Add line number in Jupyter Notebook code cell by default

Select a code cell, open browser’s JavaScript console, type and execute the following lines:

var cell = Jupyter.notebook.get_selected_cell();
var config = cell.config;
var patch = {
    CodeCell:{
        cm_config:{lineNumbers:true}
    }
}
config.update(patch)

Then reload the page. These changes will be saved in .jupyter/nbconfig. See Configuring the notebook frontend.