I would like to loop inside all dirs of a root file and write all the TH object into a .ps file and each TH being in a seperate page…
I guess, I need to look using the keys, but how to “order” one page/TH object in the output file? Can possibly ROOT do that ?
do the examples given in http://root.cern.ch/root/html/TPostScript.html( Making several pictures in the same Postscript file: case 1 & case 2) help you maybe? Both of these seem to address your use case, i.e. printing histograms to a ps file either one or also several per page. If you look for an example of how to loop through all directories of a root file, the hadd.C macro ($ROOTSYS/tutorials/io/hadd.C) might help you.
Or maybe I misunderstand what you intend to do?