grep in scripts in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of grep with multiple options

What is the output of this bash command when run on a file named data.txt containing the lines:

apple
banana
Apple pie
banana split
APPLE

Command:

grep -i 'apple' data.txt

Bash Scripting

grep -i 'apple' data.txt

A apple

B

apple
Apple pie
APPLE
banana
banana split

C

Apple pie
APPLE

D

apple
Apple pie
APPLE

💡 Hint

The -i option makes grep ignore case when matching.

The -i option matches 'apple' regardless of case, so it finds 'apple', 'Apple pie', and 'APPLE'. Lines without 'apple' are ignored.

💻 Command Output

intermediate

Using grep to count matching lines

Given a file log.txt with 10 lines, 3 of which contain the word 'error', what is the output of:

grep -c 'error' log.txt

Bash Scripting

grep -c 'error' log.txt

A 3

B

error
error
error

C 10

D 0

💡 Hint

The -c option counts matching lines instead of printing them.

The -c option returns the number of lines that contain the pattern. Since 3 lines have 'error', the output is 3.

🔧 Debug

advanced

Why does this grep command fail?

Consider this script snippet:

if grep 'pattern' file.txt
then
echo 'Found'
fi

Why might this script not print 'Found' even if 'pattern' is in the file?

Bash Scripting

if grep 'pattern' file.txt
then
echo 'Found'
fi

A grep outputs matching lines but the if condition checks exit status, so if grep outputs nothing, it returns false

B grep returns 0 exit status if it finds matches, so the if condition should print 'Found' if pattern exists

C grep command is missing the -q option to suppress output, causing the if to fail

D The script is missing a semicolon after grep command, causing syntax error

💡 Hint

Check how grep exit status works in if conditions.

grep returns exit status 0 if it finds the pattern, so the if condition is true and 'Found' is printed. The script works as intended.

📝 Syntax

advanced

Identify the syntax error in this grep usage

Which option shows the correct syntax to search for lines containing 'test' in file.txt and ignore case?

A grep -i 'test' file.txt

B grep -i test file.txt

C grep -i "test file.txt"

D grep -i test -file.txt

💡 Hint

Check how to pass the pattern and filename correctly.

Option A correctly uses quotes around the pattern and specifies the filename after the pattern. Option A works but is less safe without quotes. Options C and D have syntax errors.

🚀 Application

expert

Extract lines with numbers and save count

You want to write a bash script that extracts all lines containing digits from input.txt and saves them to numbers.txt. Then, you want to store the count of these lines in a variable count. Which snippet does this correctly?

A count=$(grep -E '[0-9]' input.txt > numbers.txt | wc -l)

B

grep -E '[0-9]' input.txt > numbers.txt
count=$(wc -l < numbers.txt)

C count=$(grep -E '[0-9]' input.txt | tee numbers.txt | wc -l)

D

grep -E '[0-9]' input.txt | wc -l > numbers.txt
count=$(cat numbers.txt)

💡 Hint

Use a command that both saves output and counts lines in a single pipeline.

Option C uses tee to save grep output to file and pipe it to wc -l to count lines, storing count in variable. Option C incorrectly pipes after redirect, so wc -l counts nothing. Option C requires two separate commands. Option C saves count to file, not lines.