maybe for the most of you is obvious, but for me it has been not so easy to be understood :roll_eyes:
The command in Linux (I use Ubuntu dis.) is the following one:

mkgmap --tdbfile *.img

Note that the .tdb and .img (as in Windows) are created in the directory from which you launch mkgmap. The name of these two files is *****000.tdb and *****000.img