publisher can only in While? no, maybe other reason.

edit

After execute the following code, the subscriber CANNOT receive msg. I start this publisher by : python demo_pub.py or rosrun demo demo_pub.py. And start the Subscriber in terminal by : rostopic echo /chatter

#!/usr/bin/env python
# license removed for brevity
import rospy
from std_msgs.msg import String

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker')
    hello_str = "hello world %s" % rospy.get_time()
    rospy.loginfo(hello_str)
    pub.publish(hello_str)

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

When I show the rqt_graph, it shows that they have been get linked.

But when I add while loop like this, it works well.

#!/usr/bin/env python
# license removed for brevity
import rospy
from std_msgs.msg import String

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker')
    rate = rospy.Rate(10) # 10hz
    while not rospy.is_shutdown():
        hello_str = "hello world %s" % rospy.get_time()
        rospy.loginfo(hello_str)
        pub.publish(hello_str)
        rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

Also, I need to restart the subscriber when I restart the demo_pub file. And how can I publish a message only once?

The code has nothing wrong.

I figure out the reason but cannot get the answer.

After I change the master_urI value within .bashrc to other IP address, and then run " roscore" in the terminal of device with that IP, then the strange problem reappeared again.

So, each time I change the master of roscore, I restart my ubuntu system will help.

I just run "source .bashrc", and restart the terminal before.