According to the POSIX standard, files named on the awk
command line must be text files; it is a fatal error if they are not.
Most versions of awk treat a directory on the command line as
a fatal error.
By default, gawk produces a warning for a directory on the
command line, but otherwise ignores it. This makes it easier to use
shell wildcards with your awk program:
$ gawk -f whizprog.awk * Directories could kill this program
If either of the --posix
or --traditional options is given, then gawk reverts
to treating a directory on the command line as a fatal error.
See Reading Directories for a way to treat directories
as usable data from an awk program.