Publishing String from Terminal?

edit

Is there any problem in my code? i just want, if i publish "q" Blue LED glow. Red for "a" and blink for "z" . i use

rostopic pub /servo std_msgs/String "data: 'z'"

command to publish topic.but nothing happening.the code running without error. my code is

#include <ros.h>
#include <std_msgs/String.h>


ros::NodeHandle nh;
void pwm( const std_msgs::String& cmd_msg)
{
  if (cmd_msg.data=="q")
  {
    digitalWrite (13, HIGH); //BLUE LED ON
  }

  else if (cmd_msg.data=="a")
  {
    digitalWrite (12, HIGH);  //RED LED ON
  }

 else if (cmd_msg.data=="z")
  {
    for(int i=0;i<1000;i++)
    { 
      digitalWrite (13, HIGH); //LED BLINKING
      delay(100);
      digitalWrite (13, LOW);
      digitalWrite (12, HIGH); 
      delay(100);
      digitalWrite (12, LOW); 
    }
  }
}
ros::Subscriber<std_msgs::String> sub("servo", pwm);


void setup() 
{

  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
   nh.initNode();
  nh.subscribe(sub);
}


void loop() 
{ 
  nh.spinOnce();
  delay(10);
}