Failed to execute echo command in system() in INIT in root file system

Question

Host : Intel pentium 4, RHEL 6

Target : ARM Cortex A9 running linux with my own small sized rootffile file system

I developed a minimal initramfs filesystem following the instructions here: Minimalist Initramfs and Initrd .

So my initrmafs has only following things:

1- console

2- init

The init is the binary having the following code:

* myinit.c
 * Build instructions:
 * ${CROSS_COMPILE}gcc -static init.c -o init
 * */

#include <stdio.h>

int
main ()
{
    printf ("\n");
    printf ("Hello world from %s!\n", __FILE__);
    while (1) { }
    return 0;
}

This works well with the linux kernel and in the log message at the end I get a hello world message.

But what I wanted is that after printing hello world, the echo command should work and do the following:

echo 10 > test.txt
echo "$(cat test.txt)"

So what I did is:

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

        int main ()
        {
           printf ("\n");
            printf ("Hello world from %s!\n", __FILE__);

system("echo 10 > test.txt");

system("echo \"$(cat test.txt)\"");

          while (1) { }
            return 0;
        }

I am cross compiling my code using the following command:

  arm-xilinx-linux-gnueabi-gcc -static echotest.c -o init

Hello world is printing correctly but echo is not working as the content of the file ie 10 is not printing.

Note: Please note that this time I have following things in my file system ie initramfs:

1- console

2- init (binary for second program)

3- test.txt (an empty file)

4- a folder bin having the binary echo

5- a folder bin having the binary cat

Aprt from these, there is nothing. The main idea was to have a minimal file system, having only things which are really needed for my aplication.

The binary cat and echo I copied from a working linux system.

Kindly help me in correctly running the echo command as above?

Update

My latest code is as under where I am mounting /bin:

#include <stdlib.h>
#include <sys/mount.h>
#include <errno.h>
#include <stdio.h>


void  mount_sys() {
    if (0 != mount("none", "/bin", "sysfs", 0, "")) {
     perror("there is an error in mounting \n");   /* handle error */
    }
    printf("mounting successful");
}

        int main ()
        {

mount_sys();
           printf ("\n");
            printf ("Hello world from %s!\n", __FILE__);

system("echo 10 > test.txt");

system("echo \"$(cat test.txt)\"");

          while (1) { }
            return 0;
        }

Unfortunately this is also not working. The last two lines in the boot log message are:

mounting successful
Hello world from echotest.c!   

Answer 1

system() requires /bin/sh to be available.

If you want really minimal, use fork / exec instead. Shell constructs like redirection will then (obviously) not be available.

As a stylistic comment, always checking system calls for errors would be a good first start for diagnosing problems, even if there will be no sane way to recover.