tr (lower case) is a
Unix command to replace or delete characters in a
string of text. The most common use is:
tr "string1" "string2"
which copies the standard
input to the standard
output, replacing each
character from "string1" with the corresponding character from "string2".
For example, this tr command will
encode what you type as
rot13:
tr 'a-zA-Z' 'n-za-mN-ZA-M'