Bash Script to Convert Uppercase to Lowercase File

Use the Bash command tr '[:upper:]' '[:lower:]' < inputfile > outputfile to convert all uppercase letters in a file to lowercase.

Examples

InputHELLO WORLD

Outputhello world

InputThis Is A Test FILE.

Outputthis is a test file.

Input1234!@#$

Output1234!@#$

How to Think About It

To convert uppercase letters to lowercase in a file, read the file content and replace each uppercase letter with its lowercase equivalent. This can be done by mapping uppercase characters to lowercase ones using a simple translation command in Bash.

Algorithm

1

Get the input file name.

2

Read the content of the input file.

3

Replace all uppercase letters with lowercase letters.

4

Write the converted content to a new output file.

5

Print a message confirming the conversion.

Code

bash

#!/bin/bash

input_file="$1"
output_file="$2"

if [[ ! -f "$input_file" ]]; then
  echo "Input file does not exist."
  exit 1
fi

tr '[:upper:]' '[:lower:]' < "$input_file" > "$output_file"
echo "Converted $input_file to lowercase and saved as $output_file."

Output

Converted SAMPLE.TXT to lowercase and saved as output.txt.

Dry Run

Let's trace converting 'HELLO WORLD' from input file SAMPLE.TXT to output file output.txt.

1

Check if input file exists

Input file SAMPLE.TXT exists.

2

Read input file content

Content read: 'HELLO WORLD'

3

Convert uppercase to lowercase

Converted content: 'hello world'

4

Write to output file

Output file output.txt now contains 'hello world'

5

Print confirmation

Printed message: Converted SAMPLE.TXT to lowercase and saved as output.txt.

Step	Action	Value
1	Input file check	SAMPLE.TXT exists
2	Read content	HELLO WORLD
3	Convert content	hello world
4	Write output	output.txt with 'hello world'
5	Print message	Converted SAMPLE.TXT to lowercase and saved as output.txt.

Why This Works

Step 1: Reading the file

The script reads the input file content using input redirection with <.

Step 2: Translating characters

The tr command replaces all uppercase letters [:upper:] with lowercase letters [:lower:].

Step 3: Saving output

The converted text is saved to the output file using output redirection with >.

Alternative Approaches

Using awk

bash

awk '{print tolower($0)}' inputfile > outputfile

This uses awk's tolower function; it is easy to read but may be slower on large files.

Using sed

bash

sed 'y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz/' inputfile > outputfile

This uses sed's transliteration; it works well but is less readable than tr.

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

Time Complexity

The script processes each character once, so time grows linearly with file size.

Space Complexity

The output file requires space proportional to the input file size; the script uses minimal extra memory.

Which Approach is Fastest?

The tr command is generally fastest and simplest for this task compared to awk or sed.

Approach	Time	Space	Best For
tr command	O(n)	O(n)	Simple and fast character translation
awk tolower	O(n)	O(n)	More readable, supports complex processing
sed transliteration	O(n)	O(n)	Works well but less intuitive