Linux: Test TCP Port when there is no telnet available

is_tcp_port_active.sh

# Copy/paste directly into bash shell or source a file with the function below:
# --- cut here ---
is_tcp_port_active () {
        if [ "$1" == "" -o "$2" == "" ]; then
                echo -e "\n usage: is_tcp_port_active <hostname> <port>\n"
                return 1
        fi
        HOST=$1;
        PORT=$2;
        echo "$HOST:$PORT";
        exec 3<>/dev/tcp/${HOST}/${PORT} && echo "Port $PORT opened" || echo "Port $PORT closed";
}
# --- end of cut ---

Usage:

usage: is_tcp_port_active

Test LDAP (389) port on host 'server01':

# is_tcp_port_active server01 389

Output with success:

server01:389
Port 389 opened

Output with error:

server01:389
-bash: server01: Name or service not known
-bash: /dev/tcp/server01/389: Invalid argument
Port 389 closed