Most awk variables are available to use for your own
purposes; they never change unless your program assigns values to
them, and they never affect anything unless your program examines them.
However, a few variables in awk have special built-in meanings.
awk examines some of these automatically, so that they enable you
to tell awk how to do certain things. Others are set
automatically by awk, so that they carry information from the
internal workings of awk to your program.
This section documents all of gawk’s predefined variables,
most of which are also documented in the chapters describing
their areas of activity.