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-#	@(#)TOUR	8.1 (Berkeley) 5/31/93
-# $FreeBSD: releng/12.1/bin/sh/TOUR 317882 2017-05-06 13:28:42Z jilles $
-
-NOTE -- This is the original TOUR paper distributed with ash and
-does not represent the current state of the shell.  It is provided anyway
-since it provides helpful information for how the shell is structured,
-but be warned that things have changed -- the current shell is
-still under development.
-
-================================================================
-
-                       A Tour through Ash
-
-               Copyright 1989 by Kenneth Almquist.
-
-
-DIRECTORIES:  The subdirectory bltin contains commands which can
-be compiled stand-alone.  The rest of the source is in the main
-ash directory.
-
-SOURCE CODE GENERATORS:  Files whose names begin with "mk" are
-programs that generate source code.  A complete list of these
-programs is:
-
-        program         input files         generates
-        -------         -----------         ---------
-        mkbuiltins      builtins.def        builtins.h builtins.c
-        mknodes         nodetypes           nodes.h nodes.c
-        mksyntax            -               syntax.h syntax.c
-        mktokens            -               token.h
-
-There are undoubtedly too many of these.
-
-EXCEPTIONS:  Code for dealing with exceptions appears in
-exceptions.c.  The C language doesn't include exception handling,
-so I implement it using setjmp and longjmp.  The global variable
-exception contains the type of exception.  EXERROR is raised by
-calling error.  EXINT is an interrupt.
-
-INTERRUPTS:  In an interactive shell, an interrupt will cause an
-EXINT exception to return to the main command loop.  (Exception:
-EXINT is not raised if the user traps interrupts using the trap
-command.)  The INTOFF and INTON macros (defined in exception.h)
-provide uninterruptible critical sections.  Between the execution
-of INTOFF and the execution of INTON, interrupt signals will be
-held for later delivery.  INTOFF and INTON can be nested.
-
-MEMALLOC.C:  Memalloc.c defines versions of malloc and realloc
-which call error when there is no memory left.  It also defines a
-stack oriented memory allocation scheme.  Allocating off a stack
-is probably more efficient than allocation using malloc, but the
-big advantage is that when an exception occurs all we have to do
-to free up the memory in use at the time of the exception is to
-restore the stack pointer.  The stack is implemented using a
-linked list of blocks.
-
-STPUTC:  If the stack were contiguous, it would be easy to store
-strings on the stack without knowing in advance how long the
-string was going to be:
-        p = stackptr;
-        *p++ = c;       /* repeated as many times as needed */
-        stackptr = p;
-The following three macros (defined in memalloc.h) perform these
-operations, but grow the stack if you run off the end:
-        STARTSTACKSTR(p);
-        STPUTC(c, p);   /* repeated as many times as needed */
-        grabstackstr(p);
-
-We now start a top-down look at the code:
-
-MAIN.C:  The main routine performs some initialization, executes
-the user's profile if necessary, and calls cmdloop.  Cmdloop
-repeatedly parses and executes commands.
-
-OPTIONS.C:  This file contains the option processing code.  It is
-called from main to parse the shell arguments when the shell is
-invoked, and it also contains the set builtin.  The -i and -m op-
-tions (the latter turns on job control) require changes in signal
-handling.  The routines setjobctl (in jobs.c) and setinteractive
-(in trap.c) are called to handle changes to these options.
-
-PARSING:  The parser code is all in parser.c.  A recursive des-
-cent parser is used.  Syntax tables (generated by mksyntax) are
-used to classify characters during lexical analysis.  There are
-four tables:  one for normal use, one for use when inside single
-quotes and dollar single quotes, one for use when inside double
-quotes and one for use in arithmetic.  The tables are machine
-dependent because they are indexed by character variables and
-the range of a char varies from machine to machine.
-
-PARSE OUTPUT:  The output of the parser consists of a tree of
-nodes.  The various types of nodes are defined in the file node-
-types.
-
-Nodes of type NARG are used to represent both words and the con-
-tents of here documents.  An early version of ash kept the con-
-tents of here documents in temporary files, but keeping here do-
-cuments in memory typically results in significantly better per-
-formance.  It would have been nice to make it an option to use
-temporary files for here documents, for the benefit of small
-machines, but the code to keep track of when to delete the tem-
-porary files was complex and I never fixed all the bugs in it.
-(AT&T has been maintaining the Bourne shell for more than ten
-years, and to the best of my knowledge they still haven't gotten
-it to handle temporary files correctly in obscure cases.)
-
-The text field of a NARG structure points to the text of the
-word.  The text consists of ordinary characters and a number of
-special codes defined in parser.h.  The special codes are:
-
-        CTLVAR              Parameter expansion
-        CTLENDVAR           End of parameter expansion
-        CTLBACKQ            Command substitution
-        CTLBACKQ|CTLQUOTE   Command substitution inside double quotes
-        CTLARI              Arithmetic expansion
-        CTLENDARI           End of arithmetic expansion
-        CTLESC              Escape next character
-
-A variable substitution contains the following elements:
-
-        CTLVAR type name '=' [ alternative-text CTLENDVAR ]
-
-The type field is a single character specifying the type of sub-
-stitution.  The possible types are:
-
-        VSNORMAL            $var
-        VSMINUS             ${var-text}
-        VSMINUS|VSNUL       ${var:-text}
-        VSPLUS              ${var+text}
-        VSPLUS|VSNUL        ${var:+text}
-        VSQUESTION          ${var?text}
-        VSQUESTION|VSNUL    ${var:?text}
-        VSASSIGN            ${var=text}
-        VSASSIGN|VSNUL      ${var:=text}
-        VSTRIMLEFT          ${var#text}
-        VSTRIMLEFTMAX       ${var##text}
-        VSTRIMRIGHT         ${var%text}
-        VSTRIMRIGHTMAX      ${var%%text}
-        VSLENGTH            ${#var}
-        VSERROR             delayed error
-
-In addition, the type field will have the VSQUOTE flag set if the
-variable is enclosed in double quotes and the VSLINENO flag if
-LINENO is being expanded (the parameter name is the decimal line
-number).  The parameter's name comes next, terminated by an equals
-sign.  If the type is not VSNORMAL (including when it is VSLENGTH),
-then the text field in the substitution follows, terminated by a
-CTLENDVAR byte.
-
-The type VSERROR is used to allow parsing bad substitutions like
-${var[7]} and generate an error when they are expanded.
-
-Commands in back quotes are parsed and stored in a linked list.
-The locations of these commands in the string are indicated by
-CTLBACKQ and CTLBACKQ+CTLQUOTE characters, depending upon whether
-the back quotes were enclosed in double quotes.
-
-Arithmetic expansion starts with CTLARI and ends with CTLENDARI.
-
-The character CTLESC escapes the next character, so that in case
-any of the CTL characters mentioned above appear in the input,
-they can be passed through transparently.  CTLESC is also used to
-escape '*', '?', '[', and '!' characters which were quoted by the
-user and thus should not be used for file name generation.
-
-CTLESC characters have proved to be particularly tricky to get
-right.  In the case of here documents which are not subject to
-variable and command substitution, the parser doesn't insert any
-CTLESC characters to begin with (so the contents of the text
-field can be written without any processing).  Other here docu-
-ments, and words which are not subject to file name generation,
-have the CTLESC characters removed during the variable and command
-substitution phase.  Words which are subject to file name
-generation have the CTLESC characters removed as part of the file
-name phase.
-
-EXECUTION:  Command execution is handled by the following files:
-        eval.c     The top level routines.
-        redir.c    Code to handle redirection of input and output.
-        jobs.c     Code to handle forking, waiting, and job control.
-        exec.c     Code to do path searches and the actual exec sys call.
-        expand.c   Code to evaluate arguments.
-        var.c      Maintains the variable symbol table.  Called from expand.c.
-
-EVAL.C:  Evaltree recursively executes a parse tree.  The exit
-status is returned in the global variable exitstatus.  The alter-
-native entry evalbackcmd is called to evaluate commands in back
-quotes.  It saves the result in memory if the command is a buil-
-tin; otherwise it forks off a child to execute the command and
-connects the standard output of the child to a pipe.
-
-JOBS.C:  To create a process, you call makejob to return a job
-structure, and then call forkshell (passing the job structure as
-an argument) to create the process.  Waitforjob waits for a job
-to complete.  These routines take care of process groups if job
-control is defined.
-
-REDIR.C:  Ash allows file descriptors to be redirected and then
-restored without forking off a child process.  This is accom-
-plished by duplicating the original file descriptors.  The redir-
-tab structure records where the file descriptors have been dupli-
-cated to.
-
-EXEC.C:  The routine find_command locates a command, and enters
-the command in the hash table if it is not already there.  The
-third argument specifies whether it is to print an error message
-if the command is not found.  (When a pipeline is set up,
-find_command is called for all the commands in the pipeline be-
-fore any forking is done, so to get the commands into the hash
-table of the parent process.  But to make command hashing as
-transparent as possible, we silently ignore errors at that point
-and only print error messages if the command cannot be found
-later.)
-
-The routine shellexec is the interface to the exec system call.
-
-EXPAND.C:  As the routine argstr generates words by parameter
-expansion, command substitution and arithmetic expansion, it
-performs word splitting on the result.  As each word is output,
-the routine expandmeta performs file name generation (if enabled).
-
-VAR.C:  Variables are stored in a hash table.  Probably we should
-switch to extensible hashing.  The variable name is stored in the
-same string as the value (using the format "name=value") so that
-no string copying is needed to create the environment of a com-
-mand.  Variables which the shell references internally are preal-
-located so that the shell can reference the values of these vari-
-ables without doing a lookup.
-
-When a program is run, the code in eval.c sticks any environment
-variables which precede the command (as in "PATH=xxx command") in
-the variable table as the simplest way to strip duplicates, and
-then calls "environment" to get the value of the environment.
-
-BUILTIN COMMANDS:  The procedures for handling these are scat-
-tered throughout the code, depending on which location appears
-most appropriate.  They can be recognized because their names al-
-ways end in "cmd".  The mapping from names to procedures is
-specified in the file builtins.def, which is processed by the
-mkbuiltins command.
-
-A builtin command is invoked with argc and argv set up like a
-normal program.  A builtin command is allowed to overwrite its
-arguments.  Builtin routines can call nextopt to do option pars-
-ing.  This is kind of like getopt, but you don't pass argc and
-argv to it.  Builtin routines can also call error.  This routine
-normally terminates the shell (or returns to the main command
-loop if the shell is interactive), but when called from a non-
-special builtin command it causes the builtin command to
-terminate with an exit status of 2.
-
-The directory bltins contains commands which can be compiled in-
-dependently but can also be built into the shell for efficiency
-reasons.  The header file bltin.h takes care of most of the
-differences between the ash and the stand-alone environment.
-The user should call the main routine "main", and #define main to
-be the name of the routine to use when the program is linked into
-ash.  This #define should appear before bltin.h is included;
-bltin.h will #undef main if the program is to be compiled
-stand-alone. A similar approach is used for a few utilities from
-bin and usr.bin.
-
-CD.C:  This file defines the cd and pwd builtins.
-
-SIGNALS:  Trap.c implements the trap command.  The routine set-
-signal figures out what action should be taken when a signal is
-received and invokes the signal system call to set the signal ac-
-tion appropriately.  When a signal that a user has set a trap for
-is caught, the routine "onsig" sets a flag.  The routine dotrap
-is called at appropriate points to actually handle the signal.
-When an interrupt is caught and no trap has been set for that
-signal, the routine "onint" in error.c is called.
-
-OUTPUT:  Ash uses its own output routines.  There are three out-
-put structures allocated.  "Output" represents the standard out-
-put, "errout" the standard error, and "memout" contains output
-which is to be stored in memory.  This last is used when a buil-
-tin command appears in backquotes, to allow its output to be col-
-lected without doing any I/O through the UNIX operating system.
-The variables out1 and out2 normally point to output and errout,
-respectively, but they are set to point to memout when appropri-
-ate inside backquotes.
-
-INPUT:  The basic input routine is pgetc, which reads from the
-current input file.  There is a stack of input files; the current
-input file is the top file on this stack.  The code allows the
-input to come from a string rather than a file.  (This is for the
--c option and the "." and eval builtin commands.)  The global
-variable plinno is saved and restored when files are pushed and
-popped from the stack.  The parser routines store the number of
-the current line in this variable.
-
-DEBUGGING:  If DEBUG is defined in shell.h, then the shell will
-write debugging information to the file $HOME/trace.  Most of
-this is done using the TRACE macro, which takes a set of printf
-arguments inside two sets of parenthesis.  Example:
-"TRACE(("n=%d0, n))".  The double parenthesis are necessary be-
-cause the preprocessor can't handle functions with a variable
-number of arguments.  Defining DEBUG also causes the shell to
-generate a core dump if it is sent a quit signal.  The tracing
-code is in show.c.