First init

Let us create the process init. It will print "hello world" as you would normally do, that is, init is a user process printing "hello world"  in user mode.
Let us make our ARM Linux runs this init process:

#include <stdio.h>
int main()
{

   while (1) { 

      printf("Hello, World; from user!\n");

   }
   return 0;
}

Do something like this:
Save what Shibata's team have done for future study:

~/Downloads/pcs3746-sistemas-operacionais/1/initramfs$ cp init.c init.c.1

edit init.c

~/Downloads/pcs3746-sistemas-operacionais/1/initramfs$ emacs init.c

I like emacs to edit files, but you may choose vim, nano, etc. Make init.c contains the code above.

Now, just do:

~/Downloads/pcs3746-sistemas-operacionais/1$ ./run.sh

The Linux source code will be compiled if it has too, the init.c will be compiled and inserted into rootfs thanks to default_cmd.sh that will make everything automatically.

That's it! You got your init running with no hassles.

In order to stop it, you may kill the container.

~/Downloads/pcs3746-sistemas-operacionais/1/initramfs$ docker ps
CONTAINER ID        IMAGE                  COMMAND                  CREATED             STATUS              PORTS               NAMES
7b18bb9eb0a2        tiagoshibata/pcs3746   "/entrypoint.sh /def…"   2 minutes ago       Up 2 minutes                            cranky_lewin
jk@jk-desktop:~/Downloads/pcs3746-sistemas-operacionais/1/initramfs$ docker kill cranky_lewin
cranky_lewin