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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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PASTE(1)						    BSD General Commands Manual 						  PASTE(1)

NAME
paste -- merge corresponding or subsequent lines of files SYNOPSIS
paste [-s] [-d list] file ... DESCRIPTION
The paste utility concatenates the corresponding lines of the given input files, replacing all but the last file's newline characters with a single tab character, and writes the resulting lines to standard output. If end-of-file is reached on an input file while other input files still contain data, the file is treated as if it were an endless source of empty lines. The options are as follows: -d list Use one or more of the provided characters to replace the newline characters instead of the default tab. The characters in list are used circularly, i.e., when list is exhausted the first character from list is reused. This continues until a line from the last input file (in default operation) or the last line in each file (using the -s option) is displayed, at which time paste begins selecting characters from the beginning of list again. The following special characters can also be used in list: newline character tab character \ backslash character Empty string (not a null character). Any other character preceded by a backslash is equivalent to the character itself. -s Concatenate all of the lines of each separate input file in command line order. The newline character of every line except the last line in each input file is replaced with the tab character, unless otherwise specified by the -d option. If '-' is specified for one or more of the input files, the standard input is used; standard input is read one line at a time, circularly, for each instance of '-'. EXIT STATUS
The paste utility exits 0 on success, and >0 if an error occurs. EXAMPLES
List the files in the current directory in three columns: ls | paste - - - Combine pairs of lines from a file into single lines: paste -s -d ' ' myfile Number the lines in a file, similar to nl(1): sed = myfile | paste -s -d ' ' - - Create a colon-separated list of directories named bin, suitable for use in the PATH environment variable: find / -name bin -type d | paste -s -d : - SEE ALSO
cut(1), lam(1) STANDARDS
The paste utility is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible. HISTORY
A paste command appeared in Version 32V AT&T UNIX. BSD
June 25, 2004 BSD
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