make string uppercase or lowercase in bash code example

Example 1: bash transform uppercase to lowercase

Just use tr command to transform lowercase letters into uppercase as:
tr a-z A-Z < file.txt	#transforms letters into uppercase in a file
echo 'HELLO' | tr A-Z a-z	#Outputs: 'hello'

Example 2: bash convert string to uppercase

var=hello #For Bash Version higher than 4.3.33 try these
echo ${var''} #Uppercase whole string
HELLO
echo ${var'} #Uppercase only first char of string
Hello
var2=BYE
echo ${var2,} #Lowercase whole string
bye
echo ${var2,,} #Lowercase only first char of string
bYE
echo $var | tr 'a-z' 'A-Z' #For lower versions of Bash just use tr command
HELLO