awk - What are NR and FNR and what does "NR==FNR" imply?

Question

I am learning file comparison using awk.

I found syntax like below,

awk 'NR==FNR{a[$1];next}$1 in a{print $1}' file1 file2

I couldn't understand what is the significance of NR==FNR in this? If I try with FNR==NR then also I get the same output?

What exactly does it do?

Answer 1

In awk, FNR refers to the record number (typically the line number) in the current file and NR refers to the total record number. The operator == is a comparison operator, which returns true when the two surrounding operands are equal.

This means that the condition NR==FNR is only true for the first file, as FNR resets back to 1 for the first line of each file but NR keeps on increasing.

This pattern is typically used to perform actions on only the first file. The next inside the block means any further commands are skipped, so they are only run on files other than the first.

The condition FNR==NR compares the same two operands as NR==FNR, so it behaves in the same way.