Assembly - trying to move bytes to memory does not work

Question

I am trying to change a string that is stored in memory by changing the byte in it. I use movb for this, but for some reason the byte in the given memory location does not change.

In the gdb debugger:

14 movb% al, (% r10) # next instr
(gdb) print / d $ al
$ 4 = 0
(gdb) print / c * $ r10
$ 5 = 47 '/'
(gdb) s
16 mov $ 59,% rax
(gdb) print / c * $ r10
$ 6 = 47 '/'

The code is simple:

.globl _start
.text

_start:
        call chamaexecve
        variaveis:
            .asciz "/bin/bashABBBBCCCC"


    chamaexecve: 

        pop %r10
        xor %rax, %rax
        movb %al, (%r10) # problem happening here

        mov $59, %rax
        mov %rsi, %rdi
        mov $0, %rsi
        mov $0, %rdx

        syscall

And it compiles with as -gstabs -o shellf.o shellf.s-ld -N -z execstack -o shellf shellf.o

, . . , , , movb $'r', %al % al , 'r' , - $0.

Linux Mint 18.1 "Serena", gnu 2.26.1. x/i $rip, :

Edit2: break to tbreak s si, , '/' '\ 000' . , s/break/gdb ?

(gdb) tb 14
1 0x4000cd: shellf.s, 14.
(gdb)
:/home/fabio/criação/nasm-tutorial/shellf

1, chamaexecve() shellf.s: 14
14 movb% al, (% r10)
(gdb) si
16 mov $59,% rax
(gdb) print/c * $rsi
0x0
(gdb) print/c * $r10
$ 1 = 0 '\ 000'
(gdb)