Bash Script to Count Vowels in a String

Use echo "$string" | grep -o -i '[aeiou]' | wc -l in Bash to count vowels in a string, where $string is your input text.

Examples

Inputhello

Output2

InputBash Scripting

Output4

Inputxyz

Output0

How to Think About It

To count vowels, look at each character in the string and check if it is a vowel (a, e, i, o, u). Count how many times vowels appear regardless of case.

Algorithm

1

Get the input string.

2

Convert the string to lowercase or ignore case.

3

Extract all vowels from the string.

4

Count the number of vowels extracted.

5

Return the count.

Code

bash

#!/bin/bash
read -p "Enter a string: " string
count=$(echo "$string" | grep -o -i '[aeiou]' | wc -l)
echo "Number of vowels: $count"

Output

Enter a string: hello
Number of vowels: 2

Dry Run

Let's trace the input 'hello' through the code

1

Input string

string = 'hello'

2

Extract vowels

echo 'hello' | grep -o -i '[aeiou]' outputs: e\no

3

Count vowels

wc -l counts 2 lines (vowels)

Vowel Found
e
o

Why This Works

Step 1: Extract vowels

The grep -o -i '[aeiou]' command finds each vowel in the string, ignoring case, and prints each on a new line.

Step 2: Count lines

The wc -l command counts how many lines (vowels) were printed, giving the total vowel count.

Alternative Approaches

Using a loop and case statement

bash

#!/bin/bash
read -p "Enter a string: " string
count=0
for (( i=0; i<${#string}; i++ )); do
  char=${string:i:1}
  case $char in
    [aeiouAEIOU]) ((count++)) ;;
  esac
done
echo "Number of vowels: $count"

This method is more manual but does not rely on external commands like grep or wc.

Using tr and grep

bash

#!/bin/bash
read -p "Enter a string: " string
count=$(echo "$string" | tr -cd 'aeiouAEIOU' | wc -c)
echo "Number of vowels: $count"

This uses <code>tr</code> to delete all characters except vowels, then counts characters with <code>wc -c</code>.

Complexity: O(n) time, O(n) space

Time Complexity

The script processes each character once to check if it is a vowel, so time grows linearly with string length.

Space Complexity

Extra space is used to hold the extracted vowels temporarily, proportional to the number of vowels.

Which Approach is Fastest?

Using grep and wc is concise and fast for typical strings; the loop method is more manual and slightly slower.

Approach	Time	Space	Best For
grep + wc	O(n)	O(n)	Quick and simple scripts
Loop + case	O(n)	O(1)	Environments without grep or wc
tr + wc	O(n)	O(n)	Simple character filtering