I have a huge text file (10GB) formatted as follow (multimodel PDB file):

Model 1

... (some text)

ENDMDL

Model 2

... (some text)

ENDMDL

Model 3

... (some text)

ENDMDL

...

Model 9999

... (some text)

ENDMDL

End

I know how to extract each model to a separate file:

while read line; do

    echo "${line}" >> model_${i}.pdb

    [[ ${line[0]} == ENDMDL ]] && ((i++))

done < $pdb

Now, I need to extract the models in a discrete way in steps of N. The idea is that if N=5, then extract the model 1, then the model 6, then model 11, etc.

A note: the number after the word Model, cannot be used as reference as can be duplicated due to standard issue with multimodels PDB files.