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)