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The awk language has evolved over the years. Full details are
provided in The Evolution of the awk Language.
The language described in this Web page
is often referred to as "new awk" (nawk).
Because of this, many systems have multiple
versions of awk.
Some systems have an awk utility that implements the
original version of the awk language and a nawk utility
for the new
version.
Others have an oawk version for the "old awk"
language and plain awk for the new one. Still others only
have one version, which is usually the new one.1
All in all, this makes it difficult for you to know which version of
awk you should run when writing your programs. The best advice
I can give here is to check your local documentation. Look for awk,
oawk, and nawk, as well as for gawk.
It is likely that you already
have some version of new awk on your system, which is what
you should use when running your programs. (Of course, if you're reading
this Web page, chances are good that you have gawk!)
Throughout this Web page, whenever we refer to a language feature
that should be available in any complete implementation of POSIX awk,
we simply use the term awk. When referring to a feature that is
specific to the GNU implementation, we use the term gawk.