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.

Practice

1. What does the character class [a-z] match in bash scripting?

easy

A. All lowercase letters from a to z

B. All uppercase letters from A to Z

C. All digits from 0 to 9

D. All special characters

Solution

1. Step 1: Understand character class notation

   The notation [a-z] means any character from 'a' to 'z' in lowercase.

2. Step 2: Identify what [a-z] matches

   This matches all lowercase letters, not uppercase letters, digits, or special characters.

3. Final Answer:

   All lowercase letters from a to z -> Option A

4. Quick Check:

   [a-z] = lowercase letters [OK]

Hint: Remember: a-z means lowercase letters only [OK]

Common Mistakes:

• Confusing lowercase with uppercase letters
• Thinking it matches digits
• Assuming it matches special characters

2. Which of the following is the correct syntax to match any digit using a character class in bash?

easy

A. [9-0]

B. [a-z]

C. [A-Z]

D. [0-9]

Solution

1. Step 1: Identify digit matching syntax

   The character class [0-9] matches any digit from 0 to 9.

2. Step 2: Check other options

   [a-z] matches lowercase letters, [A-Z] matches uppercase letters, and [9-0] is invalid because ranges must go from lower to higher.

3. Final Answer:

   [0-9] -> Option D

4. Quick Check:

   [0-9] = digits [OK]

Hint: Digits always use [0-9], not reversed ranges [OK]

Common Mistakes:

• Using reversed ranges like [9-0]
• Confusing letters with digits
• Using uppercase letter ranges for digits

3. What will be the output of this command?

echo -e "a1 b2 c3" | grep -o '[a-z][0-9]'

medium

A. a1 b2 c3

B. a1
b2
c3

C. a b c

D. 1 2 3

Solution

1. Step 1: Understand the grep command with -o and pattern

   The pattern [a-z][0-9] matches a lowercase letter followed by a digit. The -o option prints each match on a new line.

2. Step 2: Apply pattern to input string

   Input "a1 b2 c3" contains matches: "a1", "b2", and "c3". Each will be printed on its own line.

3. Final Answer:

   a1 b2 c3 -> Option B

4. Quick Check:

   Pattern matches pairs, -o prints each on new line [OK]

Hint: Use -o to print each match on a separate line [OK]

Common Mistakes:

• Expecting all matches on one line
• Matching letters or digits alone
• Ignoring the -o option effect

4. Identify the error in this grep command:

grep '[a-z-9]' file.txt

medium

A. Should use double brackets [[ ]] instead of single brackets

B. Missing quotes around the pattern

C. The character class is invalid because of the dash placement

D. No error, command is correct

Solution

1. Step 1: Analyze the character class syntax

   The pattern [a-z-9] is intended to match letters and digits, but the dash is misplaced between 'z' and '9'.

2. Step 2: Understand dash usage in character classes

   Dash (-) defines a range if placed between two characters. Here, 'z-9' is invalid because 'z' comes after '9' in ASCII order. Dash should be first or last to be literal.

3. Final Answer:

   The character class is invalid because of the dash placement -> Option C

4. Quick Check:

   Dash in ranges must go from lower to higher [OK]

Hint: Put dash at start or end to avoid range errors [OK]

Common Mistakes:

• Placing dash between characters in wrong order
• Confusing quotes usage
• Using wrong bracket types

5. You want to extract all words from a file that contain only lowercase letters and digits. Which grep pattern correctly matches such words?

grep -E '\b[?]+\b' file.txt

Choose the correct pattern to replace ?.

hard

A. [a-z0-9]+

B. [a-z]+[0-9]+

C. [a-z][0-9]

D. [0-9a-z]{2,}

Solution

1. Step 1: Understand the requirement for matching words

   We want words with only lowercase letters and digits, any order, any length (at least one character).

2. Step 2: Analyze each pattern

   [a-z0-9]+ matches one or more lowercase letters or digits in any order.
   [a-z]+[0-9]+ requires letters followed by digits only.
   [a-z][0-9] matches exactly two characters: one letter then one digit.
   [0-9a-z]{2,} matches two or more letters/digits but order is not enforced; this is valid but less clear than [a-z0-9]+.

3. Final Answer:

   [a-z0-9]+ -> Option A

4. Quick Check:

   Character class with + matches any length words [OK]

Hint: Use [a-z0-9]+ to match letters and digits in any order [OK]

Common Mistakes:

• Forcing letters then digits order
• Matching only two characters
• Ignoring word boundaries