Export QGIS 3 layout from pyqgis 3 script

Export_layout_from_pyqgis3_script.py

#!/usr/bin/env python3
import os
from qgis.core import (QgsProject, QgsLayoutExporter, QgsApplication)

QgsApplication.setPrefixPath("/usr", True)

gui_flag = False
app = QgsApplication([], gui_flag)

app.initQgis()

project_path = os.getcwd() + '/project.qgz'

project_instance = QgsProject.instance()
project_instance.setFileName(project_path)
project_instance.read()

manager = QgsProject.instance().layoutManager()
layout = manager.layoutByName("my_layout") # name of the layout
# or layout = manager.layouts()[0] # first layout

exporter = QgsLayoutExporter(layout)
exporter.exportToPdf(project_instance.absolutePath() + "/layout.pdf",
                     QgsLayoutExporter.PdfExportSettings())

app.exitQgis()