Bash Script to Replace Character in String Easily

Use Bash parameter expansion like new_string=${old_string//old_char/new_char} to replace all occurrences of a character in a string.

Examples

Inputhello world, replace o with a

Outputhella warld

Inputbanana, replace a with o

Outputbonono

Inputno change here, replace x with y

Outputno change here

How to Think About It

To replace a character in a string in Bash, think of the string as text where you want to swap one character for another everywhere it appears. Bash lets you do this easily by using a special syntax that replaces all instances of the old character with the new one without needing external tools.

Algorithm

1

Get the original string input.

2

Identify the character to replace and the new character.

3

Use Bash parameter expansion to replace all occurrences of the old character with the new character.

4

Store the result in a new variable.

5

Print the new string.

Code

bash

#!/bin/bash
old_string="banana"
old_char="a"
new_char="o"
new_string=${old_string//${old_char}/${new_char}}
echo "$new_string"

Output

bonono

Dry Run

Let's trace replacing 'a' with 'o' in 'banana' through the code

1

Set original string

old_string="banana"

2

Set characters to replace

old_char="a", new_char="o"

3

Replace characters

new_string = "banana" with all 'a' replaced by 'o' → "bonono"

4

Print result

Output: bonono

Step	Variable	Value
1	old_string	banana
2	old_char	a
2	new_char	o
3	new_string	bonono
4	output	bonono

Why This Works

Step 1: Parameter Expansion

Bash uses ${variable//search/replace} to replace all occurrences of search with replace in variable.

Step 2: Double Slashes Meaning

The double slashes // mean replace all matches, not just the first one.

Step 3: No External Commands

This method works inside Bash without calling external tools like sed or tr, making it fast and simple.

Alternative Approaches

Using sed command

bash

old_string="banana"
echo "$old_string" | sed 's/a/o/g'

Uses external tool sed; good for complex patterns but slower than parameter expansion.

Using tr command

bash

old_string="banana"
echo "$old_string" | tr 'a' 'o'

Simple character replacement; cannot replace strings longer than one character.

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

Time Complexity

The replacement scans the entire string once, so time grows linearly with string length.

Space Complexity

A new string is created to hold the result, so space also grows linearly with string length.

Which Approach is Fastest?

Bash parameter expansion is fastest as it avoids external calls; sed and tr are slower due to process overhead.

Approach	Time	Space	Best For
Bash parameter expansion	O(n)	O(n)	Simple character/string replacement inside scripts
sed command	O(n)	O(n)	Complex pattern replacements, regex support
tr command	O(n)	O(n)	Single character replacements only