Why does >/dev/null not work as expected? - bash

Question

I have written the following shell script:
#! /bin/bash
# This script is designed to find hosts with MySQL installed
nmap -sT my_IP_address -p 3306 >/dev/null -oG MySQLscan
cat MySQLscan | grep open > MySQLscan2
cat MySQLscan2
According to the script the output of nmap should be sent to /dev/null. On the other hand, the final out put should be written to the file MySQLscan2 in my pwd.
Not as I have expected, two files are written to my pwd:
MySQLscan: Contains the output of the scan that I have expected to be in MySQLscan2.
MySQLscan2: This file is empty.
Is there an mistake in my script? How can I solve the problem?
Earlier today, I managed to run the script with correct output. I am not sure if I have changed the script in some way. I checked it again and again, but cannot find, what is wrong...
I am working with Kali Linux and Oracle VM Virtual Box.

Answer 1

> /dev/null causes shell to redirect stdout, that is a file with
file descriptor to 1
to /dev/null before the command starts so in other words to discard
it. When nmap runs with -oG MySQLscan option it opens a new file and
gets a new file descriptor. You can check it with strace:
$ strace -f nmap -sT localhost -p 3306 -oG MySQLscan |& grep MySQLscan
execve("/usr/bin/nmap", ["nmap", "-sT", "localhost", "-p", "22", "-oG", "MySQLscan"], 0x7ffc88805198 /* 60 vars */) = 0
openat(AT_FDCWD, "MySQLscan", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
In this example openat() returned 4 as a new file descriptor (you
can read more about this function with man 2 openat). Since file
descriptor 4 hasn't been redirected before command started MySQLscan
gets created. Also notice that even if file descriptor that openat()
returns to open MySQLscan is redirected to /dev/null:
nmap -sT localhost -p 22 -oG MySQLscan 4>/dev/null
it doesn't prevent MySQLscan from being created because openat()
requests a new unused file descriptor from kernel every time it's
run.