Bash Script to Find Most Frequent Character

Use echo "$input" | fold -w1 | sort | uniq -c | sort -nr | head -1 | awk '{print $2}' to find the most frequent character in a string in Bash.

Examples

Inputhello

Outputl

Inputabracadabra

Outputa

Inputxyz

Outputx

How to Think About It

To find the most frequent character, split the string into single characters, count how many times each appears, then pick the one with the highest count.

Algorithm

1

Get the input string.

2

Split the string into individual characters.

3

Count how many times each character appears.

4

Sort the counts in descending order.

5

Return the character with the highest count.

Code

bash

read -p "Enter a string: " input
most_frequent_char=$(echo "$input" | fold -w1 | sort | uniq -c | sort -nr | head -1 | awk '{print $2}')
echo "$most_frequent_char"

Output

l

Dry Run

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

1

Input string

input = 'hello'

2

Split into characters

h e l l o

3

Sort characters

e h l l o

4

Count unique characters

1 e 1 h 2 l 1 o

5

Sort counts descending

2 l 1 o 1 h 1 e

6

Select most frequent

l

Count Character
2 l
1 o
1 h
1 e

Why This Works

Step 1: Splitting characters

Using fold -w1 breaks the string into one character per line.

Step 2: Counting characters

The uniq -c command counts consecutive repeated lines, so sorting first groups same characters together.

Step 3: Finding the max

Sorting counts numerically in reverse order with sort -nr puts the most frequent character on top.

Alternative Approaches

Using awk only

bash

echo "$input" | awk '{for(i=1;i<=length;i++) c[substr($0,i,1)]++} END {for(k in c) if(c[k]>max){max=c[k];maxchar=k} print maxchar}'

This uses awk to count characters without external sort commands, but is less readable for beginners.

Using grep and tr

bash

echo "$input" | tr -cd '.[:alnum:]' | fold -w1 | sort | uniq -c | sort -nr | head -1 | awk '{print $2}'

This filters only alphanumeric characters before counting, useful if input has spaces or punctuation.

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

Time Complexity

Sorting the characters dominates with O(n log n), where n is string length.

Space Complexity

Extra space is used to store characters and counts, O(n) in worst case.

Which Approach is Fastest?

The awk-only method avoids sorting and can be faster for large inputs but is less intuitive.

Approach	Time	Space	Best For
fold + sort + uniq	O(n log n)	O(n)	Simple and readable scripts
awk only	O(n)	O(n)	Performance with large inputs
grep + tr + sort	O(n log n)	O(n)	Filtered inputs with punctuation