How to convert text files to all upper or lower case

How to convert text files to all upper or lower case

How can you convert a text file to all lower case or all upper case?
As usual, in Linux, there are more than 1 way to accomplish a task.

To convert a file (input.txt) to all lower case (output.txt), choose
any ONE of the following:

* dd
$ dd if=input.txt of=output.txt conv=lcase

* tr
$ tr '[:upper:]' '[:lower:]' <> output.txt

* awk
$ awk '{ print tolower($0) }' input.txt > output.txt

* perl
$ perl -pe '$_= lc($_)' input.txt > output.txt

* sed
$ sed -e 's/\(.*\)/\L\1/' input.txt > output.txt

We use the backreference \1 to refer to the entire line and the
\L to convert to lower case.

To convert a file (input.txt) to all upper case (output.txt):

* dd
$ dd if=input.txt of=output.txt conv=ucase

* tr
$ tr '[:lower:]' '[:upper:]' <> output.txt

* awk
$ awk '{ print toupper($0) }' input.txt > output.txt

* perl
$ perl -pe '$_= uc($_)' input.txt > output.txt

* sed
$ sed -e 's/\(.*\)/\U\1/' input.txt > output.txt