BEGIN and END Rules ¶There are several (sometimes subtle) points to be aware of 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 function
without a variable (see Explicit Input with 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, all of BWK awk, mawk, and gawk
preserve the value of $0 for use in END rules. Be aware,
however, that some other implementations and many older versions
of Unix awk 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 record. Many longtime 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, because 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, because all the input has been read.
(See The next Statement and
see The nextfile Statement.)