start in on a shared_space chapter

chapters

@@ -6,6 +6,7 @@
 ./script/index.md
 ./general_purpose/index.md
 ./diff/index.md
+./shared_space/index.md
 ./web/index.md
 ./miscellany/index.md
 ./endmatter/index.md

shared_space/index.md

@@ -0,0 +1,14 @@
+7. the command line as as a shared world
+========================================
+
+In an earlier chapter, I wrote:
+
+> You can think of the shell as a kind of environment you inhabit, in much
+> the way your character inhabits an adventure game.
+
+It turns out that sometimes there are other human inhabitants of this
+environment.
+
+Unix was built on a model known as "time-sharing".  This is an idea with a lot
+of history, but the very short version is that when computers were rare and
+expensive, it made sense for lots of people to be able to use them at once.

web/index.md

@@ -1,4 +1,4 @@
-7. the command line and the web
+8. the command line and the web
 ===============================
 
 Web browsers are really complicated these days.  They're full of rendering