I have not yet found an equivalent of the `debug` command for Linux. Ppl told me `gdb` could do the same but I found debug easier to use.
More modern formats (be it PE, ELF, Mach-o, ...) are far more complex, with many different segments, running in a more complex CPU mode, generally dynamically linked, and all the other bell and whistles. So even if you had an identical debug.exe for Linux, it would still be much harder to make sense of.
This is inaccurate. A DOS .com executable has no header whatsoever. It is literally just the program code and data with absolutely no metadata at all. It is loaded at memory address 0x100 and then execution begins, also at 0x100.
It is possible to have a 0 byte .com executable and it will perform a useful function. 0 bytes are loaded to 0x100 and then execution begins at 0x100, effectively re-running the last program run. Commonly such a file was named "go.com".