Understanding esoteric awk syntax

[GitWasAMistakeItsNothingButTrash]

How to parse {$0="hello world"RS$0}7, as in awk 'NR == 1 {$0="hello world"RS$0}7' input > output? In particular, what does the "7" do?

[SteelTermite]

Well, that's a fun one! The 7 causes the records to be printed, right? How did you discover that?

I think the 7 is interpreted as a boolean expression, which evaluates to true. So it is a pattern, just like NR == 1 in your example. It seems that there is an implicit {print} after the pattern, which causes the records to be printed. If you replace 7 with {print}, the result will be the same. And if you change 7to NR % 2 == 1, only oddly-numbered records are printed. Why can you leave out the {print} then? I have no idea.

[GitWasAMistakeItsNothingButTrash]

I wonder why they chose "7" for that, it strikes me as obfuscating the code more than anything else.

I found it on Stack Exchange, after my instinctive expression 'NR == 1 { print "hello world"; }' didn't work.

[SteelTermite]

Hmm, interesting. The author of the question uses the same trick, but with 1 instead of 7:

How to add lines to the beginning of the file? I tried so

for i in $(ls); do awk 'NR==1{print "first_line" }'1 "$i" > "$i"; done

(The 1 is outside the quotes, but that does not make a difference here. Try echo 'high'5 to see that Bash merges it into a single argument.)

I also found someone describing this trick as a code golf technique here:

Remember that the default action for a matching pattern is print $0, so if you get what you want printed into $0, just use

1

[GitWasAMistakeItsNothingButTrash]

Fascinating. So is there any difference between the print commands 1 and 7? And if not, can any number be used?