02-27-2020
Yes, $2 (USER=MICKEY) is split at the equals sign into array T , and then rebuilt with the lower case username in T[2].
The !a[$0]++ is a trick (independent of $2): a[$0] evaluates to FALSE if it equals zero or doesn't exist or is created on first reference, so its negation is TRUE and triggers the default action: print. Then it is "post incremented" and will never (OK, not until reaching / crossing MAX_INT) trigger again. So any further occurrences of $0 are suppressed.
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LEARN ABOUT CENTOS
dpkg-trigger
dpkg-trigger(1) dpkg suite dpkg-trigger(1)
NAME
dpkg-trigger - a package trigger utility
SYNOPSIS
dpkg-trigger [option...] trigger-name
dpkg-trigger [option...] command
DESCRIPTION
dpkg-trigger is a tool to explicitly activate triggers and check for its support on the running dpkg.
This can be used by maintainer scripts in complex and conditional situations where the file triggers, or the declarative activate triggers
control file directive, are insufficiently rich. It can also be used for testing and by system administrators (but note that the triggers
won't actually be run by dpkg-trigger).
Unrecognized trigger name syntaxes are an error for dpkg-trigger.
COMMANDS
--check-supported
Check if the running dpkg supports triggers (usually called from a postinst). Will exit 0 if a triggers-capable dpkg has run, or 1
with an error message to stderr if not. Normally, however, it is better just to activate the desired trigger with dpkg-trigger.
-?, --help
Show the usage message and exit.
--version
Show the version and exit.
OPTIONS
--admindir=dir
Change the location of the dpkg database. The default location is /var/lib/dpkg.
--by-package=package
Override trigger awaiter (normally set by dpkg through the DPKG_MAINTSCRIPT_PACKAGE environment variable of the maintainer scripts,
naming the package to which the script belongs, and this will be used by default).
--no-await
This option arranges that the calling package T (if any) need not await the processing of this trigger; the interested package(s) I,
will not be added to T's trigger processing awaited list and T's status is unchanged. T may be considered installed even though I
may not yet have processed the trigger.
--await
This option does the inverse of --no-await (since dpkg 1.17.21). It is currently the default behavior.
--no-act
Just test, do not actually change anything.
EXIT STATUS
0 The requested action was successfully performed. Or a check or assertion command returned true.
1 A check or assertion command returned false.
2 Fatal or unrecoverable error due to invalid command-line usage, or interactions with the system, such as accesses to the database,
memory allocations, etc.
ENVIRONMENT
DPKG_ADMINDIR
If set and the --admindir option has not been specified, it will be used as the dpkg data directory.
SEE ALSO
dpkg(1), deb-triggers(5), /usr/share/doc/dpkg-dev/triggers.txt.gz.
1.19.0.5 2018-04-16 dpkg-trigger(1)