Executing standalone Python script for QGIS processing from within PowerShell script

Question

Is there a way to execute either a python script or pyhton commands from within a powershell script to do some QGIS geoprocessing on a Windows 10 machine (64 bit)?
I have a powershell script that downloads gazetteer data with cURL and copies the data into a geopackage. I want to enrich the address data with a number of other attributes from other layers. As of now, I use python processing commands for this task that need to be executed from the python console of QGIS. A third script (powershell again) exports the data to csv files, 7z-ips and copies them to the clipboard in preparation for mail delivery.
Example of processing:
processing.run("native:joinattributesbylocation", {'INPUT':'C:/Users/myusername/Documents/Parkrauminventur.gpkg|layername=Hauskoordinaten','JOIN':'//some_server/Projekte/PRM.gdb|layername=Kontrollgebiete','PREDICATE':[0,1,2,3,4,5,6],'JOIN_FIELDS':['Gebiet','Region'],'METHOD':1,'DISCARD_NONMATCHING':False,'PREFIX':'KG_','OUTPUT':'ogr:dbname='C:/Users/myusername/Spatial_Joins.gpkg' table="SJ_HK" (geom)'})

However, I would like to write one script to include all steps that runs without the need to start QGIS. I read many instructions on how to write standalone scripts but none of them seem to be assuming powershell. So I am not sure how to set the required environmental variables in powershell and how to interact with python/QGIS.
Are there directions for this scenario?

simo · Accepted Answer

You do not even need to use Python.

Since QGIS 3.14 (Pi) you are able to exexute QGIS with command line by running the qgis_process.

To get all available processings list, just type qgis_process list in terminal. Then, you will see you can execute Join Attributes by Location by typing :

qgis_process run native:joinattributesbylocation

If you need complementary information regarding a command, just type : qgis_process help native:joinattributesbylocation

Join attributes by location (native:joinattributesbylocation)

---------------- Description
---------------- Join attributes from one vector layer to another by location. This algorithm takes an input vector layer and creates a new vector layer that is an extended version of the input one, with additional attributes in its attribute table.

The additional attributes and their values are taken from a second vector layer. A spatial criteria is applied to select the values from the second layer that are added to each feature from the first layer in the resulting one.

---------------- Arguments
----------------

INPUT: Base Layer   Argument type:  source  Acceptable values:
        - Path to a vector layer 
JOIN: Join Layer    Argument type:  source  Acceptable values:
        - Path to a vector layer 
PREDICATE: Geometric predicate  Argument type:  enum    Available values:
        - 0: intersects
        - 1: contains
        - 2: equals
        - 3: touches
        - 4: overlaps
        - 5: within
        - 6: crosses    Acceptable values:
        - Number of selected option, e.g. '1'
        - Comma separated list of options, e.g. '1,3' 
JOIN_FIELDS: Fields to add (leave empty to use all fields)  Argument type:  field   Acceptable values:
        - The name of an existing field
        - ; delimited list of existing field names 
METHOD: Join type   Argument type:  enum    Available values:
        - 0: Create separate feature for each matching feature (one-to-many)
        - 1: Take attributes of the first matching feature only (one-to-one)
        - 2: Take attributes of the feature with largest overlap only (one-to-one)  Acceptable values:
        - Number of selected option, e.g. '1'
        - Comma separated list of options, e.g. '1,3' 
DISCARD_NONMATCHING: Discard records which could not be joined  Argument type:  boolean     Acceptable values:
        - 1 for true/yes
        - 0 for false/no 
PREFIX: Joined field prefix     Argument type:  string  Acceptable values:
        - String value 
OUTPUT: Joined layer    Argument type:  sink    Acceptable values:
        - Path for new vector layer 
NON_MATCHING: Unjoinable features from first layer  Argument type:  sink    Acceptable values:
        - Path for new vector layer

---------------- Outputs
----------------

OUTPUT: <outputVector>  Joined layer NON_MATCHING: <outputVector>   Unjoinable features from first layer JOINED_COUNT: <outputNumber>   Number of joined features from input table

to pass an argument just use the following syntax : --PARAM=value

Full sample :

qgis_process run native:joinattributesbylocation --INPUT="/my/folder/input.shp" --JOIN="/my/folder/tojoin.shp" --METHOD="0"  --PREDICATE="1"  --OUTPUT="/my/folder/output.shp"

Have a look on complementary resources :

Tolotos · Answer

simo, you made my day - thank you very much, indeed! Converting your example into PowerShell I got the desired result. I used "splatting" to pass the parameters like so:
##Preparation
# Locate the qgis_process.bat and store location in variable
$bat_qgis_process = Get-ChildItem -Path 'C:Program Files' -File -Recurse -Include qgis_process-qgis.bat

# Definition of parameters for qgis_process in variable (this is called "Splatting")
$Args_qgis_process =
    'run',
    'native:joinattributesbylocation',
    '--INPUT="C:/Users/myusername/Parkrauminventur.gpkg|layername=Hauskoordinaten"',
    '--JOIN="//some_server/ArcGIS-Ablage/Projekte/PRM.gdb|layername=Kontrollgebiete"',
    '--METHOD="1"',
    '--PREDICATE="0,1,2,3,4,5,6"',
    '--JOIN_FIELDS="Gebiet";"Region"',
    '--DISCARD_NONMATCHING=0',
    '--PREFIX="KG_"',
    '--OUTPUT="C:/Users/myusername/Spatial_Joins.gpkg"'

#  Execute qgis_process script with above parameters

& $bat_qgis_process.FullName @Args_qgis_process

Write-Host 'We are done here :-)!'

Read-Host -Prompt "Hit Enter to leave the script!"

The only minor drawback: As of now it seems impossible to define a specific layer name for the output geodatabase. The syntax for --INPUT is different form --OUTPUT in this regard. There is a feature request concerning this (see here).

Executing standalone Python script for QGIS processing from within PowerShell script

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