Use sed or perl to extract every nth line in a text file

I recently blogged about the use of sed to extract lines in a text file.

As examples, I showed some simple cases of using sed to extract a single line, and a block of lines in a file.

An annoymous reader asked how one would extract every nth line from a large file.

Suppose somefile contains the following lines:

$ cat > somefile
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
line 10
$

Below, I show 2 ways to extract every 4th line: lines 4 and lines 8 in somefile.

1. sed
   

   $ sed -n '0~4p' somefile
   line 4
   line 8
   $

   
   0~4 means select every 4th line, beginning at line 0.
   
   Line 0 has nothing, so the first printed line is line 4.
   
   -n means only explicitly printed lines are included in output.
   
   
2. perl
   

   $ perl -ne 'print ((0 == $. % 4) ? $_ : "")'  somefile
   line 4
   line 8
   $

   
   
   $. is the current input line number.
   
   % is the remainder operator.
   
   $_ is the current line.
   
   
   The above perl statement prints out a line if its line number
   can be evenly divided by 4 (remainder = 0).
   
   Alternatively,
   

   $ perl -ne 'print unless (0 != $. % 4)' somefile
   line 4
   line 8
   $