For the later described combination of the tools it is often very useful to read some data from pipe and add other sets from file. Therfore an additional flag -pipe is needed in the command line to force the tool to read in from pipe as well. Similar to the -pipe option is the +pipe option, but while-pipe reads the data from standard out before the data from file, +pipe reads the data after evaluating the files.
gr_spline -f test.dat -n 80 | gr_copy -pipe -f test.dat
The output will contain both, a splined version of test.dat and the original data.