Bash How to Convert Text to Lowercase

In Bash, convert a string to lowercase using ${variable,,} syntax, for example: echo "${text,,}".

Examples

InputHELLO

Outputhello

InputBash Script

Outputbash script

Input123 ABC xyz!

Output123 abc xyz!

How to Think About It

To convert text to lowercase in Bash, you take the original string and apply a built-in parameter expansion that changes all uppercase letters to lowercase. This is simple and does not require external commands, making it efficient and easy to use.

Algorithm

1

Get the input string.

2

Use Bash parameter expansion with double commas to convert all uppercase letters to lowercase.

3

Output the converted lowercase string.

Code

bash

#!/bin/bash

text="Hello World!"
lowercase_text="${text,,}"
echo "$lowercase_text"

Output

hello world!

Dry Run

Let's trace the example 'Hello World!' through the code

1

Set variable

text = 'Hello World!'

2

Convert to lowercase

lowercase_text = '${text,,}' results in 'hello world!'

3

Print result

Output: 'hello world!'

Step	Variable	Value
1	text	Hello World!
2	lowercase_text	hello world!
3	echo output	hello world!

Why This Works

Step 1: Parameter Expansion

The ${variable,,} syntax tells Bash to convert all uppercase letters in variable to lowercase.

Step 2: No External Commands

This method uses Bash's built-in features, so it is faster and simpler than calling external tools like tr.

Step 3: Works on Strings

It works on any string stored in a variable, preserving non-letter characters unchanged.

Alternative Approaches

Using tr command

bash

text="Hello World!"
echo "$text" | tr '[:upper:]' '[:lower:]'

This uses an external command and is portable but slightly slower.

Using awk

bash

text="Hello World!"
echo "$text" | awk '{print tolower($0)}'

Another external tool approach, useful if you already use awk in your script.

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

Time Complexity

The operation processes each character once, so time grows linearly with string length.

Space Complexity

A new string is created for the lowercase result, so space also grows linearly with input size.

Which Approach is Fastest?

Using Bash parameter expansion is fastest since it avoids external commands, unlike tr or awk.

Approach	Time	Space	Best For
Bash parameter expansion	O(n)	O(n)	Fastest, simplest for Bash scripts
tr command	O(n)	O(n)	Portable, works in all shells
awk command	O(n)	O(n)	When awk is already used in script