#!/bin/rm
#!/sbin/md5
#!/usr/bin/file
#!/usr/bin/du -h
#!./quine
cat $0
#!/bin/rm
#!/sbin/md5
#!/usr/bin/file
#!/usr/bin/du -h
#!./quine
cat $0
Refer to sys/kern/imgact_shell.c and ^do_execve in sys/kern/kern_exec.c, I skimmed but could not find why it ran.
$ sudo dtrace -n 'syscall::execve*:entry { printf("%s %s", execname, copyinstr(arg0)); }'
0 51617 execve:entry sh ./quine
1 51617 execve:entry sh /bin/sh
1 51617 execve:entry sh /bin/cat
My login shell is the Bourne shell. However I tried bash, csh and tcsh too, it works, so it's probably the system level code that substitutes /bin/sh (maybe because it's my login shell?) to the interpreter.