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.