Example of running standalone QGIS 3 processing algorithms in Ubuntu Linux (typical installation)

rasterstats.py

#!/usr/bin/env python3
import os
import sys
import warnings
from functools import wraps


def ignore_warnings(f):
    @wraps(f)
    def inner(*args, **kwargs):
        with warnings.catch_warnings(record=True) as w:
            warnings.simplefilter("ignore")
            response = f(*args, **kwargs)
        return response
    return inner


def import_qgis_processing():
    sys.path.append('/usr/lib/qgis')
    sys.path.append('/usr/share/qgis/python/plugins')
    from qgis.core import QgsApplication, QgsProcessingFeedback, QgsRasterLayer
    app = QgsApplication([], False)
    feedback = QgsProcessingFeedback()
    return (app, feedback, QgsRasterLayer)


app, feedback, QgsRasterLayer = import_qgis_processing()
app.initQgis()


@ignore_warnings # Ignored because we want the output of this script to be a single value, and "import processing" is noisy
def initialise_processing(app):
    from qgis.analysis import QgsNativeAlgorithms
    import processing
    from processing.core.Processing import Processing
    Processing.initialize()
    app.processingRegistry().addProvider(QgsNativeAlgorithms())
    return (app, processing,)


def check_input_validity(QgsRasterLayer):
    raster = QgsRasterLayer(sys.argv[1], 'raster', 'gdal')
    if not raster.isValid():
        raise Exception('Invalid raster')


def raster_stats():
    params = {
        'INPUT': sys.argv[1],
        'BAND': 1
    }
    return processing.run(
        "qgis:rasterlayerstatistics",
        params, feedback=feedback
    )


def get_raster_stat(stat='SUM'):
    return raster_stats().get(stat)


app, processing = initialise_processing(app)
check_input_validity(QgsRasterLayer)
print(get_raster_stat(stat=sys.argv[2]))
app.exitQgis()

You would run this as: python3 ./rasterstats.py /path/to/some/raster/file.tif SUM, and it would output the sum of all values of the input raster to stdout. MAX, MIN etc. all work too.