pnmtorast
Convert a PNM file to a Sun rasterfile.
TLDR
Convert a PNM image to a RAST image
$ pnmtorast [path/to/input.pnm] > [path/to/output.rast]
Force either RT_STANDARD or RT_BYTE_ENCODED form for the output
$ pnmtorast -[standard|rle] [path/to/input.pnm] > [path/to/output.rast]
