How to convert all text in a file from upper case to lower case and vice versa on Linux? Convert from upper case to lower case: tr '[:upper:]' '[:lower:]' < input.txt > output.txt Convert from lower case to upper case: tr '[:lower:]' '[:upper:]' < input.txt > output.txt Read more: How to convert a string to lower case in Python? How to convert between Simplified Chinese and Traditional Chinese characters in text files on Linux? How to convert PDF to text with format kept on Linux? How to convert HTML file to text on Linux How to convert a piece of HTML code to plain text in PHP? 3 Ways of Making Case Insensitive Search in Vim/Vi Vim: case insensitive search How to calculate the average of value in lines of a text file on Linux by a script?