Shell Scripting: Convert Uppercase to Lowercase

I've a small shell script and I'd like to convert all incoming user input to lowercase using a shell script. How do I convert uppercase words or strings to a lowercase or vise versa under Unix / Linux BASH shell?

Use the tr command to convert all incoming text / words / variable data from upper to lower case or vise versa (translate all uppercase characters to lowercase).

Convert All Text In a File From UPPER to lowercase:

Type the following command at shell prompt:
$ tr '[:upper:]' '[:lower:]' < input.txt > output.txt
$ cat output.txt

Task: Convert Data Stored in a Shell Variable From UPPPER to lowercase:

Type the following command:
$ echo $VAR_NAME | tr '[:upper:]' '[:lower:]'
$ echo $VAR_NAME | tr '[:lower:]' '[:upper:]'

Bash 4 Uppercase To Lowercase or Viceversa

Type the following commands to convert $y into uppercase:

 
y="this Is A test"
echo "${y^^}"
 

Sample outputs:

THIS IS A TEST

Type the following commands to convert $y into lowercase:

 
y="THIS IS a TeSt"
echo "${y,,}"
 

Sample outputs:

this is a test

Sample Shell Script

#!/bin/bash
# get filename
echo -n "Enter File Name : "
read fileName
 
# make sure file exits for reading
if [ ! -f $fileName ]; then
  echo "Filename $fileName does not exists"
  exit 1
fi
 
# convert uppercase to lowercase using tr command
tr '[A-Z]' '[a-z]' < $fileName
 
# Bash version 4 user should use builtin 

Recommended readings:

• See tr command shell scripting example for more information.

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