grppha

ftp://ftp.xray.mpe.mpg.de/xmm/service/data_analysis/EPIC_PN/spectral/spec_bin.html

input files: source.fits, backgr.fits
output file: source_grp.fits.fits
instrument: EPIC PN and MOS, IMAGING mode

We have created a source and background spectrum source.fits and backgr.fits as described on the pages for the EPIC PN detector or for the EPIC MOS detector

It will ask you for the input and output names, e.g.

Please enter PHA filename[] source.fits
Please enter output filename[] source_grp.fits

grppha will not overwrite an existing file. It will simply crash. To avoid this, either delete or rename the old file, or set clobber=yes.

Another useful page for grppha command is linked below:

http://heasarc.gsfc.nasa.gov/docs/journal/grppha4.html
There are many examples out there, eg.
To use GRPPHA in a non-interactive manner with no chatter:
unix> grppha infile='test.pha' outfile='new.pha' chatter=0 comm='group 1 2048 4&bad 1-10&exit'