I have recently come across awk seen option. I can see it is removing duplicates in files. But unsure how it works tried google but didnt find any results, need help in clarification for the same. cat tes 1 2 3 1 1 1 3 4  with awk seen output cat tes | awk '!seen[$0]++'  1 2 3 4
This one-liner removes duplicate lines from text input without pre-sorting.  For example:  $ cat &gt;f q w e w r $ awk '!a[$0]++' &lt;f q w e r $    The original code I have found on the internets read:  awk '!_[$0]++'  This was even more perplexing to me as I took _ to have a special meaning in awk, like in Perl, but it turned out to be just a name of an array.  Now, I understand the logic behind the one-liner:  each input line is used as a key in a hash array, thus, upon completion, the hash contains unique lines in the order of arrival.  What I would like to learn is how exactly this notation is interpreted by awk. E.g. what the bang sign (!) means and the other elements of this code snippet.  How does it work?
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