Modifying text contents based on operation on line number

Question

Let's say I have this data in a text file:

line1 
line2 
line3
line4
line5
...

I want to change this text file based on a pattern using line number e.g. line number mod 3,line contents:

1,line1
2,line2
3,line3
1,line4
2,line5
...

How can I achieve this (preferably using sed)?

Answer 1

This is hard to do in sed because it lacks arithmetic. However, there are a couple of ways you can do it with awk:

awk '{ printf "%d, %s\n", (linenum++ % 3) + 1, $0 }' lines.txt

or

awk '{ print (linenum++ % 3) + 1 ", " $0 }' lines.txt

In both cases, linenum is being used to count the lines, then the modulo operation is used to calculate the remainder of the line count divided by 3. The 1 is added to get the numbering scheme you requested.

printf provides a little "eye candy" to the program.

See man awk for details and other options.