Jan 9, 2020 - Using sed to edit config files


Using sed to edit config files

If you have a script which installs something, you often also want to automatically adjust a config file after installation. You can use the unix tool ‘sed’ for that.

Here are some examples, which assume that you have a parameter called ‘Test’ in the config file which is set to a value using ‘=’ character, e.g. test.conf

# This is some fake config file
Test = 123

Now you can use ‘sed’ to comment that line out, in or change the value:

# comment out
sed -i -e '/^[[:blank:]]*Test/ s/^#*/#/' test.conf

# comment in
sed -i -e '/^[[:blank:]]*#[[:blank:]]*Test/ s/^[[:blank:]]*#*//' test.conf

# set the value to 'abc'
sed -i -e '/^[[:blank:]]*Test[[:blank:]]*=/ s/=.*/= abc/' test.conf

With the ‘-i’ option the file is edited in-line. If you don’t use it the (modified) file content is printed out on the command line; good for testing.

The ‘[[:blank:]]’ fields are included so that the expression also works for messy config files, where you could have things like

  #  Test
Test    =123
   Test    = 123
# etc.

Dec 27, 2019 - Raspberry PI - Get started


Raspberry PI - Get started

Default tasks when setting up a new Raspberry PI.

Get Raspbian

Download from raspberrypi.org

Transfer it onto the SD card

Plug in the SD card and run fdisk -l to find out which device it is. If it is mounted, unmount it umount /dev/sdc1 I assume for now it is /dev/sdc. You have to use yours, make sure to get this right!!

Then write the image to the SD card:

dd if=2019-09-26-raspbian-buster-full.img of=/dev/sdc bs=512k

Setup Wifi and SSH before the first boot

Most often you don’t want to connect a keyboard and monitor. You just want plug it in the power socket somewhere and get going. Therefore you need to setup Wifi and enable the SSH server before you boot it up.

Simply remove the SD card and put it back in again. Then the /boot partition of the card should get mounted.

Run wpa_passphrase YOUR_SSID and note the output. Then create the file wpa_supplicant.conf in the boot parition and adjust it accordingly:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
country="Two letter country code, e.g. GB"

    ssid="Your SSID"
    psk="Your PASSWORD"

Enable the SSH server by creating an empty file ssh in the boot partition.

Umount the SD card partitions again, put the card into the Raspberry Pi and start it up.

Change the password

Run nmap -sn to find out the IP address of the Pi. Then log in with the default password which is ‘raspberry’: ssh pi@192.168.1.xxx

And change it: passwd

Update system

sudo apt update
sudo apt upgrade

Install X2Go (optional)

If you also want to use the desktop, install the X2Go server.

Create the file /etc/apt/sources.list.d/x2go.list:

# X2Go Repository (release builds)
deb http://packages.x2go.org/raspbian buster main
# X2Go Repository (sources of release builds)
deb-src http://packages.x2go.org/raspbian buster main

Add the key:

sudo apt-key adv --recv-keys --keyserver keys.gnupg.net E1F958385BFE2B6E


sudo apt update && sudo apt install x2goserver x2goserver-xsession

Start x2goserver and enable it at boot time:

sudo systemctl start x2goserver
sudo systemctl enable x2goserver

Install the client on your PC accordingly, the package is called x2goclient. Then you can remotely log into the desktop running on the Raspberry Pi. The default Raspbian desktop image comes with the ‘LXDE’ desktop. You can skip the ‘Setup’ tasks which pop up on the first desktop login, you’ve already done them.

Oct 24, 2019 - A simple netcat 'pong' service


A simple netcat ‘pong’ service

Given you have a machine outside your home LAN, you can create a very simple ‘pong’ service to get your external IP address. For example if you have to update a dynamic DNS entry for a server which is running inside your LAN.

I just call it ‘pong’, opposed to the wellknown ‘ping’ tool.

It simply uses netcat (you might have to install it first) to listen (-l) on a given port (-p) and dump its verbose (-v) output into a file (this will contain the IP address of the incoming request). Then extract the IP address and return it. Then wait for the next request.

On the external machine

Create the file /usr/local/bin/pong.sh



_term() {
  # Catch the interrupt signal to enable
  # a graceful exit
  fuser -k $port/tcp
  exit 0

trap _term INT

while true
  nc -lvp $port -c "grep connect $file | cut -d'[' -f 3 | cut -d']' -f 1" 2> $file
  rm $file

Create a systemd service /etc/systemd/system/pong.service

Description=Pong Service



Start it:

systemctl start pong

Respectively enable it at boot time:

systemctl enable pong

On your local machine:

Get your external IP address by running

nc [SERVER IP] 12345