tail [ -fr ] [ -n nlines ] [ -c ncharacters ] [ file ]

Copying begins at position +number measured from the beginning, or -number from the end of the input. Number is counted in lines, 1K blocks or characters, according to the appended flag l, b, or c. Default is -10l (ten ell).

The further flag r causes tail to print lines from the end of the file in reverse order; f (follow) causes tail, after printing to the end, to keep watch and print further data as it appears.

The second syntax is that promulgated by POSIX, where the numbers of lines (-n) or characters (-c) are specified separately as signed integers. Note than an unsigned value is treated as negative, ie '-n 2' is equivalent to '-n -2'.

tail file

Print the last 10 lines of a file.

tail +0f file

Print a file, and continue to watch data accumulate as it grows.
According to custom, option +number counts lines from 1, and counts blocks and characters from 0.

tail -nlines +3 file

Print a file starting at line number 3