For files with many columns:
===========================

$ cut -f2 NAMEFILE  --> display the second column
$ cut -f1 NAMEFILE  --> display first column
$ paste FILE1 FILE2  --> join the columns together of two files
$ join FILE1 FILES2  --> join separate files but only if there is common field in both files with identical values


Changing the contend of files:
==============================

CHARACTERS:
----------

$ tr "h2" "3x" < FILENAME --> changes all h to 3 and all 2 to x


WORDS:
------
$ sed expression file
$ sed '/tea/s//milk/g' FILENAME

/tea/ --> find tea word
s//milk/ --> replace (substitute) the word milk for tea
g --> make the changes globally


DUPLICATES:
-----------

Print LINES that are unique.
$ uniq FILENAME

Duplicate lines must be next to next to each other, so it's better to sort first:
$ sort FILENAME | uniq


Ex
--
$ ex FILENAME --> start the editor