Command line option parsing

Being revised

Command line option parsing is a method to set the values of different variables when running your executable from the command line.

After reading this page, you should be able to set command line options from the command line, add new ones, change the default values of the default command line options and use them for FWLite.

However, you should understand that VarParsing is recommended if you have a few command line options. If you are having an increasing number of command line options, configuring python to do the same is a lot more easier and flexible. You can find in the example at WorkBookFWLiteExamples. Also so far it's still not possible to use CRAB for FWLite scripts in a simple way at all. For both condor and CRAB you can use VarParsing. In case you want to use CRAB one should switch to EDAnalyzer.

Using VarParsing from command line

Let us start here right away with an example that shows the usage command line options with python script called This script is used to copy a given number of events, from a given data file to an output file. The number of events, name of the input data file and the output file can all be defined as the command line options shown as follows.

cmsRun    inputFiles=one.root inputFiles=two.root inputFiles=third.root     outputFile=MyOutputFile.root    maxEvents=100


cmsRun    inputFiles=one.root, two.root, third.root     outputFile=MyOutputFile.root    maxEvents=100


cmsRun    inputFiles=myFiles.txt
  outputFile=MyOutputFile.root    maxEvents=100

where myFiles.txt is a text file containing one.root, two.root and three.root data files on three different lines.

Using text file containing names of data files is useful particularly when you select them from a dataset in DBS discovery using plain option as shown below.

For where to click for plain option ( see the red circle) for a dataset in DBS , here is an example:


After you click plain, the list of files looks like below. You can simply cut and paste them in a text file and use this text file for the parameter option -inputFiles.


OR you can click on plain and copy and the link within double quotes as shown in the command below and get the files to a text file:

wget -O myListOfFiles.txt "<paste>" --no-check-certificate

Note: the link in the above example is*

You an further read about it at WorkBookDataSamples.

VarParsing in config file

To understand usage of variable parsing inside a configuration file, we look at the used above in detail. In this configuration file, another configuration file called is imported. contains the infrastructure to parse variable definitions passed to cmsRun configuration script

The variables passed to the cmsRun are defined in the configuration script as follows ( read the comments in red that are not part of the code):

import FWCore.ParameterSet.Config as cms
 The line below always has to be included to make VarParsing work 
from FWCore.ParameterSet.VarParsing import VarParsing

 In teh line below 'analysis' is an instance of VarParsing object 
options = VarParsing ('analysis')

 Here we have defined our own two VarParsing options 
# add a list of strings for events to process
options.register ('eventsToProcess',
              "Events to process")
options.register ('maxSize',
              "Maximum (suggested) file size (in Kb)")

 The (option.inputFile) in the lines below  is simply replaced by say one.root 
process = cms.Process("PickEvent")
process.source = cms.Source ("PoolSource",
     fileNames = cms.untracked.vstring (options.inputFiles),

 And here we use our own VarParsing options defined above 
if options.eventsToProcess:
    process.source.eventsToProcess =             cms.untracked.VEventRange (options.eventsToProcess)

process.maxEvents = cms.untracked.PSet(
       input = cms.untracked.int32 (options.maxEvents)

process.Out = cms.OutputModule("PoolOutputModule",
        fileName = cms.untracked.string (options.outputFile)

if options.maxSize:
    process.Out.maxSize = cms.untracked.int32 (options.maxSize)

process.end = cms.EndPath(process.Out)

Default options

The default options are defined in the are as follows:

  • maxEvents
  • totalSections
  • section
  • inputFiles
  • outputFile

Adding new options

If you need to add new options, you can define them in your configuration file. In the configuration file discussed here, the two new options added are eventsToProcess and maxSize. They are defined as follows in the file.

options.register ('eventsToProcess',
              "Events to process")
options.register ('maxSize',
              "Maximum (suggested) file size (in Kb)")

And to use the option maxSize , you can define the option as follows:

outputFile=output.root maxSize=100000 where maxSize=100000 defines the size of the output file.

Command Line Options in FWLite (PyROOT)

In FWLite you can readout the command line options in your executable as indicated in the following lines:

from [[FWCore.ParameterSet.VarParsing]] import VarParsing
options = VarParsing ('python')

and then loading the events using "options":

from DataFormats.FWLite import Events, Handle
events = Events (options)

Finally you can call the code using the options. Example:

python3 DataFormats /FWLite/examples/bin/ inputFiles=/eos/cms/store/relval/CMSSW_12_1_0_pre2/RelValPsi2SToJPsiPiPi_14/MINIAODSIM/121X_mcRun3_2021_realistic_v1-v3/10000/934b45ff-3f99-4f69-9648-bfd129ab7d90.root secondaryInputFiles=/eos/cms/store/relval/CMSSW_12_1_0_pre2/RelValPsi2SToJPsiPiPi_14/MINIAODSIM/121X_mcRun3_2021_realistic_v1-v3/10000/934b45ff-3f99-4f69-9648-bfd129ab7d90.root maxEvents=10


VarParsing is the only way to use the secondary file solution in FWLite/PyROOT %ENDCOLOR%

See examples: ( )

In CMSSW, DataFormats /FWLite unit test:

Command Line Options in FWLite

In FWLite you can readout the command line options in your executable as indicated in the following lines:

#include "CMS.PhysicsTools/FWLite/interface/CommandLineParser.h"

  // ...

  // initialize command line parser
  optutl::CommandLineParser parser ("Analyze FWLite Histograms");

  // set defaults
  parser.integerValue ("maxEvents"  ) = 1000;
  parser.integerValue ("outputEvery") =   10;
  parser.stringValue  ("outputFile" ) = "analyzeFWLiteHistograms.root";

  // parse arguments
  parser.parseArguments (argc, argv);
  int maxEvents_ = parser.integerValue("maxEvents");
  unsigned int outputEvery_ = parser.integerValue("outputEvery");
  std::string outputFile_ = parser.stringValue("outputFile");
  std::vector inputFiles_ = parser.stringVector("inputFiles");

ALERT! Note: Don't forget to make the object known to the compiler by including the proper .h file. You will also need to link the FWLite package to your executable to make the implementation known. In the first part of this example the CommandLineParser is initialised, in the second part default values for the command line options outputFile, maxEvents and outputEvery are defined. In the third part the command line parameters listed before and the additional parameter inputFiles are passed on. You can find the full executable in the file in the bin directory of the FWLite package. Have a look to WorkBookFWLiteExamples to see the command line parsing in FWLite in action. The lines above were taken from Example 2. Please refer to the experts in case of more questions.

If you are using the PythonProcessDesc to parse configuration files you must pass the argc and argv variables to the constructor so that parser has access to options given on the command line. An example is given below.

#include "FWCore/PythonParameterSet/interface/MakeParameterSets.h"

int main(int argc, char* argv[])
  // load framework libraries
  gSystem-&gt;Load( "libFWCoreFWLite" );

  // only allow one argument for this simple example which should be the
  // the python cfg file
  if ( argc &lt; 2 ) {
    std::cout &lt;&lt; "Usage : " &lt;&lt; argv[0] &lt;&lt; " []" &lt;&lt; std::endl;
    return 0;

  // get the python configuration
  // PythonProcessDesc builder(argv[1]);   /* this method prevents option parsing in the file */
  PythonProcessDesc builder(argv[1], argc, argv);  // &lt;--- use this constructor.
  edm::ParameterSet cfg = *builder.processDesc()-&gt;getProcessPSet();

  /* your analysis here */

  return 0;
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