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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LEARN ABOUT OPENDARWIN
cancel
cancel(1) cancel(1)
NAME
cancel - cancel print request
SYNOPSIS
cancel [ request-ID...] [destination...]
cancel -u user... [destination...]
The cancel utility cancels print requests. There are two forms of the cancel command.
The first form of cancel has two optional arguments: print requests (request-ID) and destinations (destination). Specifying request-ID with
destination cancels request-ID on destination. Specifying only the destination cancels the current print request on destination. If desti-
nation is not specified, cancel cancels the requested print request on all destinations.
The second form of cancel cancels a user's print requests on specific destinations.
Users can only cancel print requests associated with their username. By default, users can only cancel print requests on the host from
which the print request was submitted. If a super-user has set user-equivalence=true in /etc/printers.conf on the print server, users can
cancel print requests associated with their username on any host. Super-users can cancel print requests on the host from which the print
request was submitted. Superusers can also cancel print requests from the print server.
The print client commands locate destination information using the printers database in the name service switch. See nsswitch.conf(4),
printers(4), and printers.conf(4) for details.
The following options are supported:
-u user The name of the user for which print requests are to be cancelled. Specify user as a username.
The following operands are supported:
destination The destination on which the print requests are to be canceled. destination is the name of a printer or class of printers
(see lpadmin(1M)). If destination is not specified, cancel cancels the requested print request on all destinations. Specify
destination using atomic or POSIX-style (server:destination), names. See for information regarding using POSIX-style des-
tination names with cancel. See standards(5) for information regarding POSIX.
request-ID The print request to be canceled. Specify request-ID using LP-style request IDs (destination-number).
user The name of the user for which the print requests are to be cancelled. Specify user as a username.
The following exit values are returned:
0 Successful completion.
non-zero An error occurred.
/var/spool/print/* LP print queue.
$HOME/.printers User-configurable printer database.
/etc/printers.conf System printer configuration database.
printers.conf.byname NIS version of /etc/printers.conf.
printers.org_dir NIS+ version of /etc/printers.conf.
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Availability |SUNWpcu |
+-----------------------------+-----------------------------+
lp(1), lpq(1B), lpr(1B), lprm(1B), lpstat(1), lpadmin( 1M), nsswitch.conf(4), printers(4), printers.conf(4), attributes(5), standards(5)
POSIX-style destination names (server:destination) are treated as print requests if destination has the same format as an LP-style request-
ID. See standards(5).
Some print servers send cancelation notification to job owners when their print jobs have been cancelled. This notification usually comes
in the form of an email message. Cancelation notices cannot be disabled on a Solaris server.
23 Feb 2005 cancel(1)