How to read Linux environment variables in c++

Use the getenv() function – see http://en.cppreference.com/w/cpp/utility/program/getenv. I like to wrap this as follows:

std::string GetEnv( const std::string & var ) {
     const char * val = std::getenv( var.c_str() );
     if ( val == nullptr ) { // invalid to assign nullptr to std::string
         return "";
     }
     else {
         return val;
     }
}

which avoids problems when the environment variable does not exist, and allows me to use C++ strings easily to query the environment. Of course, it does not allow me to test if an environment variable does not exist, but in general that is not a problem in my code.