grep (search text patterns) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of grep with line numbers

Given a file data.txt containing the lines:

apple
banana
apple pie
pineapple
banana split

What is the output of this command?

grep -n 'apple' data.txt

A

2:banana
5:banana split

B

apple
apple pie
pineapple

C

1:apple
3:apple pie
4:pineapple

D

1:apple
2:banana
3:apple pie
4:pineapple
5:banana split

💡 Hint

The -n option shows line numbers of matching lines.

The command searches for lines containing 'apple' and shows their line numbers. Lines 1, 3, and 4 contain 'apple'.

💻 Command Output

intermediate

Using grep with word boundaries

Consider a file words.txt with these lines:

cat
cater
scatter
concatenate
cat
category

What is the output of:

grep -w 'cat' words.txt

A

cat
cat

B

cat
cater
scatter
concatenate
cat
category

C

cater
scatter
concatenate
category

D

cat
scatter
cat

💡 Hint

The -w option matches whole words only.

Only lines where 'cat' appears as a whole word match. Lines with 'cater', 'scatter', etc. do not match.

🔧 Debug

advanced

Why does this grep command fail?

You run this command:

grep -e apple -e banana fruits.txt

But it returns no output even though the file contains those words. Which option explains the problem?

A The command needs quotes around the patterns.

B The <code>-e</code> option cannot be used multiple times.

C The command requires the <code>-i</code> option to work.

D The file name is incorrect or the file does not exist.

💡 Hint

Check if the file is accessible and named correctly.

Using multiple -e options is valid. If no output appears, the file might be missing or named wrong.

💻 Command Output

advanced

Output of grep with inverted match and count

Given a file colors.txt with:

red
green
blue
yellow
red
blue

What is the output of:

grep -vc 'red' colors.txt

A 4

B 2

C 6

D 0

💡 Hint

The -v option inverts the match; -c counts matching lines.

Lines not containing 'red' are green, blue, yellow, blue (4 lines).

🚀 Application

expert

Find lines with exactly 3 digits using grep

You want to find lines in log.txt that contain exactly three digits anywhere in the line (not more, not less). Which grep command achieves this?

A grep -P '^(?:[^\d]*\d){3}[^\d]*$' log.txt

B grep -P '^(?:\D*\d){3}\D*$' log.txt

C grep -P '^([^\d]*\d){3}[^\d]*$' log.txt

D grep -P '^(?:\D*\d){3}\D*\d' log.txt

💡 Hint

Use Perl regex with -P and match exactly three digits with non-digit characters around.

Option B matches lines with exactly three digits by allowing any number of non-digit characters before, between, and after digits, but exactly three digits total.