From a Forum posting by Blair Jamieson, 26-11-2003:
https://twist.phys.ualberta.ca/forum/view.php?site=twist&bn=twist_software&key=1069895323

Posted by Blair Jamieson (jamieson@physics.ubc.ca) , Wednesday Nov 26, 2003
17:48

I have just committed updated versions to tree processing code,
including many more comment lines and histograms to:

$MOFIA_SOURCE/rootmacro   

A copy of this code with most of the histograms removed (to make the
logic easier to follow) has been put in:

$MOFIA_SOURCE/rootmacro/processtreeskeleton

If you want to add histograms, you need to do the following:

1)Add a pointer to the histogram in HTree.h class
   definition.  Look for lines like 
   TH1D* h50; //For 1D histograms
   TH2D* h222; // For 2D histograms

2)Add code to create the histograms in the class constructor
HTree::HTree() in HTree.h.  Look for lines like:
      h50 = new TH1D((TString("h0050")+(*ts)).Data(),"event
type",32,-0.5,31.5);

3)Add code to tell root to calculate errors correctly (also in
HTree::HTree() in HTree.h.  Look for lines like:
     h50->Sumw2();

4)Add code to fill the histogram in the file HTree.C, somewhere in the
loop code.  There are some cuts set up already in the code, or you can
add your own cuts before filling histograms.  Look for example at:
   h50->Fill( Event_type );

5) Complie the code with make

This tree loop code is already set up to input from the command line
things like which root files to loop over, and where to put the output
root histogram file.  Just run processhelixtrees with no arguments to
see the somewhat cryptic arguments it wants.

Some examples:

The following will process all of the trees from westgrid mc job 7,
second analysis.  The output root file would get put in
/twist/data13/jamieson/rootmacro/gen07/run.root

processhelixtrees 2 "/twist/tw00z/systematics/mc/gen07/anal2/root/" run "/twist/data13/jamieson/rootmacro/gen07/" -1

The following will process all of the trees in the directory
/home/jamieson/myfavouriterootfiles (if such a directory existed).
The output will go to the same directory in myfav.root.

processhelixtrees 1 "/home/jamieson/myfavoriterootfiles/" myfav "/home/jamieson/myfavoriterootfiles/" -1

-Blair