Shell Programming and Scripting

Worked like a charm:

Code:

#include <stdio.h>
#include <stdlib.h>

int main( int argc, char **argv ) { 

   FILE* fp = fopen( "/hi" , "w" );
   fprintf( fp, "%s\n", "hi there" );
   fclose ( fp );
   exit(1);
}

And then:

Code:

$ gcc hi.c
$ mv a.out sayHi
$ sudo chown root sayHi
$ sudo chgrp root sayHi
$ sudo chmod u+s sayHi
$ ls -l sayHi
-rwsr-xr-x 1 root root 8256 Nov 29 06:04 sayHi
$ ./sayHi
$ ls -l /hi
-rw-r--r-- 1 root pi 9 Nov 29 06:06 /hi
$ cat /hi
hi there
$

Alles klar.

------ Post updated at 10:23 PM ------

I wasn't sure what information was relevant. Originally I thought a shell script could be used to experiment but apparently not.
I'm using Bash v4.4 on a Raspberry Pi running Raspbian, a Debian based Linux distribution. And Bash v4.4 on Kali Linux (Stretch), also Debian based.

I'm actually not trying to overwrite a running program. It may have looked like that because originally I tried to write the string "hello" into a file with filename "hello" and the script was also named hello. My apologies for the confusion.