-c

Complement string1: replace it with a lexicographically ordered list of all other characters.

-d

Delete from input all characters in string1.

-s

Squeeze repeated output characters that occur in string2 to single characters.

In either string a noninitial sequence -x, where x is any character (possibly quoted), stands for a range of characters: a possibly empty sequence of codes running from the successor of the previous code up through the code for x. The character \ followed by 1, 2 or 3 octal digits stands for the character whose 16-bit value is given by those digits. The character sequence \x followed by 1, 2, 3, or 4 hexadecimal digits stands for the character whose 16-bit value is given by those digits. A \ followed by any other character stands for that character.

tr A-Z a-z <mixed >lower

Create a list of all the words in file1 one per line in file2, where a word is taken to be a maximal string of alphabetics. String2 is given as a quoted newline.

tr -cs A-Za-z '
' <file1 >file2