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cat
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reads file(s) and outputs, line by line

-n number output lines, starting at 1
-b number non-empty lines, starting at 1

-e display non-printing characters including \n (as $)
-t display non-printing characters (^I for tab)
-v displace non-print so that they are visible. Upper-ASCII are
   printed as M- followed by 7-bit character 

gcat has special features
-s squeeze successive empty lines

$ cat > a.txt 	#you can start typing and end entry with cntrl-D
$ cat >> a.txt  #start typing and append to a.txt. end with cntrl-D
$ cat a.txt > b.txt #copy a.txt to b.txt
$ cat a.txt b.txt > c.txt #copy a.txt and then append b.txt, all to c.txt

The following emulate cat 

$ awk 1 infile		#1 means the condition is true
$ sed -n 'p' infile     #p is print every line
$ sed -n 'l' infile     #print every line but in long form (\nl ->$ etc)