Print every n lines from a file

Question

I\'m trying to print every nth line from file, but n is not a constant but a variable.

For instance, I want to replace sed -n \'1~5p\'

Answer 1

awk can also do it in a more elegant way:

awk -v n=YOUR_NUM 'NR%n==1' file

With -v n=YOUR_NUM you indicate the number. Then, NR%n==1 evaluates to true just when the line number is on a form of 7n+1, so it prints the line.

Note how good it is to use awk for this: if you want the lines on the form of 7n+k, you just need to do: awk -v n=7 'NR%n==k' file.

Example

Let's print every 7 lines:

$ seq 50 | awk -v n=7 'NR%n==1'
1
8
15
22
29
36
43
50

Or in sed:

$ n=7
$ seq 50 | sed -n "1~$n p" # quote the expression, so that "$n" is expanded
1
8
15
22
29
36
43
50