code: public boolean isCommandInstalled(String command, String failureMessage){
          List<String> output = getProcessManager().executeAndJoinOutput(command);
          return output.size() > 0 && !output.get(0).contains(failureMessage);
      }
docs: True if the given command is installed in the shell, false otherwise.
  @param command eg brew, aptitude, sudo etc
  @param failureMessage the message expected upon failure.
  @return <tt>true</tt> if the given command is installed in the shell, false otherwise.