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

 
 Input/Output from `BEGIN' and `END' Rules
 .........................................
 
    There are several (sometimes subtle) issues involved when doing I/O
 from a `BEGIN' or `END' rule.
 
    The first has to do with the value of `$0' in a `BEGIN' rule.  Since
 `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 ( Explicit Input
 with `getline' Getline.).  Another way is to simply assign a value to
 it.
 
    The second point is similar to the first, but from the other
 direction.  Inside an `END' rule, what is the value of `$0' and `NF'?
 Traditionally, due largely to implementation issues, `$0' and `NF' were
 _undefined_ inside an `END' rule.  The POSIX standard specified that
 `NF' was available in an `END' rule, containing the number of fields
 from the last input record.  Due most probably 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.  What is the meaning of
 `print' inside a `BEGIN' or `END' rule?  The meaning 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 `print' in `BEGIN'
 and `END' rules, to mean `print ""', relying on `$0' being null.  While
 you might generally get away with this in `BEGIN' rules, in `gawk' at
 least, it is a very bad idea in `END' rules.  It is also poor style,
 since if you want an empty line in the output, you should say so
 explicitly in your program.
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(gawk.info) Using BEGIN/END
(gawk.info) BEGIN/END
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