If you’re using a linux terminal and need to convert all upper case characters to lowercase, there are a number of ways you can do it. One of the easiest is to use tr:

tr '[:upper:]' '[:lower:]'

For example:

$ echo "HeLlo WOrlD" | tr '[:upper:]' '[:lower:]'
hello world

:upper: and :lower: can be reversed if you want to create an upper case string instead:

$ echo "HeLlo WOrlD" | tr '[:lower:]' '[:upper:]'
HELLO WORLD