Make a C file executable with those few lines.

shebang.c

#if 0
set -e; [ "$0" -nt "$0.bin" ] &&
gcc -Wall -Wextra -pedantic -std=c99 "$0" -o "$0.bin"
exec "$0.bin" "$@"
#endif

#include <stdio.h>

int
main(int argc, char *argv[]) {
	int ch = getchar();
	printf("Hello %c world!\n", ch);
	return 0;
}

Yeah, it makes sense as you describe the whole behavior.

Basically I thought of TCC's and Dlang's -run flags; what I know for sure is that DMD would generate an executable at /tmp, run it, and then delete it immediately.

Of course as you have said yourself, the workarounds you have just shared (thank you by the way) are quite hacky and a bit verbose to say the least, therefore I should stick with your original preprocessor technique.

Thank you for your thorough explanation, I was reminded how script behavior works.