@title UDPShell Documentation
 @author Stephen Dawson-Haggerty
 @release public
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UDPShell is a simple text-based command processor which comes with the
ip-stack.  It is an optional, although convenient way of
implementating debugging commands on a mote.

Usage
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By default, the shell contains only a few simple commands: help, echo,
uptime, ping, and ident.  It is designed to be very easy to extend by
adding your own commands.

To include just the basic shell, include the UDPShellC component in
your application.  To augment the shell with a new shell command, use
the generic component ShellCommandC.

Example
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Suppose we want to implement `expr`, a simple arithmetic evaluator.
First, bind the 'expr' command string in your application configuration.

configuration App {} implementation {
  components AppImplP;
  components new ShellCommandC("expr") as Expr;
  AppImplP.Expr -> Expr;
}

Within AppImplP, you must implement the ShellCommand interface.  The
interface has only one event, 'eval' which has the same prototype as
main() in a typical c program.

event char *Expr.eval(int argc, char **argv) {
  static char ret[10];
  return ret;
}

If expr returns a non-null value, it is assumed to be a
null-terminated string which will be echoed back to a connected
client.  The buffer returned must obviously not be allocated on the
stack.  The shell does maintain a single buffer which components can
use to print their reply to; it can be requested with a call to
Expr.getBuffer(uint16_t len).

More running code
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Fully fleshed out examples of code using this interface are available
within the stack; see tos/lib/net/b6lowpan/shell/FlashShell[CP].nc and
tos/lib/net/b6lowpan/nwprog/NWProg[CP].nc