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'