netcat や nmap を使わずに bash だけでポートの開放をみたい場合のハック (横着) ref: https://qiita.com/kitsuyui/items/7d79fd8940d8910e3862

file0.txt

$ exec 7<> /dev/tcp/8.8.8.8/53

file1.txt

$ exec 7<> /dev/tcp/8.8.8.8/54
(しばらく待たされる)
-bash: connect: Operation timed out
-bash: /dev/tcp/8.8.8.8/54: Operation timed out

file2.txt

$ exec 7<> /dev/tcp/127.0.0.1/10000
-bash: connect: Connection refused
-bash: /dev/tcp/127.0.0.1/10000: Connection refused

file3.txt

$ check_tcp_port() (bash -c 'trap "exit 2" SIGTERM; (exec 7<> /dev/tcp/'"$1"'/'"$2"' & (sleep 1 && kill 0) & wait %1)' &> /dev/null)

file4.txt

$ check_tcp_port 8.8.8.8 53 ; echo $?
0
$ check_tcp_port 8.8.8.8 54 ; echo $?
2
$ check_tcp_port 127.0.0.1 10000 ; echo $?
1