How does one join split files with extensions like ".001 .002 .003 .004…" back together into one file again?
Split files like these are likely made by Windows users using the QuickPar program, and as such linux users can rejoin them into the original file using command line options of the pararchive package. Using the following command par will see the split files as additional blocks and it will check, repair and concatenate (aka joining) the files into the single original file or files.
par2 r myfile.par2 myfile.*