I found the best bkg function: fit_mjj_full_calotrijet2016_3.pdf - it the 6 parametric functions that was used by atlas and ua2 collaboration to fit the dijet mass spectrum.
The script RunBias.py executes all the commands that are needed for the bias study:
First command: writes the data card need to run combine tool in the following steps using the data file (prepared), nominal signal file (prepared), signal files with systematic uncertainties (prepared).
Second command: generates N pseudo experiments using as input:
the output data card from the first step
the function to generate toys
parameters from the dijet mass spectrum fit using the same function as for the generation
user-defined signal strength
Third command: fit using the combine tool with the maximum likelihood method all of the toys using user-defined function and compute signal strength from the fit.