Bash Script to Convert String to Uppercase

Use echo "$string" | tr '[:lower:]' '[:upper:]' or Bash's built-in ${string^^} to convert a string to uppercase in a script.

Examples

Inputhello world

OutputHELLO WORLD

InputBash123

OutputBASH123

Inputalready UPPER

OutputALREADY UPPER

How to Think About It

To convert a string to uppercase in Bash, you can either use the tr command to translate all lowercase letters to uppercase or use Bash's built-in parameter expansion ${string^^} which changes all letters in the variable to uppercase. Both methods take the input string and produce the uppercase version.

Algorithm

1

Get the input string from the user or variable.

2

Use a method to convert all lowercase letters to uppercase.

3

Output the converted uppercase string.

Code

bash

#!/bin/bash

input="Hello Bash"

# Method 1: Using parameter expansion
uppercase1=${input^^}
echo "$uppercase1"

# Method 2: Using tr command
uppercase2=$(echo "$input" | tr '[:lower:]' '[:upper:]')
echo "$uppercase2"

Output

HELLO BASH
HELLO BASH

Dry Run

Let's trace the input "Hello Bash" through the code using parameter expansion and tr command.

1

Set input string

input = "Hello Bash"

2

Convert using parameter expansion

uppercase1 = ${input^^} = "HELLO BASH"

3

Print uppercase1

Output: HELLO BASH

4

Convert using tr command

uppercase2 = $(echo "Hello Bash" | tr '[:lower:]' '[:upper:]') = "HELLO BASH"

5

Print uppercase2

Output: HELLO BASH

Step	Operation	Value
1	input	Hello Bash
2	parameter expansion	HELLO BASH
4	tr command	HELLO BASH

Why This Works

Step 1: Parameter Expansion

The ${string^^} syntax tells Bash to convert all lowercase letters in string to uppercase directly.

Step 2: tr Command

The tr command translates characters from lowercase to uppercase by mapping the character sets [:lower:] to [:upper:].

Alternative Approaches

awk command

bash

input="Hello Bash"
uppercase=$(echo "$input" | awk '{print toupper($0)}')
echo "$uppercase"

Uses awk's toupper function; slightly slower but useful if awk is preferred.

sed command

bash

input="Hello Bash"
uppercase=$(echo "$input" | sed 's/.*/\U&/')
echo "$uppercase"

Uses sed to convert to uppercase; less common but works in GNU sed.

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

Time Complexity

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

Space Complexity

A new uppercase string is created, so space usage is proportional to the input string size.

Which Approach is Fastest?

Bash parameter expansion ${string^^} is fastest as it avoids spawning external commands.

Approach	Time	Space	Best For
Bash parameter expansion	O(n)	O(n)	Fastest, simplest, no external commands
tr command	O(n)	O(n)	Works in all shells, external command
awk command	O(n)	O(n)	When awk is preferred or needed
sed command	O(n)	O(n)	GNU sed environments, less common