sort and uniq in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of combined sort and uniq commands

What is the output of the following command sequence on the file fruits.txt containing:

apple
banana
apple
orange
banana
apple

?

Linux CLI

sort fruits.txt | uniq

A

apple
apple
banana
banana
orange

B

apple
banana
orange

C

apple
banana
apple
orange
banana
apple

D

banana
apple
orange

💡 Hint

Remember that sort orders lines alphabetically and uniq removes adjacent duplicates.

The sort command arranges the lines alphabetically, so the file becomes:
apple
apple
apple
banana
banana
orange. Then uniq removes repeated adjacent lines, leaving one of each unique line in order.

💻 Command Output

intermediate

Effect of uniq without sorting

Given a file colors.txt with content:

red
blue
red
green
blue
red

What is the output of the command uniq colors.txt?

Linux CLI

uniq colors.txt

A

red
blue
red
green
blue
red
red

B

red
blue
green

C

red
blue
red
green
blue

D

red
blue
red
green
blue
red

💡 Hint

uniq only removes adjacent duplicate lines.

Since the file is not sorted, duplicate lines are not adjacent. uniq does not remove non-adjacent duplicates, so the output is the same as the input.

📝 Syntax

advanced

Correct usage of uniq to count duplicates

Which command correctly counts how many times each unique line appears in data.txt?

A sort data.txt | uniq -c

B uniq -c data.txt

C sort -c data.txt | uniq -c

D uniq data.txt | sort -c

💡 Hint

Counting duplicates requires sorting first so duplicates are adjacent.

uniq -c counts adjacent duplicates only. Sorting first groups duplicates together, so sort data.txt | uniq -c correctly counts all duplicates.

🔧 Debug

advanced

Why does this command not remove all duplicates?

A user runs uniq -u data.txt to get unique lines from data.txt but sees duplicates still present. Why?

A Because <code>uniq</code> only removes adjacent duplicates; the file is not sorted.

B Because <code>-u</code> option is invalid and ignored.

C Because <code>uniq</code> removes all duplicates regardless of order.

D Because the file contains hidden characters causing errors.

💡 Hint

Think about how uniq works with adjacent lines.

uniq -u prints only lines that are unique and not repeated adjacent lines. If the file is not sorted, duplicates may not be adjacent, so they remain.

🚀 Application

expert

Extract unique sorted IP addresses from a log

You have a log file access.log with many lines, each starting with an IP address. Which command extracts all unique IP addresses sorted alphabetically?

A sort access.log | cut -d' ' -f1 | uniq

B uniq access.log | cut -d' ' -f1 | sort

C cut -d' ' -f1 access.log | sort | uniq

D cut -d' ' -f1 access.log | uniq | sort

💡 Hint

Extract the first field, then sort, then remove duplicates.

First, cut -d' ' -f1 extracts the IP addresses. Then sort arranges them alphabetically, grouping duplicates. Finally, uniq removes duplicates.