Character classes ([a-z], [0-9]) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of grep with character class [a-z]

What is the output of this command when run on a file containing the lines:

apple
Banana
cherry
123
date

grep '^[a-z]' filename

Assume the file is named filename and contains exactly the lines above.

Bash Scripting

grep '^[a-z]' filename

A

apple
Banana
cherry
date

B

apple
Banana
cherry
123
date

C

Banana
123

D

apple
cherry
date

💡 Hint

The pattern '^[a-z]' matches lines starting with a lowercase letter.

The grep command with '^[a-z]' matches lines starting with any lowercase letter from a to z. Lines starting with uppercase letters or digits are excluded.

💻 Command Output

intermediate

Matching digits with grep [0-9]

Given a file numbers.txt with the following lines:

abc123
456def
789
no_digits
123abc456

What lines will this command output?

grep '^[0-9]' numbers.txt

Bash Scripting

grep '^[0-9]' numbers.txt

A

456def
789
123abc456

B

abc123
456def
789
123abc456

C

789
no_digits

D

abc123
no_digits

💡 Hint

The pattern '^[0-9]' matches lines starting with a digit.

Only lines starting with digits 0-9 are matched. So lines starting with letters or no digits at start are excluded.

🔧 Debug

advanced

Why does this grep command fail to match digits?

You run this command:

grep '[0-9]$' file.txt

But it does not match lines ending with digits as expected. What is the most likely reason?

Bash Scripting

grep '[0-9]$' file.txt

A The character class [0-9] is invalid in grep.

B The pattern is missing a caret ^ at the start.

C The file has Windows line endings, so the $ matches before the carriage return, not the digit.

D The file is empty, so no lines match.

💡 Hint

Check the file line endings if the pattern looks correct.

If the file has Windows-style line endings (\r\n), the $ matches before the \r, so the digit is not at the end of the line, causing no match.

📝 Syntax

advanced

Identify the syntax error in this character class usage

Which option contains a syntax error in the grep pattern using character classes?

Bash Scripting

grep '^[a-z0-9' file.txt

A grep '^[a-z0-9' file.txt

B grep '^[a-z0-9-' file.txt

C grep '^[a-z0-9]' file.txt

D txt.elif ']9-0z-a[^' perg

💡 Hint

Check if the character class is properly closed with a bracket.

Option A is missing the closing bracket ] for the character class, causing a syntax error.

🚀 Application

expert

Count lines starting with lowercase letters and digits

You want to count how many lines in data.txt start with either a lowercase letter or a digit. Which command will give the correct count?

A grep -c '^[a-z]|^[0-9]' data.txt

B grep -c '^[a-z0-9]' data.txt

C grep -c '^[a-z][0-9]' data.txt

D grep -c '^[a-z0-9]$' data.txt

💡 Hint

Use a single character class to match either lowercase letters or digits at line start.

Option B correctly uses a character class [a-z0-9] to match lines starting with either a lowercase letter or digit and counts them. Option B uses alternation incorrectly inside grep basic regex. Option B matches lines starting with a lowercase letter followed immediately by a digit. Option B matches lines with exactly one character that is lowercase letter or digit.