qgis - Using processing using other output (result from another algorithm) as input

I have a problem. I am developing a standalone app in Python and I need to use processing algorithms.

I was able to use one algorithm without errors but I need to run other processing algorithm which input is the output of the first one. So, my program stops in the first algorithm. Why it doesn't read the second one?

CODE:

import sys

from qgis.core import QgsApplication, QgsVectorLayer
from PyQt4.QtGui import QApplication
from PyQt4.QtCore import QFileInfo
import GdalTools_utils
import ftools_utils
import osgeo
import ogr

import gdal
app = QApplication([])
QgsApplication.setPrefixPath("/usr", True)
QgsApplication.initQgis()

from processing.core.Processing import Processing
Processing.initialize()

inputLayer = QgsVectorLayer(...)
inputMask = QgsVectorLayer(...)


extent_rect = inputMask.extent()

xmin = extent_rect.xMinimum()
xmax = extent_rect.xMaximum()
ymin = extent_rect.yMinimum()
ymax = extent_rect.yMaximum()
extent = str(xmin) + "," + str(xmax) + "," + str(ymin) + "," + str(ymax)

cellsize = 30

outrains10 = '...'

count = inputLayer.featureCount()

rain_10 = 'rains10'
days_10 = 'days10'

Processing.runAlgorithm("grass:v.surf.idw", None, inputLayer, count, 2.0,rain_10, False,extent, cellsize, -1.0, 0.0001, outrains10)

Processing.runAlgorithm("saga:clipgridwithpolygon", None, outrains10 , inputMask, outclip_rains10)

So the first algorithm grass is OK. The output is fine. But I want to use it as input in the second one, but the code stops there.

UPDATE

The code stops because I didn't know that I have to install SAGA to use in Linux. Now I have SAGA installed (I run from QGIS and everything is OK) but I still don't know how to configure SAGA to read through processing, such as the line:

Processing.runAlgorithm("saga:clipgridwithpolygon", None, outrains10 , inputMask, outclip_rains10)

UPDATE

I still can't get SAGA working but with many many tries, I found that, when I run the script, an error (Segmentation fault) and others are returned. So debugging my code I found that is the input raster which gives the error:

example:

grid = QgsRasterLayer('/.../raster.tif')

Does anyone knows why this is happen?? Can you help me please?