Character classes ([a-z], [0-9]) in Bash Scripting - Mini Project: Build & Apply

Filter Filenames Using Character Classes in Bash

📖 Scenario: You have a folder with many files. Some files have names with only lowercase letters, some have numbers, and others have mixed characters. You want to find files that have only lowercase letters in their names.

🎯 Goal: Build a bash script that lists filenames containing only lowercase letters using character class [a-z].

📋 What You'll Learn

Create a list of filenames stored in a variable.

Create a pattern variable using character classes to match lowercase letters.

Use a loop and conditional to filter filenames matching the pattern.

Print the filtered filenames.

💡 Why This Matters

🌍 Real World

Filtering filenames based on character patterns helps organize files, automate backups, or process specific file groups in scripts.

💼 Career

Understanding character classes and regex matching in bash is essential for system administrators and automation engineers to write efficient scripts.

1

Create a list of filenames

Create a variable called files that contains these exact filenames separated by spaces: apple, banana123, cherry, 12345, date, fig9.

Bash Scripting

# Create a variable called files with the exact filenames
# Your code here

Hint

Use double quotes and separate filenames with spaces inside the variable.

2

Create a pattern for lowercase letters

Create a variable called pattern and set it to the regular expression ^[a-z]+$ to match filenames with only lowercase letters.

Bash Scripting

files="apple banana123 cherry 12345 date fig9"
# Create a variable called pattern with the regex ^[a-z]+$
# Your code here

Hint

The pattern should start with ^, end with $, and use [a-z]+ to match one or more lowercase letters.

3

Filter filenames with only lowercase letters

Use a for loop with variable file to iterate over $files. Inside the loop, use an if statement with [[ $file =~ $pattern ]] to check if the filename matches the pattern. If it matches, add the filename to a variable called filtered separated by spaces.

Bash Scripting

files="apple banana123 cherry 12345 date fig9"
pattern='^[a-z]+$'
filtered=""
# Use a for loop to check each file against the pattern
# Your code here

Hint

Use double square brackets and the =~ operator to match regex in bash.

4

Print the filtered filenames

Print the variable filtered to display the filenames that contain only lowercase letters.

Bash Scripting

files="apple banana123 cherry 12345 date fig9"
pattern='^[a-z]+$'
filtered=""
for file in $files; do
    if [[ $file =~ $pattern ]]; then
        filtered+="$file "
    fi
done
# Print the filtered filenames
# Your code here

Hint

Use printf "%s\n" "$filtered" to print the filenames separated by spaces.

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