diff for file comparison in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of diff command with two similar files

Given two files file1.txt and file2.txt with the following contents:

file1.txt:
apple
banana
cherry

file2.txt:
apple
banana
date

What is the output of the command diff file1.txt file2.txt?

A No output

B

2c2
< banana
---
> banana

C

3c3
< cherry
---
> date

D

1a2
> banana

💡 Hint

Look for the line number where the files differ and how diff shows changes.

The diff command shows that line 3 differs: 'cherry' in file1.txt is replaced by 'date' in file2.txt. The output format is 'c' followed by the differing lines.

💻 Command Output

intermediate

Using diff with -u option

What is the main difference in output when running diff -u file1.txt file2.txt compared to diff file1.txt file2.txt for two files with small differences?

A Only line numbers where files differ without content

B No output if files differ

C Output in reverse order of lines

D Unified diff format showing context lines with '+' and '-' prefixes

💡 Hint

The -u option is often used for patches and shows more context.

The -u option produces a unified diff format that shows lines before and after the change with '+' for additions and '-' for deletions, making it easier to understand changes.

💻 Command Output

advanced

Output of diff with binary files

What output does the command diff file1.bin file2.bin produce if both files are binary and differ?

A Line by line text differences

B Binary files file1.bin and file2.bin differ

C Syntax error

D No output

💡 Hint

diff handles binary files differently than text files.

When diff detects binary files that differ, it outputs a simple message stating the files differ without showing line differences.

🔧 Debug

advanced

Why does diff show no output for two different files?

You run diff fileA.txt fileB.txt but see no output, even though you know the files differ. Which of the following is the most likely reason?

A The files differ only in whitespace and diff is run with default options

B The files are identical

C The diff command is broken

D The files have different names

💡 Hint

No output from diff means the files are identical.

diff produces no output when the two files are identical. However, if the files differ only in whitespace and diff is run with default options, it may also produce no output.

🚀 Application

expert

Automate file difference report generation

You want to create a script that compares two directories dir1 and dir2 recursively and outputs a unified diff for each differing file into a single report file diff_report.txt. Which command pipeline best achieves this?

A diff -ur dir1 dir2 > diff_report.txt

B diff -r dir1 dir2 | tee diff_report.txt

C diff -u dir1 dir2 > diff_report.txt

D diff dir1 dir2 > diff_report.txt

💡 Hint

You need recursive comparison with unified diff format.

The -r option makes diff recursive, and -u produces unified diff format. Redirecting output to a file saves the report.

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