docs: add rfork/env explanations, fix table ordering in Shell.nw

Add export/isolation explanation to rfork section, footnote in Intro.nw
linking to it, exec builtin explanation, initialization chapter row
in code organization table, reorder table rows to match chapter order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: aryx

shells/Intro.nw

@@ -211,12 +211,12 @@ basic command interpreter, not a full scripting language like [[rc]].
 \l if/goto (and glob) were external programs
 
 %claude:
-The small size of Ken Thompson's shell reflects the \unix philosophy
-of keeping the shell minimal: [[if]] and [[goto]] were separate
-programs (not builtins), and wildcard expansion was handled by
-an external [[glob]] program. This made the shell itself trivial
-at the cost of some awkwardness---for example, you could not
-write a loop without creating a script file for [[goto]] to jump into.
+%The small size of Ken Thompson's shell reflects the \unix philosophy
+%of keeping the shell minimal: [[if]] and [[goto]] were separate
+%programs (not builtins), and wildcard expansion was handled by
+%an external [[glob]] program. This made the shell itself trivial
+%at the cost of some awkwardness---for example, you could not
+%write a loop without creating a script file for [[goto]] to jump into.
 
 %history:
 \item The Bourne shell~\cite{sh}, also called [[sh]], superseded 
@@ -261,14 +261,14 @@ as in [[rc]], but instead a complex recursive descent parser.
 \l design of sh by bourne: https://www.youtube.com/watch?v=2kEJoWfobpA
 
 %claude:
-Part of the difficulty comes from the source code itself: Bourne
-wrote extensive C macros to make the code look like Algol
-([[IF]], [[THEN]], [[BEGIN]], [[END]], etc.), which allegedly
-inspired the International Obfuscated C Code Contest.
-More fundamentally, the Bourne shell has no formal grammar---Tom Duff
-once remarked that ``nobody knows the grammar of [[sh]]''---so
-the parser is full of special cases and context-dependent
-disambiguation that make it hard to reason about.
+%Part of the difficulty comes from the source code itself: Bourne
+%wrote extensive C macros to make the code look like Algol
+%([[IF]], [[THEN]], [[BEGIN]], [[END]], etc.), which allegedly
+%inspired the International Obfuscated C Code Contest.
+%More fundamentally, the Bourne shell has no formal grammar---Tom Duff
+%once remarked that ``nobody knows the grammar of [[sh]]''---so
+%the parser is full of special cases and context-dependent
+%disambiguation that make it hard to reason about.
 
 %\item C shell lots of interactive features.
 %\item Korn shell lots of interactive features.
@@ -1263,7 +1263,10 @@ variable stringification, see Section~\ref{sec:quoting}).
 %claude:
 The [[boot.rc]] script also assigns [[objtype=arm]].
 This is not just a local shell variable: in \plan, all shell
-variables are automatically exported to the environment.
+variables are automatically exported to the environment---there
+is no need for an [[export]] command like in [[bash]]%
+\footnote{If you need environment isolation, [[rfork e]] creates
+a private copy of [[/env/]]; see Section~\ref{sec:rfork}.}.
 When [[mk]] (the build tool) is later run from this shell,
 it inherits [[$objtype]] and uses it to select the correct
 compiler and assembler (e.g., [[5c]], [[5a]], [[5l]] for ARM).
@@ -1303,6 +1306,10 @@ would be unaffected.
 The [[exec]] builtin replaces the current shell process
 with the specified command.  Here it replaces the boot
 shell with an interactive instance of [[rc]] (the [[-i]] flag).
+Without [[exec]], the boot shell would fork a child [[rc]],
+wait for it to finish, and then continue---but there is
+nothing left to do, so the extra process would just waste
+a process slot until the interactive shell exits.
 See Chapter~\ref{chap:builtins} for the full list of builtins.
 
 \subsection{Other features}

shells/Shell.nw

@@ -172,12 +172,10 @@ other bytecodes                & \ref{chap:compile-interpret}  & [[exec.c]]
 \midrule
 shell builtins                 & \ref{chap:builtins}           & [[builtins.c]] & [<execcd()>] [<execexit()>] [<execdot()>]       & \\ % execshift execeval execflag [execwait()>]  [execwhatis()>]
 shell environment              & \ref{chap:environment}        & [[env.c]]      & [<addenv()>] [<Vinit()>]                        & \\ % Updenv
+signal management              & \ref{chap:signals}            & [[trap.c]]     & [<dotrap()>] [<Trapinit()>]                     & \\ %
+initialization (bootstrap)     & \ref{chap:initialization-real} & [[main.c]]     & [<Rcmain>] [<dotcmds>]                          & \\ %
 wildcard matching              & \ref{chap:globbing}           & [[glob.c]]     & [<glob()>] [<match()>]                          & \\ % globlist Xglob
-signal management              & \ref{chap:signals}            & [[trap.c]]     & [<dotrap()>]                                    & \\ %
-
-\midrule
-% initialization chapter?
-here documents & \ref{chap:advanced-topics}    & [[here.c]]     & [<readhere()>] [<heredoc()>]                    & \\ %
+here documents                 & \ref{chap:advanced-topics}    & [[here.c]]     & [<readhere()>] [<heredoc()>]                    & \\ %
 
 \midrule
 AST dumper                     & \ref{chap:debugging-appendix} & [[pcmd.c]]     & [<pcmd()>]                                      & \\ %
@@ -711,7 +709,7 @@ is responsible for this transformation.
 
 %toc:
 The token kinds in [[rc]] are declared below using
-the [[yacc]] syntax [[\%token]] (see \book{Compiler}).
+the [[yacc]] syntax \verb+%token+ (see \book{Compiler}).
 %
 I have grouped them in categories:
 the keyword tokens ([[FOR]], [[WHILE]], [[IF]], etc.),
@@ -9336,9 +9334,8 @@ Executable(char *file)
 
 
 \subsection{[[$ rfork]]}
-%$
+\label{sec:rfork}
 
-%rfork
 %claude:
 The [[rfork]] builtin exposes Plan~9's [[rfork(2)]]
 system call, which selectively shares or unshares
@@ -9350,6 +9347,28 @@ file descriptors.
 This is the Plan~9 equivalent of Linux containers
 and namespaces---but from 1992.
 
+%claude:
+The environment flag is particularly interesting.
+As mentioned in Chapter~\ref{chap:environment},
+all [[rc]] variables are automatically exported to [[/env/]]---there
+is no [[export]] command like in [[bash]].
+But this raises the question: how do you {\em prevent}
+a variable from being visible to children?
+The answer is [[rfork]]:
+[[rfork e]] creates a {\em copy} of the current [[/env/]],
+so subsequent changes in either the parent or the child
+are invisible to the other;
+[[rfork E]] creates an entirely {\em empty} [[/env/]],
+starting with a clean slate.
+%
+Note that the lowercase flags ([[n]], [[e]], [[f]]) copy
+the resource, while the uppercase flags ([[N]], [[E]], [[F]])
+clear it entirely.
+%
+When called with no arguments, [[rfork]] defaults
+to [[RFENVG|RFNAMEG|RFNOTEG]], creating new
+environment, mount namespace, and note groups all at once.
+
 <<function [[execnewpgrp]]>>=
 void
 execnewpgrp(void)