Command line argument passing in rosrun

edit

it only works when you write this ros::NodeHandle nh("~");

Just to complement, I think your original code is a bit confusing. You are expecting to have a parameter with the type "string", instead of expecting a specific value for a given parameter.

It's important to pay attention to the return of nh.getParam(). For example, if you run with _blue:=true or _blue:=1, you have false return because it expects to fill a string variable, not a boolean or an integer. But if you run _blue:=some_text, you have true return, because the type matches.

Just to complement, you can remove the parameter param or you can use it and remove check, it gets cleaner. I tried in my local environment and it worked like that:

#include<ros/ros.h>
#include<iostream>

using namespace std;

 int main(int argc, char *argv[])
 {  
    std::string param;
    ros::init(argc, argv, "node_name");
    ros::NodeHandle nh("~");
    nh.getParam("param", param);
    ROS_INFO("Got parameter : %s", param.c_str());

    if(param.compare("blue") == 0)
    {
        cout << "blue " << endl;
    }
    else if(param.compare("green") == 0)
    {
        cout << "green " << endl;
    }
    else
    {
        cout << "Don't run anything !! " << endl;
    }
    return 0;
}

According to rosrun documentation you can do following:

rosrun my_package my_node _my_param:=value

Concrete in your case to run "blue" code should be:

rosrun my_package my_node _blue:=true

Add _ (underscore) before the parameter name to make it identify and remap as a variable inside the node. Adding nothing will help the ROS to identify it as a topic and remap the same.

Syntax

rosrun package node _variable:=value

rosrun package node topic:=/topic_name