What it does

tac is a Linux command that allows you to see a file line-by-line backwards. It is named by analogy with cat.

Mandatory arguments to long options are mandatory for short options too:

-b, --before	attach the separator before instead of after
-r, --regex	interpret the separator as a regular expression
-s, --separator=STRING
	use STRING as the separator instead of newline

Example

Input file:

0 1 2 3 4 5 # 6 7 8 9

default settings:

9 8 7 6 # 5 4 3 2 1 0

with option -s 5:

# 6 7 8 9 0 1 2 3 4 5

with option -b and -s 5:

5 # 6 7 8 9 0 1 2 3 4