According to the POSIX standard, files named on the
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.
gawk produces a warning for a directory on the
command line, but otherwise ignores it. If either of the --posix
or --traditional options is given, then
to treating a directory on the command line as a fatal error.