/**
* Executes a command and sends the result to the specified output stream.
*
* @param cmd server command
* @param arg argument
* @param os target output stream
* @return string
* @throws IOException I/O exception
*/
protected String exec(final ServerCmd cmd, final String arg, final OutputStream os)
throws IOException {
final OutputStream o = os == null ? new ArrayOutput() : os;
sout.write(cmd.code);
send(arg);
sout.flush();
final BufferInput bi = new BufferInput(sin);
ClientSession.receive(bi, o);
if (!ClientSession.ok(bi)) throw new BaseXException(bi.readString());
return o.toString();
}
