With tr
:
# Converts upper to lower case
$ tr '[:upper:]' '[:lower:]' < input.txt > output.txt
# Converts lower to upper case
$ tr '[:lower:]' '[:upper:]' < input.txt > output.txt
Or, sed
on GNU (but not BSD or Mac as they don’t support \L
or \U
):
# Converts upper to lower case
$ sed -e 's/\(.*\)/\L\1/' input.txt > output.txt
# Converts lower to upper case
$ sed -e 's/\(.*\)/\U\1/' input.txt > output.txt