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Top Forums UNIX for Beginners Questions & Answers Need advise/tip if there is more efficient way of doing this cut/paste/awk after changing a field Post 303044636 by MadeInGermany on Friday 28th of February 2020 12:21:43 PM
Old 02-28-2020
A more detailed explanation follows.
The main awk code runs for each input line.
!a[$0]++ is ultra-condensed, quick and dirty.
A bit more explicit is !($0 in A) { A[$0]; print }:
If not $0 in array A (A[$0] not defined) then define A[$0] (no A[$0]=value needed here) and print $0.
The array A is associative (string-addressed). So if the same $0 will occur in another input line it will see a defined A[$0] and won't print.
If there is a pre-condition and no { action code } following then the default for a true condition is { print }, and print without arguments defaults to print $0.

Now to the quick and dirty !A[$0]++:
Define A[$0] with value 0 if undefined, if the negated value is non-zero (true) then default-print. Also post-increment A[$0].
If the same $0 will occur then the A[$0] value will be 1, negated 0 (false), won't print, but post-incremented.
If the same $0 will occur then the A[$0] value will be 2, negated 0 (false), won't print, but post-incremented.
...
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PRINT(3)								 1								  PRINT(3)

print - Output a string

SYNOPSIS
int print (string $arg) DESCRIPTION
Outputs $arg. print is not actually a real function (it is a language construct) so you are not required to use parentheses with its argument list. PARAMETERS
o $arg - The input data. RETURN VALUES
Returns 1, always. EXAMPLES
Example #1 print examples <?php print("Hello World"); print "print() also works without parentheses."; print "This spans multiple lines. The newlines will be output as well"; print "This spans multiple lines. The newlines will be output as well."; print "escaping characters is done "Like this"."; // You can use variables inside a print statement $foo = "foobar"; $bar = "barbaz"; print "foo is $foo"; // foo is foobar // You can also use arrays $bar = array("value" => "foo"); print "this is {$bar['value']} !"; // this is foo ! // Using single quotes will print the variable name, not the value print 'foo is $foo'; // foo is $foo // If you are not using any other characters, you can just print variables print $foo; // foobar print <<<END This uses the "here document" syntax to output multiple lines with $variable interpolation. Note that the here document terminator must appear on a line with just a semicolon no extra whitespace! END; ?> NOTES
Note Because this is a language construct and not a function, it cannot be called using variable functions. SEE ALSO
echo(3), printf(3), flush(3), Heredoc syntax. PHP Documentation Group PRINT(3)
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