BEGINFILE
and ENDFILE
Special PatternsThis section describes a gawk-specific feature.
Two special kinds of rule, BEGINFILE
and ENDFILE
, give
you “hooks” into gawk's command-line file processing loop.
As with the BEGIN
and END
rules (see BEGIN/END), all
BEGINFILE
rules in a program are merged, in the order they are
read by gawk, and all ENDFILE
rules are merged as well.
The body of the BEGINFILE
rules is executed just before
gawk reads the first record from a file. FILENAME
is set to the name of the current file, and FNR
is set to zero.
The BEGINFILE
rule provides you the opportunity for two tasks
that would otherwise be difficult or impossible to perform:
You do this by checking if the ERRNO
variable is not the empty
string; if so, then gawk was not able to open the file. In
this case, your program can execute the nextfile
statement
(see Nextfile Statement). This causes gawk to skip
the file entirely. Otherwise, gawk exits with the usual
fatal error.
The ENDFILE
rule is called when gawk has finished processing
the last record in an input file. For the last input file,
it will be called before any END
rules.
The ENDFILE
rule is executed even for empty input files.
Normally, when an error occurs when reading input in the normal input
processing loop, the error is fatal. However, if an ENDFILE
rule is present, the error becomes non-fatal, and instead ERRNO
is set. This makes it possible to catch and process I/O errors at the
level of the awk program.
The next
statement (see Next Statement) is not allowed inside
either a BEGINFILE
or and ENDFILE
rule. The nextfile
statement (see Nextfile Statement) is allowed only inside a
BEGINFILE
rule, but not inside an ENDFILE
rule.
The getline
statement (see Getline) is restricted inside
both BEGINFILE
and ENDFILE
. Only the ‘getline
variable < file’ form is allowed.
BEGINFILE
and ENDFILE
are gawk extensions.
In most other awk implementations, or if gawk is in
compatibility mode (see Options), they are not special.