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`BEGIN' and `END' Special Patterns

   `BEGIN' and `END' are special patterns.  They are not used to match
input records.  Rather, they are used for supplying start-up or
clean-up information to your `awk' script.  A `BEGIN' rule is executed,
once, before the first input record has been read.  An `END' rule is
executed, once, after all the input has been read.  For example:

     awk 'BEGIN { print "Analysis of `foo'" }
          /foo/ { ++foobar }
          END   { print "`foo' appears " foobar " times." }' BBS-list

   This program finds the number of records in the input file `BBS-list'
that contain the string `foo'.  The `BEGIN' rule prints a title for the
report.  There is no need to use the `BEGIN' rule to initialize the
counter `foobar' to zero, as `awk' does this for us automatically
(see Variables.).

   The second rule increments the variable `foobar' every time a record
containing the pattern `foo' is read.  The `END' rule prints the value
of `foobar' at the end of the run.

   The special patterns `BEGIN' and `END' cannot be used in ranges or
with boolean operators (indeed, they cannot be used with any operators).

   An `awk' program may have multiple `BEGIN' and/or `END' rules.  They
are executed in the order they appear, all the `BEGIN' rules at
start-up and all the `END' rules at termination.

   Multiple `BEGIN' and `END' sections are useful for writing library
functions, since each library can have its own `BEGIN' or `END' rule to
do its own initialization and/or cleanup.  Note that the order in which
library functions are named on the command line controls the order in
which their `BEGIN' and `END' rules are executed.  Therefore you have
to be careful to write such rules in library files so that the order in
which they are executed doesn't matter.  See Invoking `awk': Command Line, for more information on using library functions.

   If an `awk' program only has a `BEGIN' rule, and no other rules,
then the program exits after the `BEGIN' rule has been run.  (Older
versions of `awk' used to keep reading and ignoring input until end of
file was seen.)  However, if an `END' rule exists as well, then the
input will be read, even if there are no other rules in the program.
This is necessary in case the `END' rule checks the `NR' variable.

   `BEGIN' and `END' rules must have actions; there is no default
action for these rules since there is no current record when they run.