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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 section 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 section Invoking awk, 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.

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