sort and uniq in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sort command do in Linux?

The sort command arranges lines of text in a file or input alphabetically or numerically. It helps organize data so it's easier to read or process.

beginner

What is the purpose of the uniq command?

uniq filters out repeated lines that are next to each other in a file or input. It shows only one copy of each repeated line.

beginner

Why do you often use sort before uniq?

Because uniq only removes duplicates that are next to each other, sorting first groups duplicates together so uniq can remove all repeated lines.

intermediate

How do you count how many times each line appears using uniq?

Use uniq -c. It adds a number before each line showing how many times that line appeared consecutively.

beginner

What does the command sort file.txt | uniq do?

It sorts the lines in file.txt alphabetically, then removes any duplicate lines, showing only unique lines in order.

What happens if you run uniq file.txt without sorting first?

A Only consecutive duplicate lines are removed

B All duplicate lines are removed regardless of position

C The file is sorted alphabetically

D Nothing happens

uniq only removes duplicates that are next to each other. Without sorting, duplicates separated by other lines stay.

Which command shows how many times each line appears in a sorted file?

A uniq -c file.txt

B sort file.txt | uniq -c

C sort -c file.txt

D uniq file.txt

Sorting first groups duplicates, then uniq -c counts each group.

What does sort -r do?

A Sorts lines in reverse order

B Removes duplicates

C Counts lines

D Sorts lines randomly

-r option sorts lines in reverse (descending) order.

How can you remove duplicate lines ignoring case differences?

A sort | uniq

B sort -n

C uniq -c

D sort -f | uniq -i

sort -f ignores case, and uniq -i ignores case when filtering duplicates.

What is the output of echo -e "apple\nApple\napple" | sort | uniq?

A apple

B apple Apple apple

C Apple apple

D Apple

Sorting puts 'Apple' before 'apple'. uniq sees them as different because of case, so both appear.

Explain how to use sort and uniq together to find unique lines in a file.

Think about why sorting helps uniq work better.

Describe how to count the number of times each unique line appears in a text file using Linux commands.

Counting needs grouping first.

Practice

1. What does the uniq command do when used alone on a text file?

easy

A. Deletes all empty lines

B. Removes repeated adjacent lines

C. Counts all lines in the file

D. Sorts all lines alphabetically

Solution

1. Step 1: Understand the uniq command behavior

   The uniq command filters out repeated lines only if they are next to each other.

2. Step 2: Compare with other options

   It does not sort lines or count all lines; it only removes adjacent duplicates.

3. Final Answer:

   Removes repeated adjacent lines -> Option B

4. Quick Check:

   uniq removes adjacent duplicates = D [OK]

Hint: uniq only removes duplicates if they are next to each other [OK]

Common Mistakes:

• Thinking uniq sorts lines
• Assuming uniq removes all duplicates anywhere
• Confusing uniq with wc (word count)

2. Which of the following is the correct syntax using a pipe to sort a file named data.txt and remove duplicate lines?

easy

A. sort data.txt | uniq

B. uniq data.txt | sort

C. sort -u data.txt

D. uniq -s data.txt

Solution

1. Step 1: Understand the order of commands

   To remove duplicates correctly, lines must be sorted first so duplicates become adjacent.

2. Step 2: Check each option

   sort data.txt | uniq sorts then pipes to uniq, which removes adjacent duplicates. uniq data.txt | sort runs uniq first, which won't remove all duplicates if unsorted. sort -u data.txt uses sort -u which is valid but not the pipe syntax asked. uniq -s data.txt is invalid syntax.

3. Final Answer:

   sort data.txt | uniq -> Option A

4. Quick Check:

   Sort before uniq to remove duplicates = A [OK]

Hint: Always sort before uniq to remove duplicates correctly [OK]

Common Mistakes:

• Running uniq before sort
• Using invalid uniq options
• Confusing sort -u with uniq

3. Given a file names.txt with contents:

bob
alice
bob
carol
alice
bob

What is the output of sort names.txt | uniq?

medium

A. alice
bob
carol

B. bob
alice
carol

C. alice
bob
bob
carol
alice
bob

D. bob
carol
alice

Solution

1. Step 1: Sort the file lines

   Sorting the lines alphabetically gives: alice, alice, bob, bob, bob, carol.

2. Step 2: Apply uniq to remove adjacent duplicates

   uniq removes repeated adjacent lines, so only one of each remains: alice, bob, carol.

3. Final Answer:

   alice bob carol -> Option A

4. Quick Check:

   Sort then uniq removes duplicates = alice, bob, carol [OK]

Hint: Sort first, then uniq removes duplicates in order [OK]

Common Mistakes:

• Not sorting before uniq
• Expecting uniq to remove all duplicates anywhere
• Confusing output order

4. You run sort data.txt | uniq but notice duplicates remain. What is the most likely cause?

medium

A. sort command does not work on text files

B. uniq command is misspelled

C. The file contains lines with trailing spaces or tabs

D. The file is empty

Solution

1. Step 1: Understand how sort and uniq treat whitespace

   sort and uniq treat lines with trailing spaces or tabs as different lines, so duplicates with extra spaces won't be adjacent identical.

2. Step 2: Check other options

   uniq misspelling would cause error, not duplicates remaining. sort works on text files. Empty file means no duplicates.

3. Final Answer:

   The file contains lines with trailing spaces or tabs -> Option C

4. Quick Check:

   Trailing spaces cause uniq to miss duplicates = C [OK]

Hint: Check for trailing spaces if duplicates remain after sort|uniq [OK]

Common Mistakes:

• Assuming uniq always removes duplicates regardless of spaces
• Ignoring trailing whitespace differences
• Thinking sort fails on text files

5. You want to count how many unique IP addresses appear in access.log. Which command correctly sorts, removes duplicates, and counts unique lines?

hard

A. sort access.log | uniq -c

B. uniq access.log | wc -l

C. uniq -c access.log | sort

D. sort -u access.log | wc -l

Solution

1. Step 1: Understand the goal

   You want the count of unique IP addresses, so you need to remove duplicates and count the unique lines.

2. Step 2: Analyze each option

   sort access.log | uniq -c counts occurrences but does not sum total unique lines. uniq -c access.log | sort counts duplicates before sorting, so misses some. sort -u access.log | wc -l uses sort -u to get unique lines, then counts lines with wc -l, which gives the unique count. uniq access.log | wc -l counts lines after uniq without sorting, so duplicates may remain.

3. Final Answer:

   sort -u access.log | wc -l -> Option D

4. Quick Check:

   sort -u then wc -l counts unique lines = B [OK]

Hint: Use sort -u then wc -l to count unique lines fast [OK]

Common Mistakes:

• Using uniq -c without sorting first
• Counting lines before removing duplicates
• Confusing uniq -c output with total unique count