diff for file comparison in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the diff command do in Linux?

The diff command compares two files line by line and shows the differences between them.

beginner

How do you use diff to compare two files named file1.txt and file2.txt?

Run diff file1.txt file2.txt. It will show lines that differ between the two files.

beginner

What does the output starting with < and > mean in diff?

Lines starting with < are from the first file, and lines starting with > are from the second file. They show what is different.

intermediate

What option can you add to diff to ignore case differences?

Use diff -i file1.txt file2.txt to ignore case differences when comparing.

beginner

How can diff help in real life?

It helps you quickly see what changed between two versions of a file, like comparing two drafts of a letter or code.

What does the diff command compare?

A Two users

B Two folders

C Two files line by line

D Two processes

diff compares two files line by line to show differences.

Which symbol in diff output shows lines from the second file?

A <

B >

C #

D *

Lines starting with > come from the second file.

How do you ignore case differences when using diff?

A -i

B -v

C -r

D -c

The -i option ignores case differences.

What will diff file1.txt file2.txt show if files are identical?

A Error message

B All lines

C File sizes

D No output

If files are the same, diff shows no output.

Which command shows differences between two files?

A diff

B cat

C ls

D grep

diff is used to compare files and show differences.

Explain how the diff command helps you compare two text files.

Think about how you spot changes between two versions of a note.

Describe a real-life situation where using diff would be useful.

Imagine you wrote a letter twice and want to see what you changed.

Practice

1. What does the diff command do in Linux?

easy

A. It copies files from one directory to another.

B. It deletes duplicate files automatically.

C. It merges two files into one.

D. It compares two files line by line and shows differences.

Solution

1. Step 1: Understand the purpose of diff

   The diff command is used to compare files line by line.

2. Step 2: Identify what diff outputs

   It shows the differences between two files, not deleting, merging, or copying.

3. Final Answer:

   It compares two files line by line and shows differences. -> Option D

4. Quick Check:

   diff compares files [OK]

Hint: Remember: diff shows differences, it does not merge or delete [OK]

Common Mistakes:

• Thinking diff merges files
• Confusing diff with copy or delete commands
• Assuming diff modifies files

2. Which of the following is the correct **basic** syntax to compare two files file1.txt and file2.txt using diff?

easy

A. diff file1.txt file2.txt

B. diff -c file1.txt file2.txt

C. diff file1.txt > file2.txt

D. diff -r file1.txt file2.txt

Solution

1. Step 1: Identify basic diff syntax

   The basic syntax to compare two files is diff file1 file2.

2. Step 2: Understand options

   diff -c file1.txt file2.txt uses -c (context), which is valid but not basic syntax. diff file1.txt > file2.txt redirects output incorrectly. diff -r file1.txt file2.txt (-r) is for directories.

3. Final Answer:

   diff file1.txt file2.txt -> Option A

4. Quick Check:

   Basic diff syntax [OK]

Hint: Basic diff syntax: diff file1 file2 [OK]

Common Mistakes:

• Using redirection instead of arguments
• Confusing directory options with file comparison
• Adding unnecessary flags for simple comparison

3. Given two files with these contents:

file1.txt:
apple
banana
cherry

file2.txt:
apple
banana
date

What will be the output of diff file1.txt file2.txt?

medium

A. 2c2
< banana
---
> banana

B. No output (files are identical)

C. 3c3
< cherry
---
> date

D. Error: files not found

Solution

1. Step 1: Compare lines one by one

   Lines 1 and 2 are the same (apple, banana). Line 3 differs: 'cherry' vs 'date'.

2. Step 2: Understand diff output format

   diff shows line number and change type: '3c3' means line 3 changed. Then shows old line with '<' and new line with '>'.

3. Final Answer:

   3c3 < cherry --- > date -> Option C

4. Quick Check:

   Line 3 differs [OK]

Hint: Look for first differing line number in diff output [OK]

Common Mistakes:

• Assuming diff shows all lines
• Confusing line numbers in output
• Expecting no output for different files

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

medium

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

B. You used wrong syntax for diff command.

C. file1.txt is empty.

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

Solution

1. Step 1: Analyze the error message

   The error says file2.txt: No such file or directory, meaning file2.txt is missing or path is wrong.

2. Step 2: Check other options

   Wrong syntax would cause different errors. Empty file1.txt or permission issues on file1.txt won't cause missing file2.txt error.

3. Final Answer:

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

4. Quick Check:

   Missing file error [OK]

Hint: Check file existence if diff says 'No such file' [OK]

Common Mistakes:

• Assuming syntax error causes missing file message
• Ignoring file path or directory
• Confusing permission errors with missing files

5. You want to compare two directories dir1 and dir2 recursively to find all file differences. Which diff command should you use?

hard

A. diff -c dir1 dir2

B. diff -r dir1 dir2

C. diff dir1 dir2

D. diff -i dir1 dir2

Solution

1. Step 1: Understand recursive comparison

   The -r option tells diff to compare directories recursively.

2. Step 2: Check other options

   -c is context output, not recursive. No option means file comparison only. -i ignores case, not recursive.

3. Final Answer:

   diff -r dir1 dir2 -> Option B

4. Quick Check:

   Recursive directory diff [OK]

Hint: Use -r to compare directories recursively with diff [OK]

Common Mistakes:

• Using -c expecting recursion
• Omitting -r for directory comparison
• Confusing -i with recursive option