AWK – How to Remove Duplicate Lines While Keeping Empty Lines

Here is a "intuitive" answer, for a more in depth explanation of awk's mechanism see either @Cuonglm's

In this case, !a[$0]++, the post-increment ++ can be set aside for a moment, it does not change the value of the expression. So, look at only !a[$0]. Here:

a[$0]

uses the current line $0 as key to the array a, taking the value stored there. If this particular key was never referenced before, a[$0] evaluates to the empty string.

!a[$0]

The ! negates the value from before. If it was empty or zero (false), we now have a true result. If it was non-zero (true), we have a false result. If the whole expression evaluated to true, meaning that a[$0] was not set to begin with, the whole line is printed as the default action.

Also, regardless of the old value, the post-increment operator adds one to a[$0], so the next time the same value in the array is accessed, it will be positive and the whole condition will fail.

I doubt it will make a difference but, just in case, here's how to do the same thing in Perl:

perl -ne 'print if ++$k{$_}==1' out.txt

If the problem is keeping the unique lines in memory, that will have the same issue as the awk you tried. So, another approach could be:

cat -n out.txt | sort -k2 -k1n  | uniq -f1 | sort -nk1,1 | cut -f2-

How it works:

1. On a GNU system, cat -n will prepend the line number to each line following some amount of spaces and followed by a <tab> character. cat pipes this input representation to sort.

2. sort's -k2 option instructs it only to consider the characters from the second field until the end of the line when sorting, and sort splits fields by default on white-space (or cat's inserted spaces and <tab>).
   When followed by -k1n, sort considers the 2nd field first, and then secondly—in the case of identical -k2 fields—it considers the 1st field but as sorted numerically. So repeated lines will be sorted together but in the order they appeared.

3. The results are piped to uniq—which is told to ignore the first field (-f1 - and also as separated by whitespace)—and which results in a list of unique lines in the original file and is piped back to sort.
4. This time sort sorts on the first field (cat's inserted line number) numerically, getting the sort order back to what it was in the original file and pipes these results to cut.
5. Lastly, cut removes the line numbers that were inserted by cat. This is effected by cut printing only from the 2nd field through the end of the line (and cut's default delimiter is a <tab> character).

To illustrate:

$ cat file
bb
aa
bb
dd
cc
dd
aa
bb
cc
$ cat -n file | sort -k2 | uniq -f1 | sort -k1 | cut -f2-
bb
aa    
dd
cc