and file2 contents and writes to newfile.

2. Step 2: Analyze each option

   cat chapter1.txt chapter2.txt > book.txt correctly concatenates chapter1.txt and chapter2.txt into book.txt. Options A, C, and D misuse redirection or file order, causing errors or overwrites.

3. Final Answer:

   cat chapter1.txt chapter2.txt > book.txt -> Option A

4. Quick Check:

   Use cat file1 file2 > newfile to combine files [OK]

Hint: Use > after files to save combined output [OK]

Common Mistakes:

• Misplacing > causing overwrite
• Using > multiple times incorrectly
• Confusing input and output files