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/ { ++n }
> END { print "\"foo\" appears " n " times." }' BBS-list
-| Analysis of "foo"
-| "foo" appears 4 times.
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 n to zero, as awk does this
automatically (see section Variables).
The second rule increments the variable n every time a
record containing the pattern `foo' is read. The END rule
prints the value of n 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.
BEGIN and END rules may be intermixed with other rules.
This feature was added in the 1987 version of awk, and is included
in the POSIX standard. The original (1978) version of awk
required you to put the BEGIN rule at the beginning of the
program, and the END rule at the end, and only allowed one of
each. This is no longer required, but it is a good idea in terms of
program organization and readability.
Multiple BEGIN and END rules are useful for writing
library functions, since each library file can have its own BEGIN and/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 section Command Line Options, for more information on
using library functions.
See section A Library of awk Functions,
for a number of useful 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.
(The original version of awk used to keep reading and ignoring input
until end of file was seen.) However, if an END rule exists,
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
FNR and NR variables (d.c.).
BEGIN and END rules must have actions; there is no default
action for these rules since there is no current record when they run.
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