Bash How to Convert Text to Uppercase

In bash, convert a string to uppercase using ${variable^^} or the tr '[:lower:]' '[:upper:]' command.

Examples

Inputhello

OutputHELLO

InputBash Scripting

OutputBASH SCRIPTING

Input123abc!@

Output123ABC!@

How to Think About It

To convert text to uppercase in bash, you can use built-in parameter expansion which changes all lowercase letters to uppercase. Alternatively, you can pipe the text through the tr command to translate lowercase characters to uppercase. Both methods handle letters only and leave other characters unchanged.

Algorithm

1

Get the input string.

2

Use bash parameter expansion <code>${variable^^}</code> to convert all lowercase letters to uppercase.

3

Or pipe the string to <code>tr '[:lower:]' '[:upper:]'</code> to translate lowercase letters to uppercase.

4

Print or return the converted uppercase string.

Code

bash

#!/bin/bash
input="Hello World!"
# Using parameter expansion
uppercase1=${input^^}
echo "$uppercase1"

# Using tr command
echo "$input" | tr '[:lower:]' '[:upper:]'

Output

HELLO WORLD!
HELLO WORLD!

Dry Run

Let's trace the input 'Hello World!' through the uppercase conversion code

1

Set input variable

input = 'Hello World!'

2

Convert using parameter expansion

uppercase1 = ${input^^} = 'HELLO WORLD!'

3

Convert using tr command

echo 'Hello World!' | tr '[:lower:]' '[:upper:]' = 'HELLO WORLD!'

Step	Input	Output
1	Hello World!	Hello World!
2	Hello World!	HELLO WORLD!
3	Hello World!	HELLO WORLD!

Why This Works

Step 1: Parameter Expansion

The ${variable^^} syntax tells bash to convert all lowercase letters in variable to uppercase.

Step 2: tr Command

The tr command translates characters from one set to another; here it changes all lowercase letters to uppercase.

Alternative Approaches

awk command

bash

echo "hello world" | awk '{print toupper($0)}'

Uses awk's toupper function; useful if awk is preferred but slower than bash built-in.

sed command

bash

echo "hello world" | sed 's/.*/\U&/'

Uses sed to convert to uppercase; less common and may not work in all sed versions.

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

Time Complexity

The conversion processes each character once, so time grows linearly with input length.

Space Complexity

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

Which Approach is Fastest?

Parameter expansion is fastest as it is built into bash; external commands like tr or awk add overhead.

Approach	Time	Space	Best For
Parameter Expansion	O(n)	O(n)	Fastest, simplest in bash 4+
tr Command	O(n)	O(n)	Works in older shells, external command
awk Command	O(n)	O(n)	When awk is preferred, slower
sed Command	O(n)	O(n)	Less common, compatibility varies