This has nothing to do with foreground and background processes; it only has to do with the currently running process. When the kernel has to answer the question “What does /proc/self point to?”, it simply picks the currently-scheduled pid, i.e. the currently running process (on the current logical CPU). The effect is that /proc/self always points to the asking program's pid; if you run

ls -l /proc/self

you'll see ls's pid, if you write code which uses /proc/self that code will see its own pid, etc.

use strings

$ cat /proc/self/cmdline | strings -1
cat
/proc/self/cmdline

There are functions called _NSGetArgv and _NSGetArgc in crt_externs.h. However, wouldn't it be simpler to just store the argc/argv in a variable at the start of main and be portable to all three platforms?

NOTE: These functions are not directly documented, but they are mentioned in the documentation for NSApplicationMain in the Application Kit Functions Reference, and the related _NSGetEnviron is mentioned in the environ(7) manpage.

The detail which you are extracting from /proc/self/status is found in /proc/self/comm (without scripting).

For the intended question: to determine which processes are interpreted and which are native, you cannot do this without knowing which processes are intended to be interpreters. After all, any process can rename itself via an exec call, using whatever it likes to the resulting process's argument list. You could guess by comparing /proc/self/exe (which points to the executable) to the argument list, but that would only be a guess, since there are examples where a non-interpreter renames its descendants just to make their use more apparent.

Further reading:

• Getting Process Invocations: The Hard Way
• The /proc filesystem
• Howto change a UNIX process and child process name by modifying argv[0]

Python, 64 bytes

-2 bytes thanks to corvus_192

import os
os.execl(*"aa",*int(input()).to_bytes(4).split(b'\0'))

Uses big endian and assumes binary is named "a".