!!! Print Nth Line of File
{{{
$ sed -n '10p' file
}}}

This prints line number 10.

Ranges are also possible:

{{{
$ sed -n '3,5p' infile
}}}

prints lines 3, 4 and 5.

!!! Print Every Nth Line of File

To print lines 1, 11, 21, etc do

{{{
$ awk '{if (count++%10==0) print $0;}' input_file
}}}

To print lines 10, 20, 30, etc do

{{{
$ awk '{if (++count%10==0) print $0;}' input_file
}}}

You can also use the following to print every 5th line starting at line 2.  This doesn't appear to work for me on Mac.

{{{
$ sed -n '2~5p' input_file
}}}

To
----
[Shell | CategoryComputing.Shell] - [Linux.Shell | CategoryComputing.Linux.Shell] - [Mac.Shell | CategoryComputing.Mac.Shell]