sort and uniq in Linux CLI - Commands & Configuration

Introduction

Sometimes you have a list of items with duplicates and want to organize them neatly and remove repeated entries. The sort and uniq commands help you do this by arranging items in order and filtering out duplicates.

When you have a list of names and want to see them in alphabetical order without repeats.

When you want to count how many unique IP addresses accessed your server from a log file.

When you need to clean up a list of installed packages by removing duplicate entries.

When you want to prepare a sorted list of unique words from a text file for analysis.

When you want to quickly find unique error messages from a system log.

Commands

Create a file named fruits.txt with a list of fruit names, some repeated, to use for sorting and removing duplicates.

Terminal

echo -e "apple\nbanana\napple\ncherry\nbanana" > fruits.txt

Expected Output

No output (command runs silently)

Sort the contents of fruits.txt alphabetically. This arranges the list so duplicates are next to each other, which uniq needs to work properly.

Terminal

sort fruits.txt

Expected Output

apple

apple

banana

banana

cherry

Sort the list and then pass it to uniq to remove duplicate lines, showing only unique fruit names in order.

Terminal

sort fruits.txt | uniq

Expected Output

apple

banana

cherry

Sort the list and use uniq with the -c flag to count how many times each unique item appears.

Terminal

sort fruits.txt | uniq -c

Expected Output

      2 apple

      2 banana

      1 cherry

→

-c - Count occurrences of each unique line

Key Concept

If you remember nothing else from this pattern, remember: uniq only removes duplicates that are next to each other, so always sort your data first.

Common Mistakes

Running uniq without sorting the input first.

uniq only removes adjacent duplicate lines, so duplicates separated by other lines will not be removed.

Always sort the input before piping it to uniq to group duplicates together.

Using uniq without understanding it only filters adjacent duplicates.

This leads to unexpected results where duplicates remain if they are not next to each other.

Use sort before uniq or use uniq -u or other options carefully depending on the goal.

Summary

Use sort to arrange lines alphabetically or numerically so duplicates are next to each other.

Pipe sorted output to uniq to remove duplicate lines or count occurrences with -c.

Remember uniq only works on adjacent duplicates, so sorting first is essential.

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