source: mainline/uspace/app/bdsh/README@ a56f4b2

BDSH - The Brain Dead Shell | Design Documentation
--------------------------------------------------

Overview:
=========

BDSH was written as a drop in command line interface for HelenOS to permit
interactive access to persistent file systems in development. BDSH was
written from scratch with a very limited userspace standard C library in
mind. Much like the popular Busybox program, BDSH provides a very limited
shell with limited common UNIX creature comforts built in.

Porting Busybox (and by extension ASH) would have taken much longer to
complete, much less make stable due to stark differences between Linux and
Spartan with regards to IPC, term I/O and process creation. BDSH was written
and made stable within the space of less than 30 days.

BDSH will eventually evolve and be refined into the HelenOS equivalent
of Busybox. While BDSH is now very intrinsic to HelenOS, its structure and
use of strictly lower level functions makes it extremely easy to port.

Design:
=======

BDSH is made up of three basic components:

 1. Main i/o, error handling and task management
 2. The builtin sub system
 3. The module sub system

The main part handles user input, reports errors, spawns external tasks and
provides a convenient entry point for built-in and modular commands. A simple
structure, cliuser_t keeps track of the user's vitals, such as their current
working directory (and eventually uid, home directory, etc if they apply).

This part defines and exposes all functions that are not intrinsic to a
certain built in or modular command. For instance: string handlers,
module/builtin search and launch functions, error handlers and other things
can be found here.

Builtin commands are commands that must have access to cliuser_t, which is
not exposed to modular commands. For instance, the 'cd' command must update
the current working directory, which is stored in cliuser_t. As such, the
entry types for builtin commands are slightly different.

Modular commands do not need anything more than the basic functions that are
exposed by default. They do not need to modify cliuser_t, they are just self
contained. A modular command could very easily be made into a stand alone
program, likewise any stand alone program could easily become a modular
command.

Both modular and builtin commands share two things in common. Both must have
two entry points, one to invoke the command and one to invoke a help display
for the command. Exec (main()) entry points are int * and are expected to
return a value. Help entry points are void *, no return value is expected.

They are typed as such (from cmds.h):

/* Types for module command entry and help */
typedef int * (* mod_entry_t)(char **);
typedef void * (* mod_help_t)(unsigned int);

/* Built-in commands need to be able to modify cliuser_t */
typedef int * (* builtin_entry_t)(char **, cliuser_t *);
typedef void * (* builtin_help_t)(unsigned int);

As you can see, both modular and builtin commands expect an array of
arguments, however bulitins also expect to be pointed to cliuser_t.

Both are defined with the same simple structure:

/* Module structure */
typedef struct {
        char *name;         /* Name of the command */
        char *desc;         /* Description of the command */
        mod_entry_t entry;  /* Command (exec) entry function */
        mod_help_t help;    /* Command (help) entry function */
        int restricted;     /* Restricts to interactive/non-interactive only */
} module_t;

NOTE: Builtin commands may grow in this respect, that is why they are
defined separately.

Builtins, of course, would use the builtin_entry_t type. The name of the
command is used to associate user input to a possible entry point. The
description is a short (40 - 60 chars) summary of what the command does. Both
entry points are then defined, and the restrict value is used to determine a
commands availability.

Restriction levels are easy, a command is either available exclusively within
interactive mode, exclusively within non-interactive mode or both. If you are
looking at a prompt, you are in interactive mode. If you issue a command like
this:

/sbin/bdsh command [arg1] [arg2]

... you are in non interactive mode. This is done when you need to force the
parent shell to be the one who actually handles the command, or ensure that
/sbin/ls was used in lieu of the built in 'ls' when in non-interactive mode.

The values are:
  0 : Unrestricted
 -1 : Interactive only
  1 : Non-interactive only

A script to generate skeletal files for a new command is included, it can be
found in cmds/mknewcmd. To generate a new modular command named 'foo', which
should also be reachable by typing 'f00', you would issue this command:

./mknewcmd -n foo -a f00 -t module

This generates all needed files and instructs you on how to include your new
command in the build and make it accessible. By default, the command will be
unrestricted. Builtin commands can be created by changing 'module' to
'builtin'

There are more options to mknewcmd, which allow you to specify the
description, entry point, help entry point, or restriction. By default, names
just follow the command such as cmd_foo(), help_cmd_foo(), 'The foo command',
etc. If you want to see the options and explanations in detail, use
./mknewcmd --help.

When working with commands, keep in mind that only the main and help entry
points need to be exposed. If commands share the same functions, put them
where they are exposed to all commands, without the potential oops of those
functions going away if the command is eliminated in favor of a stand alone
external program.

The util.c file is a great place to put those types of functions.

Also, be careful with globals, option structures, etc. The compiler will
generally tell you if you've made a mistake, however declaring:

volatile int foo

... in a command will allow for anything else to pick it up. Sometimes
this could be desirable .. other times not. When communicating between
builtins and the main system, try to use cliuser_t. The one exception
for this is the cli_quit global, since everything may at some point
need to check it. Modules should only communicate their return value.

Symbolic constants that everything needs should go in the config.h file,
however this is not the place to define shared macros.

Making a program into a module
==============================

If you have some neat program that would be useful as a modular command,
converting it is not very hard. The following steps should get you through
the process easily (assuming your program is named 'foo'):

1: Use mknewcmd to generate the skeletal files.

2: Change your "usage()" command as shown:
     -- void usage(...)
     ++ void * help_cmd_foo(unsigned int level)
     -- return;
     ++ retrn CMD_VOID;

     'level' is either 0 or 1, indicating the level of help requested.
     If the help / usage function currently exits based on how it is
     called, you'll need to change it.

3: Change the programs "main()" as shown:
     -- int main(int argc, char **argv)
     ++ int * cmd_foo(char **argv)
     -- return 1;
     ++ return CMD_FAILURE;
     -- return 0;
     ++ return CMD_SUCCESS;

     If main() returns an int that is not 1 or 0 (e.g. 127), cast it as
     such:

     -- return 127;
     ++ return (int *) 127;

     NOTE: _ONLY_ the main and help entry points need to return int * or
     void *, respectively. Also take note that argc has changed. The type
     for entry points may soon change.

     NOTE: If main is void, you'll need to change it and ensure that its
     expecting an array of arguments, even if they'll never be read or
     used. I.e.:

     -- void main(void)
     ++ int * cmd_foo(char **argv)

     Similararly, do not try to return CMD_VOID within the modules main
     entry point. If somehow you escape the compiler yelling at you, you
     will surely see pretty blue and yellow fireworks once its reached.

4: Don't expose more than the entry and help points:
     -- void my_function(int n)
     ++ static void my_function(int n)

5: Copy/paste to the stub generated by mknewcmd then add your files to the
   Makefile. Be sure to add any directories that you made to the SUBDIRS so
   that a 'make clean' will clean them.

Provided that all functions that your calling are available in the
userspace C library, your program should compile just fine and appear
as a modular command.

Overcoming userspace libc obstacles
===================================

A quick glance through the util.c file will reveal functions like
cli_strdup(), cli_strtok(), cli_strtok_r() and more. Those are functions
that were missing from userspace libc when BDSH was born. Later, after
porting what was needed from FBSD/NBSD, the real functions appeared in
the userspace libc after being tested in their cli_* implementations.

Those functions remain because they guarantee that bdsh will work even
on systems that lack them. Additionally, more BDSH specific stuff can
go into them, such as error handling and reporting when malloc() fails.

You will also notice that FILE, fopen() (and all friends), ato*() and
other common things might be missing. The HelenOS userspace C library is
still very young, you are sure to run into something that you want/need
which is missing.

When that happens, you have three options:

1 - Implement it internally in util.c , when its tested and stable send a
patch to HelenOS asking for your function to be included in libc. This is
the best option, as you not only improve BDSH .. but HelenOS as a whole.

2 - Work around it. Not everyone can implement / port fopen() and all of
its friends. Make open(), read(), write() (etc) work if at all possible.

3 - Send an e-mail to the HelenOS development mailing list. Explain why you
need the function and what its absence is holding up.

If what you need is part of a library that is typically a shared object, try
to implement a 'mini' version of it. Currently, all userspace applications
are statically linked. Giving up creature comforts for size while avoiding
temporary 'band aids' is never frowned upon.

Most of all, don't get discouraged .. ask for some help prior to giving up
if you just can't accomplish something with the limited means provided.

Contributing
============

I will take any well written patch that sanely improves or expands BDSH. Send
me a patch against the trunk revision, or, if you like a Mercurial repository
is also maintained at http://echoreply.us/hg/bdsh.hg and kept in sync with
the trunk.

Please be sure to follow the simple coding standards outlined at
http://www.helenos.eu/cstyle (mostly just regarding formatting), test your
changes and make sure your patch applies cleanly against the latest revision.

All patches submitted must be your original code, or a derivative work of
something licensed under the same 3 clause BSD license as BDSH. See LICENSE
for more information.

When sending patches, you agree that your work will be published under the
same 3 clause BSD license as BDSH itself. Failure to ensure that anything
you used is not under the same or less restrictive license could cause major
issues for BDSH in the future .. please be sure. Also, please don't forget
to add yourself in the AUTHORS file, as I am horrible about keeping such
things up to date.