cat (display file contents) in Linux CLI - Step-by-Step Execution

Concept Flow - cat (display file contents)

Start

↓

Run 'cat filename'

↓

Shell reads file

↓

File contents sent to stdout

↓

Display contents on screen

↓

End

The 'cat' command reads a file and shows its contents on the screen step-by-step.

Execution Sample

Linux CLI

cat example.txt

This command shows the contents of 'example.txt' on the screen.

Execution Table

Step	Action	Input	Output	Notes
1	Run command	cat example.txt	None	Command starts
2	Shell opens file	example.txt	File data stream	Reads file contents
3	Send data to stdout	File data stream	Text lines	Data flows to screen
4	Display output	Text lines	Visible file contents	User sees file text
5	End	None	None	Command finishes

💡 File fully read and displayed, command ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
file_data	None	File contents loaded	Sent to stdout	Displayed on screen

Key Moments - 2 Insights

Why does 'cat' show the whole file at once instead of line by line?

The execution_table row 3 shows 'cat' reads the entire file data stream before sending it to the screen, so it outputs all content continuously.

What happens if the file does not exist?

The shell cannot open the file (step 2 fails), and 'cat' shows an error message instead of file contents.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 3?

A Text lines

B Visible file contents

C None

D Error message

💡 Hint

Check the 'Output' column at step 3 in the execution_table.

At which step does the file contents become visible on the screen?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Notes' column for when the user sees the file text.

If the file is empty, what would the output be at step 4?

A Error message

B Visible file contents with text

C No output (blank screen)

D Partial file contents

💡 Hint

Refer to variable_tracker for file_data state and execution_table step 4 output.

Concept Snapshot

cat filename

- Reads the whole file named 'filename'
- Sends contents to screen (stdout)
- Shows file text exactly as is
- If file missing, shows error
- Useful to quickly view file contents

Full Transcript

The 'cat' command in Linux reads a file and displays its contents on the screen. When you run 'cat filename', the shell opens the file, reads all its data, sends it to the screen, and then finishes. If the file does not exist, an error message appears instead. This command is simple and useful to quickly see what's inside a file.

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