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ROS_INFO("%s", msg.data);

For std_msgs::Int8, the data field is of the int8 type (see the msg doc). The %s format string is for strings (or char*), so that won't work. Try %d.

PS: are you sure you aren't ignoring some compiler warning?

PPS: the question title says you cannot publish an Int8, but in your actual question it would seem you only have difficulty logging the data field using ROS_INFO(..). Perhaps you should update the title to reflect your actual issue.