The GNU Awk User's Guide
7.5 Built-in Variables
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
awk how to do certain things. Others are set
awk, so that they carry information from the
internal workings of
awk to your program.
This section documents all the built-in variables of
gawk, most of which are also documented in the chapters
describing their areas of activity.