Switch between WiFi networks without reboot, command line

Today I have a chalange to set up WiFi connection on Raspbian Stretch Lite operating system.

While I testing it is nice to have:

• Right keyboard layout by default
• Auto login pi user

For fast wifi connection I can put the configuration on /boot/first/wpa_supplicant.conf

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
network={
ssid="first-network"
psk="AwwEsom3"
}

and also I have second configuration on /boot/second/wpa_supplicant.conf

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
network={
ssid="second-network"
psk="Hello123"
}

Here is what I have learned:

Every file which will be placed in /boot/wpa_supplicant.conf will be automatically moved to /etc/wpa_supplicant/wpa_supplicant.conf
So I will place my configurations in subdirectory :)

By default /etc/network/interface is empty like this:

# interfaces(5) file used by ifup(8) and ifdown(8)

# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and 'man dhcpcd.conf'

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

and "/etc/network/interfaces.d" actually is empty directory So how do I move to another wifi network without rebooting pi?

Let's replace all content in "/etc/network/interfaces" with something else:

cat > /etc/network/interfaces <<EOF

#let wifi interface request IP address by himself
iface wlan0 inet dhcp

#tell where all wifi credentials are in
wpa_conf /etc/wpa_supplicant/wpa_supplicant.conf

#execute wlan0 configuration everytime network service is restarted. very important
auto wlan0

EOF

Now I can switch between networks with

cp /boot/first/wpa_supplicant.conf /etc/wpa_supplicant
/etc/init.d/networking restart

or

cp /boot/second/wpa_supplicant.conf /etc/wpa_supplicant
/etc/init.d/networking restart

If something is not working at all then kill wpa_supplicant:

rm /var/run/wpa_supplicant/wlan0
pkill wpa_supplicant