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Many of the GNU tools are reimplementation of already existing tools.

For example the initial implementation of AWK was in 1977 [1], a few years before GNU even existed [2], so it _is_ a Unix tool.

[1] https://en.wikipedia.org/wiki/AWK#History [2] https://en.wikipedia.org/wiki/GNU#History

The GNU coreutils are a reimplementation of the Unix utilities. With some caveats, the same tools are available on BSD and Solaris derivatives which are both Unix.

So while GNU is obviously an important project, it would not be correct to say "xargs" is not a Unix tool.

I and many others take “Unix” to mean any UNIX or UNIX-like operating system, whereas “UNIX” with all capital letters means only the ones that are certified UNIX.

awk is in the POSIX norm. It is a Unix tool.

GNU awk is one of the popular awk implementation (usually referred as gawk). I personnally prefer mawk. awk is not GNU.

awk predates GNU by at least 7 years. It is most certainly a UNIX tool.