Delete every Nth line in shell

Question

I'm trying to delete a line after N lines using awk and I can't seem to get it right. The file format is like this

YYYYYY
XXXXXX
XXXXXX
YYYYYY
XXXXXX
XXXXXX

The real example would be

office3
3
1
office3
6
1
office3
6
3
office3
1
1

How can I delete the YY lines or the lines that say "office". I need to delete a line every two lines regardless of their content.

Solution 1

If you have GNU sed, you could use the n~m (n skip m) address notation

sed '1~3d' file

which deletes every third line, starting at the first.

Solution 2

To select only lines modulo N with awk try

awk '!(NR%2)' file

or

awk 'NR%3==0' file

Here NR denotes number of rows processed so far.

---

In your specific case (remove all lines with Y):

$ echo 'YYYYYY
XXXXXX
XXXXXX
YYYYYY
XXXXXX
XXXXXX' | awk '!(NR%3==1)'
XXXXXX
XXXXXX
XXXXXX
XXXXXX

Comments

[Ciro Santilli Путлер Капут 六四事]

stackoverflow.com/questions/9894986/… || delete: superuser.com/questions/396536/…

[Angel Todorov]

in this example, we want to delete lines 1 and 4, so NR%3==1

[mormaii2]

This solved it for me. I didn't know abot that sed function. Thank you!

[Stéphane Chazelas]

This solution has the advantage of being standard and portable to all POSIX systems (even pre-POSIX ones as except for the first one, it works in the original awk from 1979)