Pipe operator (|) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command pipeline?

Consider the following command pipeline:

echo -e "apple\nbanana\ncherry" | grep 'a' | wc -l

What is the output?

Linux CLI

echo -e "apple\nbanana\ncherry" | grep 'a' | wc -l

A 1

B 3

C 2

D 0

💡 Hint

Count lines containing the letter 'a' after filtering.

The echo command outputs three lines: apple, banana, cherry. grep 'a' filters lines containing 'a': apple and banana. wc -l counts these lines, resulting in 2.

💻 Command Output

intermediate

How many lines are printed by this pipeline?

What is the output count of lines from this command?

seq 5 | awk '{print $1 * 2}' | grep -v '^4$'

How many lines are printed?

Linux CLI

seq 5 | awk '{print $1 * 2}' | grep -v '^4$'

A 3

B 4

C 5

D 2

💡 Hint

Check which numbers are filtered out by grep -v '^4$'.

seq 5 outputs 1 to 5. awk multiplies each by 2: 2,4,6,8,10. grep -v '^4$' removes line '4' only. So lines printed are 2,6,8,10 → 4 lines.

📝 Syntax

advanced

Which pipeline produces the output 'hello' correctly?

You want to print 'hello' using a pipe. Which command works correctly?

A echo hello | grep world

B echo hello | echo world

C echo hello && echo world

D echo hello | cat

💡 Hint

Remember what the pipe operator does: it sends output of left command as input to right command.

Option D pipes 'hello' into cat, which outputs 'hello'. Option D: echo world ignores stdin and outputs 'world'. Option D uses && (not pipe) and outputs 'hello\nworld'. Option D: grep finds no match, no output.

🔧 Debug

advanced

Why does this pipeline produce no output?

Examine this command:

cat file.txt | grep 'pattern' | grep 'nomatch'

Assuming 'file.txt' contains lines with 'pattern' but none with 'nomatch', why is there no output?

Linux CLI

cat file.txt | grep 'pattern' | grep 'nomatch'

A Because the second grep filters out all lines, resulting in no output

B Because cat command fails to read the file

C Because grep 'pattern' produces an error

D Because the pipe operator is used incorrectly

💡 Hint

Think about how grep filters lines and how pipes pass output.

The first grep filters lines containing 'pattern'. The second grep looks for 'nomatch' in those lines. Since none contain 'nomatch', no lines pass through, so output is empty.

🚀 Application

expert

Which pipeline counts unique words in a file correctly?

You want to count how many unique words appear in 'document.txt'. Which pipeline produces the correct count?

A cat document.txt | tr ' ' '\n' | sort | uniq | wc -l

B cat document.txt | sort | uniq | wc -l

C cat document.txt | tr '\n' ' ' | sort | uniq | wc -l

D cat document.txt | tr -s ' ' | uniq | wc -l

💡 Hint

Think about how to split words into separate lines before sorting and counting unique entries.

Option A replaces spaces with newlines, so each word is on its own line. Then sorting and uniq count unique words correctly. Other options either don't split words properly or misuse commands.

Practice

1. What does the pipe operator | do in Linux command line?

easy

A. Sends the output of one command as input to another command

B. Runs two commands simultaneously

C. Redirects output to a file

D. Comments out a line in a script

Solution

1. Step 1: Understand the pipe operator function

   The pipe operator | takes the output from the command on its left side.

2. Step 2: Connect output to input of next command

   This output is then passed as input to the command on the right side of the pipe.

3. Final Answer:

   Sends the output of one command as input to another command -> Option A

4. Quick Check:

   Pipe operator = send output to next command [OK]

Hint: Pipe connects output to input of next command [OK]

Common Mistakes:

• Thinking pipe runs commands in parallel
• Confusing pipe with output redirection >
• Assuming pipe comments code lines

2. Which of the following is the correct syntax to use the pipe operator to list files and then count them?

easy

A. ls & wc -l

B. ls > wc -l

C. ls || wc -l

D. ls | wc -l

Solution

1. Step 1: Identify correct pipe syntax

   The pipe operator is |, which connects commands by passing output to input.

2. Step 2: Check command usage

   ls | wc -l lists files and counts lines, which is correct syntax.

3. Final Answer:

   ls | wc -l -> Option D

4. Quick Check:

   Pipe syntax uses | between commands [OK]

Hint: Use single | to pipe commands, not > or & [OK]

Common Mistakes:

• Using > which redirects output to file
• Using & which runs commands in background
• Using || which is logical OR, not pipe

3. What is the output of the command: echo -e "apple\nbanana\ncherry" | grep "an"?

medium

A. apple cherry

B. apple banana cherry

C. banana

D. No output

Solution

1. Step 1: Understand the echo command output

   echo -e "apple\nbanana\ncherry" outputs three lines: apple, banana, cherry.

2. Step 2: Apply grep filter with "an"

   grep "an" filters lines containing "an", which matches only "banana".

3. Final Answer:

   banana -> Option C

4. Quick Check:

   grep filters lines with 'an' = banana [OK]

Hint: grep filters lines containing the pattern [OK]

Common Mistakes:

• Expecting all words printed on one line
• Ignoring newline characters in echo
• Thinking grep matches partial words only

4. Identify the error in this command: cat file.txt | | grep "error"

medium

A. No error, command runs fine

B. Extra pipe operator causes syntax error

C. grep command is misspelled

D. Missing filename after cat

Solution

1. Step 1: Analyze the pipe usage

   The command has two pipe operators | | consecutively, which is invalid syntax.

2. Step 2: Understand correct pipe usage

   Only one pipe operator should separate commands; extra pipes cause errors.

3. Final Answer:

   Extra pipe operator causes syntax error -> Option B

4. Quick Check:

   Double pipes cause syntax error [OK]

Hint: Use single | between commands, no doubles [OK]

Common Mistakes:

• Ignoring extra pipe and expecting output
• Thinking missing filename is the error
• Assuming grep is misspelled

5. You want to find how many lines in log.txt contain the word "fail" but ignore case. Which command correctly uses pipes to do this?

hard

A. cat log.txt | grep -i fail | wc -l

B. grep fail log.txt | wc -l

C. cat log.txt > grep -i fail | wc -l

D. grep -i fail | cat log.txt | wc -l

Solution

1. Step 1: Use cat to output file content

   cat log.txt outputs the file content to pipe.

2. Step 2: Use grep with -i to ignore case

   grep -i fail filters lines containing "fail" ignoring case.

3. Step 3: Count lines with wc -l

   wc -l counts the filtered lines.

4. Final Answer:

   cat log.txt | grep -i fail | wc -l -> Option A

5. Quick Check:

   Pipe commands to filter and count lines [OK]

Hint: Pipe cat to grep -i then to wc -l for case-insensitive count [OK]

Common Mistakes:

• Using > instead of | breaks pipe chain
• Placing commands in wrong order
• Omitting -i for case-insensitive search