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  • BTW, this awk script is run once and processes both files in one pass. this is as fast as it gets. Your while-read-awk loop runs awk once for every line in in.txt, this is about as slow as it gets - partly because of the overhead of starting up awk and partly because reading text in a shell while loop is slow. Commented Oct 24, 2021 at 6:59
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    I gave up on NR==FNR after one of my clients passed an empty first file, and my awk tried to build a 40GB array from the second file. I moved to awk 'myAwk' fSeq=1 A.file fSeq=2 B.file. This also works for multiple reference files. Commented Oct 24, 2021 at 8:15
  • @Paul_Pedant GIGO. BTW, technically an empty file isn't a text file - it doesn't have a line ending in \n Commented Oct 24, 2021 at 10:23
  • @cas thanks,,,so technically it will consume less cpu right...i compare both side by side awk consumes high cpu... Commented Oct 24, 2021 at 14:47
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    @UncleBilly that statement is so unjustifiably smug and arrogant that I don't even think it's possible for a human. How about using your god-like genius to correct the literally hundreds of answers here on this site that use FNR==NR? What's lame is that awk doesn't deal with it sanely - it should treat an empty file as if it had one line, even if it doesn't have a newline. Either that, or it should have a file-counter variable, perhaps FC, that can be used instead. Commented Oct 25, 2021 at 0:28