How can I get a range of line every nth interval using awk, sed, or other unix command?

Question

I know how to get a range of lines by using awk and sed. I also do know how to print out every nth line using awk and sed.

However, I don't know how to combined the two.

For example, I have a file with 1780000 lines.

For every 17800th line, I would like to print 17800th line plus the two after that.

So if I have a file with 1780000 lines and it starts from 1 and ends at 1780000, this will print:

1
2
3
17800
17801
17802
35600
35601
35602
# ... and so on.

Does anyone know how to get a range of line every nth interval using awk, sed, or other unix command?

Answer 1

Using GNU sed:

sed -n '0~17800{N;N;p}' input

Meaning,

For every 17800th line: 0~17800
  Read two lines: {N;N;
  And print these out: p}

We can also add the first three lines:

sed -n -e '1,3p' -e '0~17800{N;N;p}' input

Using Awk, this would be simpler:

awk 'NR%17800<3 || NR==3 {print}' input