(gawk.info.gz) I/O And BEGIN/END

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 6.1.4.2 Input/Output from `BEGIN' and `END' Rules
 .................................................
 
 There are several (sometimes subtle) points to remember when doing I/O
 from a `BEGIN' or `END' rule.  The first has to do with the value of
 `$0' in a `BEGIN' rule.  Because `BEGIN' rules are executed before any
 input is read, there simply is no input record, and therefore no
 fields, when executing `BEGIN' rules.  References to `$0' and the fields
 yield a null string or zero, depending upon the context.  One way to
 give `$0' a real value is to execute a `getline' command without a
 variable ( Getline).  Another way is simply to assign a value to
 `$0'.
 
    The second point is similar to the first but from the other
 direction.  Traditionally, due largely to implementation issues, `$0'
 and `NF' were _undefined_ inside an `END' rule.  The POSIX standard
 specifies that `NF' is available in an `END' rule. It contains the
 number of fields from the last input record.  Most probably due to an
 oversight, the standard does not say that `$0' is also preserved,
 although logically one would think that it should be.  In fact, `gawk'
 does preserve the value of `$0' for use in `END' rules.  Be aware,
 however, that Unix `awk', and possibly other implementations, do not.
 
    The third point follows from the first two.  The meaning of `print'
 inside a `BEGIN' or `END' rule is the same as always: `print $0'.  If
 `$0' is the null string, then this prints an empty line.  Many long
 time `awk' programmers use an unadorned `print' in `BEGIN' and `END'
 rules, to mean `print ""', relying on `$0' being null.  Although one
 might generally get away with this in `BEGIN' rules, it is a very bad
 idea in `END' rules, at least in `gawk'.  It is also poor style, since
 if an empty line is needed in the output, the program should print one
 explicitly.
 
    Finally, the `next' and `nextfile' statements are not allowed in a
 `BEGIN' rule, because the implicit
 read-a-record-and-match-against-the-rules loop has not started yet.
 Similarly, those statements are not valid in an `END' rule, since all
 the input has been read.  ( Next Statement, and see 
 Nextfile Statement.)
 

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