00 Running CMSSW

Initial setup

Assert that CVMFS is installed in the machine. Then:

export CMS_PATH=/cvmfs/cms.cern.ch
source $CMS_PATH/cmsset_default.sh
export SCRAM_ARCH=el8_amd64_gcc10

(I suggest you add those lines to your .bashrc of .bash_profile)

Setting up a CMSSW development area

The version for CMSSW that we are working with for the time being is CMSSW_12_5_2_patch1. So:

cmsrel CMSSW_12_5_2_patch1
cd CMSSW_12_5_2_patch1/src
cmsenv

The cmsrel command creates the area, and needs to be run only once. The cmsenv command should be used every time to setup the environment development (correct versions of gcc, python, ...)

Information about the CMSSW computing model and framework

• https://twiki.cern.ch/twiki/bin/view/CMSPublic/WorkBookComputingModel
• https://twiki.cern.ch/twiki/bin/view/CMSPublic/WorkBookCMSSWFramework

The five basic ideas of CMSSW. All of those are only approximately correct, but it is a good approximation:

1. There is only one CMSSW executable, cmsRun.
2. Configure the CMSSW job with a Python file.
3. The Event is where everything happens. It is a big container, with all the data products (either real data or simulation).
4. The cmsRun configuration makes the event goes through a series of modular plugins (Modules), that process the event sequentially.
5. Modules cannot directly affect each other. They can only communicate writing and reading data in the event.

Running a simple premade configuration

Check the file test_WToMuNu.py