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be used for working with the ARC server from the command line, resp. to start the
server.

=head1 SYNOPSIS

This module is part of the module suite ARCv2.

This is the command module from ARCv2. If we would use C++, we would say
this is an abstract class of an ARC Command. All commands used by ARCv2 should
derive from this class.

=head1 HOW DOES IT WORKS

This abstract class is the base class for all existing ARCv2 commands and 
should be the base class for all new ARCv2 commands.
 
When starting the ARCv2 Server you have to pass a important hash:

 $arc = new Arc::Server ( 
    [..] Arc::Server vars [..],
   connection_vars => { 
      commands => { 
        'pv' => 'Arc::Command::Pv'
      } 
   }
 )  
 
 resp.

 $arc->{connection_vars}->{commands}

This hash describes the assignment of B<Command Name> and B<Command Class>. 
When a client has authenticated and wants to run a command, it will send 
the B<Command Name> and suitable, optional parameters. The server will look into 
the commands hash and creates an object of the B<Command Class> 
associated with B<Command Name>.

 my $perlcmd = $this->{commands}->{$cmd};
 [..]
 eval "require $perlcmd;"
 [..]
 (fork)
 my $ret = eval
 {
  my $object = new $perlcmd(
    _username => $this->{_username},
    _peeraddr => $this->{peeraddr},
    _peerport => $this->{peerport},
    _peername => $this->{peername},
    _cmd => $cmd,
    logfileprefix => "command",
  );
  $object->Execute(@a);
  $cmderr = $object->IsError();

  return -1;
 };
 
When everything went alright, the command will be executed. The command runs
in a separate process. Therefore STDIN, STDOUT and STDERR are duped to two
pipes, one for the in, one for the out direction. In the parent process data
from the encrypted network command connection is read from and written to these pipes.
Same situation on the client side, STDIN and STDOUT are used to put and get the 
data through the network.
 
                   encrypted
          /--->>---| net- |--->>-----\   
        / /---<<---| work |---<<-----\ \
      / /                              \ \
     | |     in                         | |    p2
  |--------|->>--\                  |--------|->>--\ 
  | Client | out   \                | Server | p1    \
  |--------|-<<-\    \              |--------|-<<-\    \ 
                /|\  \|/                          /|\  \|/
               |--------|                       |-----------|
               |  User  |                       |  Command  |
               |--------|                       |-----------|
 
This design makes it easy for ARCv2 Commands to get input and produce output.

B<Example>:
 sub Execute 
 {
  while ($_ = <STDIN>) { # ends on EOF
     s/a/b/g; print;
  }
 }
 
If you want to implement a new Command for ARCv2 you have to derive from 
Arc::Command and override the sub C<Execute>. See existing Arc::Command::* 
classes for examples. To get your Command recognised you have to assign a 
B<Command Name> to your command class. ARCv2 ignores the return code of
B<Execute>. If your command runs into an error use the _SetError function 
and return immediately. This is what ARCv2 will evaluate and send to the
client.

B<Example>:
 sub Execute
 {
  my $this = shift;
  my $pw = <>;
  if ($pw ne "klaus") {
	  return $this->_SetError("Wrong password.");
  }
 }
 
In ARCv2 some standard commands are already implemented: C<Arc::Command::Get>,
C<Arc::Command::Put>, C<Arc::Command::Uptime>, C<Arc::Command::Whoami>,
C<Arc::Command::Test>.

By default, these classes are mapped to B<Command Names> as follows (in the 
default arcxd.conf for arcxd):
  uptime => Arc::Command::Uptime,
  whoami => Arc::Command::Whoami,
  copy   => Arc::Command::Get,
  cp     => Arc::Command::Get,
  get    => Arc::Command::Get,
  put    => Arc::Command::Put,
  test   => Arc::Command::Test,
  help   => Arc::Command::Help,
 
B<Caution>: Especially take care of the C<Arc::Command::Get> and 
C<Arc::Command::Put> in production environment. As ARCv2 will probably
run as root and by default the Get and Put command do NOT have an access