How to read from and write to files using NASM for x86-64bit - linux

Question

I have a NASM program for 64bit Linux system which works with standard I/O devices and it looks something like that:
section .data
prompt db "Enter your text: ", 10
length equ $ - prompt
text times 255 db 0
textSize equ $ - text
section .text
global main
main:
mov rax, 1
mov rdi, 1
mov rsi, prompt
mov rdx, length
syscall ;print prompt
mov rax, 0
mov rdi, 0
mov rsi, text
mov rdx, textSize
syscall ;read text input from keyboard
mov rcx, rax ; rcx - character counter
mov rsi, text ; a pointer to the current character starting from the beginning.
****
exit:
mov rax, 60
mov rdi, 0
syscall
I need the program to read from and write to the files, but I can't find anywhere which syscalls has to be used and how they should be used to achieve these results. So, I am wondering if someone of you could help me. Thanks in advance.

Answer 1

Use system calls "open" and "close":
Open a file under 64-bit Linux:
rax = 2
rdi = pointer to NUL-terminated filename
rsi = something like O_WRONLY
rdx = file flags if creating a file (e.g. 0644 = rw-r--r--)
syscall
now rax contains the file hanle
Close a file:
rax = 3
rdi = file handle
syscall
Reading/writing from/to a file:
rax = 0 or 1 (like keyboard/screen in/output)
rdi = file handle (instead of 0/1)
...

Answer 2

;this program is only overwrite the result.txt
;to write it on bottom you need to lseek to the end
section .data
prompt db "Enter your text: ",10
length equ $ - prompt
text times 255 db 0
textsize equ $-text
fname db "result.txt",0
fd dq 0
thefox dq 0
global _start
section .text
_start:
mov al,1
mov dil,al
mov esi,prompt
mov dl,length
syscall
mov al,0
mov dil,al
mov rsi,text
mov rdx,textsize
syscall
mov [thefox],rax
mov rax,2
mov rdi,fname
mov rsi,0102o
mov rdx,0666o
syscall
mov [fd],rax
mov rdx,[thefox]
mov rsi,text
mov rdi,[fd]
mov rax,1
syscall
mov rdi,[fd]
mov rax,3
syscall
mov rax,60
syscall