src/TOUR

#	@(#)TOUR	8.1 (Berkeley) 5/31/93

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         intput files        generates
        -------         ------------        ---------
        mkbuiltins      builtins            builtins.h builtins.c
        mkinit          *.c                 init.c
        mknodes         nodetypes           nodes.h nodes.c
        mksignames          -               signames.h signames.c
        mksyntax            -               syntax.h syntax.c
        mktokens            -               token.h
        bltin/mkexpr    unary_op binary_op  operators.h operators.c

There are undoubtedly too many of these.  Mkinit searches all the
C source files for entries looking like:

        INIT {
              x = 1;    /* executed during initialization */
        }

        RESET {
              x = 2;    /* executed when the shell does a longjmp
                           back to the main command loop */
        }

It pulls this code out into routines which are when particular
events occur.  The intent is to improve modularity by isolating
the information about which modules need to be explicitly
initialized/reset within the modules themselves.

Mkinit recognizes several constructs for placing declarations in
the init.c file.
        INCLUDE "file.h"
includes a file.  The storage class MKINIT makes a declaration
available in the init.c file, for example:
        MKINIT int funcnest;    /* depth of function calls */
MKINIT alone on a line introduces a structure or union declara-
tion:
        MKINIT
        struct redirtab {
              short renamed[10];
        };
Preprocessor #define statements are copied to init.c without any
special action to request this.

INDENTATION:  The ash source is indented in multiples of six
spaces.  The only study that I have heard of on the subject con-
cluded that the optimal amount to indent is in the range of four
to six spaces.  I use six spaces since it is not too big a jump
from the widely used eight spaces.  If you really hate six space
indentation, use the adjind (source included) program to change
it to something else.

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 uninterruptable 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 folloing 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 is
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 -j 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
three tables:  one for normal use, one for use when inside single
quotes, and one for use when inside double quotes.  The tables
are machine dependent because they are indexed by character vari-
ables 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              Variable substitution
        CTLENDVAR           End of variable substitution
        CTLBACKQ            Command substitution
        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}

The name of the variable comes next, terminated by an equals
sign.  If the type is not VSNORMAL, then the text field in the
substitution follows, terminated by a CTLENDVAR byte.

Commands in back quotes are parsed and stored in a linked list.
The locations of these commands in the string are indicated by
the CTLBACKQ character.

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 splitting and file name
generation, have the CTLESC characters removed during the vari-
able and command substitution phase.  Words which are subject
splitting and file name generation have the CTLESC characters re-
moved 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 to 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 be 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:  Arguments are processed in three passes.  The first
(performed by the routine argstr) performs variable and command
substitution.  The second (ifsbreakup) performs word splitting
and the third (expandmeta) performs file name generation.  If the
"/u" directory is simulated, then when "/u/username" is replaced
by the user's home directory, the flag "didudir" is set.  This
tells the cd command that it should print out the directory name,
just as it would if the "/u" directory were implemented using
symbolic links.

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.
There are two consequences of this.  First, if an assignment to
PATH precedes the command, the value of PATH before the assign-
ment must be remembered and passed to shellexec.  Second, if the
program turns out to be a shell procedure, the strings from the
environment variables which preceded the command must be pulled
out of the table and replaced with strings obtained from malloc,
since the former will automatically be freed when the stack (see
the entry on memalloc.c) is emptied.

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, which is processed by the mkbuil-
tins 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 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 makefile in this directory compiles these programs
in the normal fashion (so that they can be run regardless of
whether the invoker is ash), but also creates a library named
bltinlib.a which can be linked with ash.  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 pro-
gram is to be compiled stand-alone.

CD.C:  This file defines the cd and pwd builtins.  The pwd com-
mand runs /bin/pwd the first time it is invoked (unless the user
has already done a cd to an absolute pathname), but then
remembers the current directory and updates it when the cd com-
mand is run, so subsequent pwd commands run very fast.  The main
complication in the cd command is in the docd command, which
resolves symbolic links into actual names and informs the user
where the user ended up if he crossed a symbolic link.

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 it's 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.