Among Unixheads, the Seventh Edition (V7) holds a special, almost mystical, place. Many consider it the best of the various Unix versions. In his Minix book, Tanenbaum remarks, “It is sometimes said Version 7 was not only an improvement over all its predecessors, but also over all it successors.” The source code has long been available and I have always found it the easiest to read and learn from.
Chris Siebenmann has a blog post with his own take on the importance of V7. He agrees with most of what I’ve written above but notes that V7 is historically important because it is the base from which (almost) all subsequent Unices are derived. If you understand V7, you’re well on your way to understanding everything that came afterwards.
Siebenmann goes further and says that V7 was the last pubic release from the CSRC (the Bell Labs researchers who invented Unix) and thus represents the last release made by those who really understood the Unix way. Those who came later, Siebenmann says, brought their own sensibilities to table and lacked the appreciation and understanding of Unix that its originators had.
If you like Unix and are interested in its history, you should definitely take a look at Siebenmann’s post. It’s interesting and reasonably short.