tr (Translate)

One thing before we start: tr does not take a filename. It reads whatever you feed it, and if you do not feed it anything it reads your keyboard and sits there waiting. That is why the examples below type a line and get a line back. When you have finished typing, press Ctrl-D to end the input, or Ctrl-C to give up. Most of the time you will feed it from another command with a pipe instead:

$ echo hello | tr a-z A-Z
HELLO

The tr (translate) command allows you to translate a set of characters into another set of characters. Let's try an example of translating all lower case characters to uppercase characters.

$ tr a-z A-Z
hello
HELLO

As you can see we made the ranges of a-z into A-Z and all text we type that is lowercase gets uppercased.