## Introduction

### Welcome!

Hi there. Congratulations on being interested enough in the process of learning about measuring distance using ultrasonic audio with the Raspberry Pi to have gotten your hands on this book.

Hopefully this will be a journey of discovery for both of us. The goal is to experiment with computers and use them to know a bit more about what is happening in the physical environment. This sort of thing has been done already by others, but I have an ulterior motive. I write books to learn and document what I’ve done. The hope is that by sharing the journey perhaps others can learn something from my efforts :-).

Ambitious? Maybe :-). But if you’re reading this, I managed to make some headway. I dare say that like other books I have written (or are in the process of writing) it will remain a work in progress. They are living documents, open to feedback, comment, expansion, change and improvement. Please feel free to provide your thoughts on ways that I can improve things. Your input would be much appreciated.

You will find that I have typically eschewed a simple “Do this approach” for more of a story telling exercise. This means that some explanations are longer and more flowery than might be to everyone’s liking, but there you go, that’s my way :-).

There’s a lot of information in the book. There’s a vast amount of ‘stuff’ that people with even a passing interest in computing will find excessive. Sorry about that. I’ve deliberately gathered together a lot of the content from other books I’ve written to create a guide that is as full of usable information as possible to help people who could conceivably be using the Pi and coding for the first time. This is ONE fairly simple project that might otherwise be described in 5 pages stretched out into a lot more. But I’m working on the principle that if I need to recreate what I have here from scratch, this guide will leave nothing out. It will also form a basis for other similarly explained books (as books before this one have done). Evolving the description as the OS versions and Raspberry Pi’s are updated.

I’m sure most authors try to be as accessible as possible. I’d like to do the same, but be warned… There’s a good chance that if you ask me a technical question I may not know the answer. So please be gentle with your emails :-).

Email: d3noobmail+ultra@gmail.com

Cover photo via Good Free Photos and Acer220.

### What are we trying to do?

Put simply, we are going to examine the wonder that is the Raspberry Pi computer and use it to accomplish something.

In this specific case we will be connecting an ultrasonic module to the Pi, measuring the values that it returns, recording them in a database and then making those values available via web a interface!

Along the way we’ll;

• Look at the Raspberry Pi and its history.
• Work out how to get software loaded onto the Pi.
• Learn about networking and configure the Pi accordingly.
• Install and configure a web server and a database.
• Write some code to interface with our ultrasonic module.

### Who is this book for?

You!

Just by virtue of taking an interest and getting hold of a copy of this book you have demonstrated a desire to learn, to explore and to challenge yourself. That’s the most important criteria you will want to have when trying something new. Your experience level will come second place to a desire to learn.

Having said that, it may be useful to be comfortable using the Windows operating system (I’ll be using Windows 7 for the set-up of the devices), you should be aware of Linux as an alternative operating system, but you needn’t have tried it before. The best thing to remember is that before you learn anything new, it pretty much always appears indistinguishable from magic, but once you start having a play, the mystery quickly falls away.

### What will we need?

Well, you could just read the book and learn a bit. By itself that’s not a bad thing, but trust me when I say that actually experimenting with physical computers is fun and rewarding.

The list below is pretty flexible in most cases and will depend on how you want to measure the distances.

• A Raspberry Pi (I’m using a Raspberry Pi Model B 2 / 3)
• Probably a case for the Pi
• A MicroSD card
• A power supply for the Pi
• A keyboard and monitor that you can plug into the Pi (there are a few options here, read on for details)
• A remote computer (like your normal desktop PC that you can use to talk to connect to the Pi). This isn’t strictly necessary, but it makes the experience way cooler.
• A JSN-SR04T Ultrasonic Module. They are available from lots of places for around $10 US. Google is your friend. There is a version 2.0 of the module. I have not used this. • A 1k and 2k Ohm resisters with some soldering equipment • Some dupont connectors (that’s what I used, but you could connect to the Pi in different ways) • An Internet connection for getting and updating the software. As we work through the book we will be covering off the different parts required and you should get a good overview of what your options are in different circumstances. ### Why on earth did I write this rambling tome? That’s a really good question. This is a project that I use in the real world (to measure the height of the water in my water tank). Over the years of owning a house with a water tank I have had several instances where (for a range of reasons) the tank has emptied and I didn’t realise (and even once where the tank over filled!). I generated a previous book on multiple temperature measurements and that was an enjoyable process, so I thought that I’d repeat it and perhaps continue to adapt the book for subsequent projects by simply adapting whatever the proceeding one was. Will this work? Who knows. What I can tell you is that I now have a way of measuring the height of the water in my water tank and you have a book that tells you how I did it :-). Included is a bunch of information from my books on the Raspberry Pi, Linux and d3.js. I hope you find it useful. ### Where can you get more information? The Raspberry Pi as a concept has provided an extensible and practical framework for introducing people to the wonders of computing in the real world. At the same time there has been a boom of information available for people to use them. The following is a far from exhaustive list of sources, but from my own experience it represents a useful subset of knowledge. raspberrypi.org Google+ reddit Raspberry Pi Stack Exchange ## The History of the Raspberry Pi The story of how the Raspberry Pi came to be started in 2006. Eben Upton, Rob Mullins, Jack Lang and Alan Mycroft, who were based at the University of Cambridge’s Computer Laboratory, became concerned at the decline in the volume and skills of students applying to study Computer Science. They saw part of the problem being that a typical student applicant was not arriving with a history of hobby programming and tinkering with hardware. Instead they were coming on board with some web design experience, but little else. They established that the way that kids were interacting with computers had changed. A school curriculum that was pre-loaded to emphasise working with Word and Excel and building web pages meant that the focus was more on working with applications. Games consoles were replacing the traditional hobbyist computer platforms that had existed in the era when the Amiga, Apple II, ZX Spectrum and the ‘build your own’ approach were king. So, in 2006, Eben and the team began to design and prototype a platform that was cheap, simple, booted into a programming environment and most of all, one which would inspire the next generation of computer enthusiasts to recover the joy of experimenting with computers. Between 2006 and 2008, they developed a number of prototypes based on the Atmel ATmega644 microcontroller. By 2008 processors designed for mobile devices were becoming affordable and powerful enough to support a range of multimedia features. This allowed the boards to support an graphical environment which they believed would make the board more attractive to children who were looking for an alternative to a purely programming-oriented device. Eben, Rob, Jack and Alan, then teamed up with Pete Lomas, and David Braben to form the Raspberry Pi Foundation to bring the project to the world. The Foundation’s goal was to offer two versions of the board, priced at US$25 and US$35. In August 2011 50 alpha boards are manufactured. These boards were functionally identical to the model B. Based on these, twenty-five model B Beta boards were assembled in December 2011 with the same component layout as what would be the production boards. Interest in the project escalated as they were demonstrated booting Linux, playing a 1080p movie trailer and running benchmarking programs. During the first week of 2012, the first 10 boards were put up for auction on eBay. One was bought anonymously and donated to the museum at The Centre for Computing History in Suffolk, England. While the ten boards together raised over 16,000 Pounds (about$25,000 USD) the last to be auctioned (serial number No. 01) raised 3,500 Pounds by itself.

The Raspberry Pi Model B entered mass production through licensed manufacturing deals with element 14/Premier Farnell and RS Electronics. They started accepting orders for the model B on the 29th of February 2012. It was quickly apparent that they had identified a need in the marketplace as their servers struggled to cope with the load placed by watchers repeatedly refreshing their browsers. The official Raspberry Pi Twitter account reported that Premier Farnell sold out within few minutes of the initial launch, while RS Components took over 100,000 pre orders on the first day of sales.

Within two years they had sold over two million units.

The the lower cost model A went on sale for $25 on 4 February 2013. By that stage the Raspberry Pi was already a hit with the model B being manufactured at a rate of 4000 units per day and the amount of on-board ram had been increased to 512MB. The official Raspberry Pi blog reported that the three millionth Pi shipped in early May 2014 and in July 2014 announced the Raspberry Pi Model B+, “the final evolution of the original Raspberry Pi. For the same price as the original Raspberry Pi model B, but incorporating numerous small improvements”. In November of the same year the even lower cost (US$20) A+ was announced. Like the A, it would have no Ethernet port, and just one USB port, but like the B+, it would have lower power requirements, a micro-SD-card slot and 40-pin HAT compatible GPIO.

On 2 February 2015 the official Raspberry Pi blog announced that the Raspberry Pi 2 was available. With the same form factor and connector layout as the Model B+, it has a 900 MHz quad-core ARMv7 Cortex-A7 CPU, twice the memory (for a total of 1 GB) and complete compatibility with the original generation of Raspberry Pis.

## Operating Systems

An operating system is software that manages computer hardware and software resources for computer applications. For example Microsoft Windows could be the operating system that will allow the browser application Firefox to run on our desktop computer.

Variations on the Linux operating system are the most popular on our Raspberry Pi. Often they are designed to work in different ways depending on the function of the computer.

Linux is a computer operating system that is can be distributed as free and open-source software. The defining component of Linux is the Linux kernel, an operating system kernel first released on 5 October 1991 by Linus Torvalds.

Linux was originally developed as a free operating system for Intel x86-based personal computers. It has since been made available to a huge range of computer hardware platforms and is one of the most popular operating systems on servers, mainframe computers and supercomputers. Linux also runs on embedded systems, which are devices whose operating system is typically built into the firmware and is highly tailored to the system; this includes mobile phones, tablet computers, network routers, facility automation controls, televisions and video game consoles. Android, the most widely used operating system for tablets and smart-phones, is built on top of the Linux kernel. In our case we will be using a version of Linux that is assembled to run on the ARM CPU architecture used in the Raspberry Pi.

The development of Linux is one of the most prominent examples of free and open-source software collaboration. Typically, Linux is packaged in a form known as a Linux ‘distribution’, for both desktop and server use. Popular mainstream Linux distributions include Debian, Ubuntu and the commercial Red Hat Enterprise Linux. Linux distributions include the Linux kernel, supporting utilities and libraries and usually a large amount of application software to carry out the distribution’s intended use.

A distribution intended to run as a server may omit all graphical desktop environments from the standard install, and instead include other software to set up and operate a solution stack such as LAMP (Linux, Apache, MySQL and PHP). Because Linux is freely re-distributable, anyone may create a distribution for any intended use.

### Welcome to Raspbian

The Raspbian Linux distribution is based on Debian Linux. At the time of writing there have been three different editions published. ‘Wheezy’, ‘Jessie’ and ‘Stretch’. Debian is a widely used Linux distribution that allows Raspbian users to leverage a huge quantity of community based experience in using and configuring software. The Wheezy edition is the earlier of the three and was the stock edition from the inception of the Raspberry Pi till the end of 2015. From that point Jessie was the default distribution until mid 2017 when Stretch took over.

The best place to source the latest version of the Raspbian Operating System is to go to the raspberrypi.org page; http://www.raspberrypi.org/downloads/. We will download the ‘Lite’ version (which doesn’t use a desktop GUI). If you’ve never used a command line environment, then good news! You’re about to enter the World of ‘real’ computer users :-).

You can download via bit torrent or directly as a zip file, but whatever the method you should eventually be left with an ‘img’ file for Raspbian.

To ensure that the projects we work on can be used with either the B+, B2 or B3 models we need to make sure that the version of Raspbian we download is from 2015-01-13 or later. Earlier downloads will not support the more modern CPU of the B2 or B3. To support the newer CPU of the B3+ (and all the previous CPUs) we will need a version of Raspbian from 2018-03-13 or later.

We should always try to download our image files from the authoritative source and we can normally do so in a couple of different ways. We can download via bit torrent or directly as a zip file.

#### Writing the Operating System image to the SD Card

Once we have an image file we need to get it onto our SD card.

We will work through an example using Windows 7 but the process should be very similar for other operating systems as we will be using the excellent open source software Etcher which is available for Windows, Linux and MacOS.

Select the img file that you want to install.

You will need an SD card reader capable of accepting your MicroSD card (you may require an adapter or have a reader built into your desktop or laptop). Place the card in the reader and you should see Etcher automatically select it for writing (Etcher is very good at presenting options for installing that are only SD cards).

Then click on ‘Flash!’ to burn the card.

Etcher will write the image to the SD card. The time taken can vary a little, but it should only take about 3-4 minutes with a class 10 SD card.

Once written, Etcher will validate the write process (this can be disabled if desired).

When the process is finished Etcher will unmount the SD card.

#### Enabling Secure Shell Access

One of the awesome things when learning to use a Raspberry Pi comes when you begin to access it remotely from another computer. This is a bit of an ‘Ah Ha!’ moment for some people as they begin to appreciate just how networks and the Internet is built. We are going to enable and use remote access via what is called ‘SSH’. We’ll start using it later in the book, but for now we can take the opportunity to enable it for later use. We do this by creating a file called ‘ssh’ on our freshly written SD card. Then, when the Pi then boots up it sees the file and automatically knows to enable SSH.

SSH used to be enabled by default, but doing so presents a potential security concern, so it has been disabled by default as of the end of 2016. In our case it’s a feature that we want to use.

Eject the card from the computer and then re-insert it. When the computer recognises the card, open it and right-click in the folder to create a new file. This can be a simple txt file so long as the file prefix is ‘ssh’. It doesn’t need to have anything in it, there just needs to be a file there.

Now we can unmount the SD card and eject it again.

#### Powering On

Insert the card into the slot on the Raspberry Pi and turn on the power.

You will see a range of information scrolling up the screen before eventually being presented with a login prompt.

#### The Command Line interface

Because we have installed the ‘Lite’ version of Raspbian, when we first boot up, the process should automatically re-size the root file system to make full use of the space available on your SD card. If this isn’t the case, the facility to do it can be accessed from the Raspberry Pi configuration tool (raspi-config) that we will look at in a moment.

Once the reboot is complete (if it occurs) you will be presented with the console prompt to log on;

Congratulations, you have a working Raspberry Pi and are ready to start getting into the thick of things!

Firstly we’ll do a bit of house keeping.

##### Raspberry Pi Software Configuration Tool

We will use the Raspberry Pi Software Configuration Tool to change the locale and keyboard configuration to suit us. This can be done by running the following command;

Use the up and down arrow keys to move the highlighted section to the selection you want to make then press tab to highlight the <Select> option (or <Finish> if you’ve finished).

Lets change the settings for our operating system to reflect our location for the purposes of having the correct time, language and WiFi regulations. These can all be located via selection ‘4 Localisation Options’ on the main menu.

Select this and work through any changes that are required for your installation based on geography.

Once you exit out of the raspi-config menu system, if you have made a few changes, there is a probability that you will be asked if you want to re-boot the Pi. That’s a pretty good idea.

Once the reboot is complete you will be presented with the console prompt to log on again;

After configuring our Pi we’ll want to make sure that we have the latest software for our system. This is a useful thing to do as it allows any additional improvements to the software we will be using to be enhanced or security of the operating system to be improved. This is probably a good time to mention that we will need to have an Internet connection available.

Type in the following line which will find the latest lists of available software;

You should see a list of text scroll up while the Pi is downloading the latest information.

The Pi should tell you the lists of packages that it has identified as suitable for an upgrade along with the amount of data that will be downloaded and the space that will be used on the system. It will then ask you to confirm that you want to go ahead. Tell it ‘Y’ and we will see another list of details as it heads off downloading software and installing it.

## Power Up the Pi

To configure the Raspberry Pi for our purpose we will extend our Pi a little. This makes configuring and using the device easier and to be perfectly honest, making life hard for ourselves is so exhausting! Let’s not do that.

As we established earlier, enabling remote access is a really useful thing. This will allow us to configure and operate our raspberry Pi from a separate computer. To do so we will want to assign our Raspberry Pi a static IP address.

An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication.

There is a strong likelihood that our Raspberry Pi already has an IP address and it should appear a few lines above the ‘login’ prompt when you first boot up;

The My IP address... part should appear just above or around 15 lines above the login line, depending on the version of Raspbian we’re using. In this example the IP address 10.1.1.25 belongs to the Raspberry Pi.

This address will probably be a ‘dynamic’ IP address and could change each time the Pi is booted. For the purposes of using the Raspberry Pi with a degree of certainty when logging in to it remotely it’s easier to set a fixed IP address.

This description of setting up a static IP address makes the assumption that we have a device running on a network that is assigning IP addresses as required. This sounds like kind of a big deal, but in fact it is a very common service to be running on even a small home network and it will be running on an ADSL modem/router or similar. This function is run as a service called DHCP (Dynamic Host Configuration Protocol). You will need to have access to this device for the purposes of knowing what the allowable ranges are for a static IP address.

A common feature for home modems and routers that run DHCP devices is to allow the user to set up the range of allowable network addresses that can exist on the network. At a higher level we should be able to set a ‘netmask’ which will do the job for us. A netmask looks similar to an IP address, but it allows you to specify the range of addresses for ‘hosts’ (in our case computers) that can be connected to the network.

A very common netmask is 255.255.255.0 which means that the network in question can have any one of the combinations where the final number in the IP address varies. In other words with a netmask of 255.255.255.0 the IP addresses available for devices on the network 10.1.1.x range from 10.1.1.0 to 10.1.1.255 or in other words any one of 256 unique addresses.

##### CIDR Notation

An alternative to specifying a netmask in the format of ‘255.255.255.0’ is to use a system called Classless Inter-Domain Routing, or CIDR. The concept is that you can add a specification in the IP address itself that indicates the number of significant bits that make up the netmask.

For example, we could designate the IP address 10.1.1.17 as associated with the netmask 255.255.255.0 by using the CIDR notation of 10.1.1.17/24. This means that the first 24 bits of the IP address given are considered significant for the network routing.

Using CIDR notation allows us to do some very clever things to organise our network, but at the same time it can have the effect of freaking people out by introducing a pretty complex topic when all they want to do is get their network going :-). So for the sake of this explanation we can assume that if we wanted to specify an IP address and a netmask, it could be accomplished by either specifying each seperatly (IP address = 10.1.1.17 and netmask = 255.255.255.0) or in CIDR format (10.1.1.1/24)

#### Distinguish Dynamic from Static

The other service that our DHCP server will allow is the setting of a range of addresses that can be assigned dynamically. In other words we will be able to declare that the range from 10.1.1.20 to 10.1.1.255 can be dynamically assigned which leaves 10.1.1.0 to 10.1.1.19 which can be set as static addresses.

You might also be able to reserve an IP address on your modem / router. To do this you will need to know what the MAC (or hardware address) of the Raspberry Pi is. To find the hardware address on the Raspberry Pi type;

(For more information on the ifconfig command check out the Linux commands section)

This will produce an output which will look a little like the following;

The figures 00:08:C7:1B:8C:02 are the Hardware or MAC address.

Because there are a huge range of different DHCP servers being run on different home networks, I will have to leave you with those descriptions and the advice to consult your devices manual to help you find an IP address that can be assigned as a static address. Make sure that the assigned number has not already been taken by another device. In a perfect World we would hold a list of any devices which have static addresses so that our Pi’s address does not clash with any other device.

For the sake of the upcoming projects we will assume that the address 10.1.1.160 is available.

#### Default Gateway

Before we start configuring we will need to find out what the default gateway is for our network. A default gateway is an IP address that a device (typically a router) will use when it is asked to go to an address that it doesn’t immediately recognise. This would most commonly occur when a computer on a home network wants to contact a computer on the Internet. The default gateway is therefore typically the address of the modem / router on your home network.

We can check to find out what our default gateway is from Windows by going to the command prompt (Start > Accessories > Command Prompt) and typing;

This should present a range of information including a section that looks a little like the following;

The default router gateway is therefore ‘10.1.1.1’.

#### Lets edit the dhcpcd.conf file

On the Raspberry Pi at the command line we are going to start up a text editor and edit the file that holds the configuration details for the network connections.

The file is /etc/dhcpcd.conf. That is to say it’s the dhcpcd.conf file which is in the etc directory which is in the root (/) directory.

To edit this file we are going to type in the following command;

The nano file editor will start and show the contents of the dhcpcd.conf file which should look a little like the following;

The file actually contains some commented out sections that provide guidance on entering the correct configuration.

We are going to add the information that tells the network interface to use eth0 at our static address that we decided on earlier (10.1.1.160) along with information on the netmask to use (in CIDR format) and the default gateway of our router. To do this we will add the following lines to the end of the information in the dhcpcd.conf file;

Here we can see the IP address and netmask (static ip_address=10.1.1.160/24), the gateway address for our router (static routers=10.1.1.1) and the address where the computer can also find DNS information (static domain_name_servers=10.1.1.1).

Once you have finished press ctrl-x to tell nano you’re finished and it will prompt you to confirm saving the file. Check your changes over and then press ‘y’ to save the file (if it’s correct). It will then prompt you for the file-name to save the file as. Press return to accept the default of the current name and you’re done!

To allow the changes to become operative we can type in;

This will reboot the Raspberry Pi and we should see the (by now familiar) scroll of text and when it finishes rebooting you should see;

Which tells us that the changes have been successful (bearing in mind that the IP address above should be the one you have chosen, not necessarily the one we have been using as an example).

### Remote access

To allow us to work on our Raspberry Pi from our normal desktop we can give ourselves the ability to connect to the Pi from another computer. The will mean that we don’t need to have the keyboard / mouse or video connected to the Raspberry Pi and we can physically place it somewhere else and still work on it without problem. This process is called ‘remotely accessing’ our computer .

To do this we need to install an application on our windows desktop which will act as a ‘client’ in the process and have software on our Raspberry Pi to act as the ‘server’. There are a couple of different ways that we can accomplish this task, but because we will be working at the command line (where all we do is type in our commands (like when we first log into the Pi)) we will use what’s called SSH access in a ‘shell’.

#### Remote access via SSH

Secure Shell (SSH) is a network protocol that allows secure data communication, remote command-line login, remote command execution, and other secure network services between two networked computers. It connects, via a secure channel over an insecure network, a server and a client running SSH server and SSH client programs, respectively (there’s the client-server model again).

In our case the SSH program on the server is running sshd and on the Windows machine we will use a program called ‘PuTTY’.

##### Setting up the Server (Raspberry Pi)

SSH is already installed and operating but to check that it is there and working type the following from the command line;

The Pi should respond with the message that the program sshd is active (running).

If it isn’t, run the following command;

Use the up and down arrow keys to move the highlighted section to the selection you want to make then press tab to highlight the <Select> option (or <Finish> if you’ve finished).

To enable SSH select ‘5 Interfacing Options’ from the main menu.

From here we select ‘P2 SSH’

And we should be done!

##### Setting up the Client (Windows)

The client software we will use is called ‘Putty’. It is open source and available for download from here.

On the download page there are a range of options available for use. The best option for us is most likely under the ‘For Windows on Intel x86’ heading and we should just download the ‘putty.exe’ program.

Save the file somewhere logical as it is a stand-alone program that will run when you double click on it (you can make life easier by placing a short-cut on the desktop).

Once we have the file saved, run the program by double clicking on it and it will start without problem.

The first thing we will set-up for our connection is the way that the program recognises how the mouse works. In the ‘Window’ Category on the left of the PuTTY Configuration box, click on the ‘Selection’ option. On this page we want to change the ‘Action of mouse’ option from the default of ‘Compromise (Middle extends, Right paste)’ to ‘Windows (Middle extends, Right brings up menu)’. This keeps the standard Windows mouse actions the same when you use PuTTY.

Now select the ‘Session’ Category on the left hand menu. Here we want to enter our static IP address that we set up earlier (10.1.1.160 in the example that we have been following, but use your one) and because we would like to access this connection on a frequent basis we can enter a name for it as a saved session (In the screen-shot below it is imaginatively called ‘Raspberry Pi’). Then click on ‘Save’.

Now we can select our raspberry Pi Session (per the screen-shot above) and click on the ‘Open’ button.

The first thing you will be greeted with is a window asking if you trust the host that you’re trying to connect to.

In this case it is a pretty safe bet to click on the ‘Yes’ button to confirm that we know and trust the connection.

Once this is done, a new terminal window will be shown with a prompt to login as: . Here we can enter our user name (‘pi’) and then our password (if it’s still the default, the password is ‘raspberry’).

There you have it. A command line connection via SSH. Well done.

If this is the first time that you’ve done something like this it can be a very liberating feeling. To complete the feeling of freedom let’s set up a wireless network connection.

### Setting up a WiFi Network Connection

Our set-up of the Raspberry Pi will allow us to carry out all the (computer interface) interactions via a remote connection. However, the Raspberry Pi is currently making that remote connection via a fixed network cable. It could be argued that the lower number of connections that we need to run to our machine the better. The most obvious solution to this conundrum is to enable a wireless connection.

It should be noted that enabling a wireless network will not be a requirement for everyone, and as such, I would only recommend it if you need to. If you’re using a model B3, B3+ or Zero W you have WiFi built in, otherwise you will need to purchase a USB WiFi dongle and correctly configure it.

#### Built in WiFi Enabling

We need to edit the file wpa_supplicant.conf at /etc/wpa_supplicant/wpa_supplicant.conf. This looks like the following;

Use the nano command as follows;

We need to add the ssid (the wireless network name) and the password for the WiFi network here so that the file looks as follows (using your ssid and password of course);

##### Make the changes operative

To allow the changes to become operative we can type in;

Once we have rebooted, we can check the status of our network interfaces by typing in;

This will display the configuration for our wired Ethernet port, our ‘Local Loopback’ (which is a fancy way of saying a network connection for the machine that you’re using, that doesn’t require an actual network (ignore it in the mean time)) and the wlan0 connection which should look a little like this;

This would indicate that our wireless connection has been assigned the dynamic IP address 10.1.1.99.

We should be able to test our connection by connecting to the Pi via SSH and ‘PuTTY’ on the Windows desktop using the address 10.1.1.99.

In theory you are now the proud owner of a computer that can be operated entirely separate from all connections except power!

##### Make the built in WiFi IP address static

In the same way that we would edit the /etc/dhcpcd.conf file to set up a static IP address for our physical connection (eth0) we will now edit it with the command…

This time we will add the details for the wlan0 connection to the end of the file. Those details (assuming we will use the 10.1.1.17 IP address) should look like the following;

Our wireless lan (wlan0) is now designated to be a static IP address (with the details that we had previously assigned to our wired connection) and we have added the ‘ssid’ (the network name) of the network that we are going to connect to and the password for the network.

##### Make the changes operative

To allow the changes to become operative we can type in;

We’re done!

#### WiFi Via USB Dongle

Using an external USB WiFi dongle can be something of an exercise if not done right. In my own experience, I found that choosing the right wireless adapter was the key to making the job simple enough to be able to recommend it to new users. Not all WiFi adapters are well supported and if you are unfamiliar with the process of installing drivers or compiling code, then I would recommend that you opt for an adapter that is supported and will work ‘out of the box’. There is an excellent page on elinux.org which lists different adapters and their requirements. I eventually opted for the Edimax EW-7811Un which literally ‘just worked’ and I would recommend it to others for it’s ease of use and relatively low cost (approximately \$15 US).

To install the wireless adapter we should start with the Pi powered off and install it into a convenient USB connection. When we turn the power on we will see the normal range of messages scroll by, but if we’re observant we will note that there are a few additional lines concerning a USB device. These lines will most likely scroll past, but once the device has finished powering up and we have logged in we can type in…

… which will show us a range of messages about drivers that are loaded to support discovered hardware.

Somewhere in that list (hopefully towards the end) will be a series of messages that describe the USB connectors and what is connected to them. In particular we could see a group that looks a little like the following;

That is our USB adapter which is plugged into USB slot 2 (which is the ‘2’ in usb 1-1.2:). The manufacturer is listed as ‘Realtek’ as this is the manufacturer of the chip-set in the adapter that Edimax uses.

##### Editing files

We need to edit two files. The first is the file wpa_supplicant.conf at /etc/wpa_supplicant/wpa_supplicant.conf. This looks like the following;

Use the nano command as follows;

We need to add the ssid (the wireless network name) and the password for the WiFi network here so that the file looks as follows (using your ssid and password of course);

##### Make the changes operative

To allow the changes to become operative we can type in;

Once we have rebooted, we can check the status of our network interfaces by typing in;

This will display the configuration for our wired Ethernet port, our ‘Local Loopback’ (which is a fancy way of saying a network connection for the machine that you’re using, that doesn’t require an actual network (ignore it in the mean time)) and the wlan1 connection which should look a little like this;

This would indicate that our wireless connection has been assigned the dynamic IP address 10.1.1.97.

We should be able to test our connection by connecting to the Pi via SSH and ‘PuTTY’ on the Windows desktop using the address 10.1.1.97.

##### Make USB WiFi IP address static

In the same way that we would edit the /etc/dhcpcd.conf file to set up a static IP address for our physical connection (eth0) we will now edit it with the command…

This time we will add the details for the wlan1 connection to the end of the file. Those details (assuming we will use the 10.1.1.160 IP address) should look like the following;

##### Make the changes operative

To allow the changes to become operative we can type in;

We’re done!

### Reconnecting to the wireless network automatically

I have found with experience that in spite of my best intentions, sometimes when setting up a Raspberry Pi to maintain a WiFi connection, if it disconnects for whatever reason it may not reconnect automatically.

To solve this problem we’re going to write a short script that automatically reconnects our Pi to a WiFi network. The script will check to see if the Pi is connected to our local network and, if it’s off-line, will restart the wireless network interface. We’ll use a cron job to schedule the execution of this script at a regular interval.

#### Let’s write a script

First, we’ll need to check if the Pi is connected to the network. This is where we’ll try to ping an IP address on our local network (perhaps our gateway address?). If the ping command succeeds in getting a response from the IP address, we have network connectivity. If the command fails, we’ll turn off our wireless interface (wlan1) and then turn it back on (yes, the timeless solution of turning it off and on).

The script looks a little like this;

Use nano to create the script, name it something like wifistart.sh, and save it in /usr/local/bin.

We also need to make sure it’s executable by running chmod (using sudo) as follows;

#### Lets run our script on a regular schedule

To make our WiFi checking script run automatically, we’ll schedule a cron job using crontab;

… and add this line to the bottom:

This runs the script every 5 minutes with sudo permissions, writing its output to /dev/null so it doesn’t spam syslog.

#### Let’s test it

To test that the script works as expected, we will want to take down the wlan1 interface and wait for the script to bring it back up. Before taking down wlan1, we might want to adjust the interval in crontab to 1 minute. And fair warning, when we disconnect wlan1, we will lose that network interface, so we will need to either have a local keyboard / monitor connected, have another network interface set up or be really confident that we’ve got everything set up right first time.

To take down wlan1 to confirm the script works, run:

After waiting for 5 (or 1) minutes, we could try ssh-ing back into the Raspberry Pi or if we’re keen we could have a ping command running on another server checking the interface to show when it stops and when it (hopefully) starts again. Assuming everything works, our Pi should reconnect seamlessly.

## Setting up the Raspberry Pi Software

While the Raspberry Pi is a capable computer, we still need to install software on it to allow us to gather, store and present our data.

The software we will be using is based on the Linux Operating System. Because this is potentially unfamiliar territory (for those who haven’t used Linux or had some practical computing experience), we will take our time and explain things as we go.

### Web Server, PHP and Database

Because we want to be able to present the data we will be collecting, we need to set up a web server that will return measurements to other computers that will be browsing within the network (remembering that this is not intended to be connected to the Internet, just inside your home network). This type of connection is called a RESTful service.

At the same time as setting up a web server on the Pi we will install PHP. PHP is a scripting language that is widely used in developing web pages. And because we will want to store our data somewhere we will add in the SQLite database. SQLite is a self-contained database engine that reads and writes directly to ordinary disk files. A complete SQL database is contained in a single file which can be transferred between platforms for backup or restoration purposes. SQLite is touted as the most widely used database engine in the world.

#### Install NGINX and PHP

The web server that we will use is called NGINX (pronounced “Engine X”). NGINX is an open-source web server that is often recommended for its performance and low resource consumption. This obviously makes it ideal for hardware such as the Raspberry Pi. In spite of being targeted as something of a ‘light’ application, it is extremely powerful, capable and it is widely used in large scale applications.

We can start the install process using the following;

We’re familiar with apt-get already, but this time we’re including more than one package in the installation process. Specifically we’re including nginx and php-fpm.

‘nginx’ is obviously the name of the NGINX web server and php-fpm is for PHP.

The Raspberry Pi will advise you of the range of additional packages that will be installed at the same time (to support those we’re installing (these additional packages are ‘dependencies’)). Agree to continue and the installation will proceed. This should take a few minutes or more (depending on the speed of your Internet connection).

##### Configuration

Firstly we should edit the default file that will get displayed as a web page if none is specified. What we want to do is to include the option to redirect to an index.php file.:

Replace the line:

with

Now we want to edit the portion of the file that will handle all requests for resources that end in .php.

Replace the section of the file that looks like this;

With this;

NGINX has its default web page location at /var/www/html on Raspbian. We are going to change the permissions / ownership of that folder by running following two commands:

This is necessary to let the ‘pi’ user edit the files in that location easily.

Now lets create a suitable index.php test file with the following command:

Now lets restart the NGINX service so that the changes we have made can take effect:

Now we can go to to the IP address of our Pi in a browser (in the example we are using it’s 10.1.1.160) and type it in to the URL area to test. Something like the following should be displayed;

Marvellous.

#### Database

As mentioned earlier, we will use a SQLite database to store the information that we collect.

SQLite is incredibly easy to install.

And that’s it!

Because SQLite does not rely on a client-server relationship, applications that interact with the SQLite database read and write directly to the database file (or files). It therefore relies on the security of the permissions on the operating system to provide separation. This means that accessing the database from a separate computer is problematic, but it simplifies interaction for the user that is operating the database locally.

#### Create a database and a table

When we read data from our sensor, we will record it in a database. SQLite is a database program, but we still need to set up a database file that SQLite will read. In fact when we come to record and explore our data we will be dealing with a ‘table’ of data that will exist inside a database.

We will create a database called ‘measurements’ and in that database we will create a table called ‘distance’ That table will record regular distance measurements and the time that they were taken.

Creating our database and initiating interaction with it is done as follows;

Once the program is started we are presented with a prompt for the database;

From this prompt we can begin to provide commands and when we are finished we can exit from SQLite by typing in .quit. At any stage if we forget what command we should be running we can type in .help and the program will give us a list of commands.

We will create a table called ‘distance’ which will contain a date time group field called ‘dtg’.

For the distance value we will name the field ‘ultrasonic’ and we will use the ‘real’ type.

Enter each of the following lines at the sqlite> prompt. The semicolon represents the end of the command.

We can then confirm that the table exists using the command ‘.table’;

There we have it! A simple database and a table ready to go.

## Ultrasonic Distance Measurement

This project will measure distance using an ultrasonic sensor module. We will use the JSN-SR04T sensor since it utilises a single waterproof send/receive audio module that will be useful for my particular application, but an alternative sensor called the HC-SR04 is a drop in replacement that combines the electronic and audio modules. It also breaks out the send and receive components which allows for measuring closer distances down to 2cm.

### The JSN-SR04T Sensor

The JSN-SR04T is an ultrasonic ranging sensor that comprises an electronics module and a waterproof audio module connected by a long cable. It is designed to measure distance by sending out a high frequency pattern of pulses when triggered and to return a signal that corresponds to the length of time that it has taken for the pulses to travel out and be reflected back to the audio module.

#### JSN-SR04T Specifications

Specifications:

• Operating voltage: DC 5V
• Quiescent current: 5mA
• Total current work: 30mA
• Acoustic emission frequency: 40khz
• Farthest distance: 4.5m
• Shortest distance: 25cm
• Module size: 41mm x 28.5mm
• Angle: less than 50 degrees
• Working temperature: -10 to 70°C

Wiring:

• +5V (Power supply)
• Trig (Receives the trigger to operate)
• Echo (Signals time taken for audio send/receive trip)
• GND (Ground)

The device is touted as a drop-in replacement for the HC-SR04 which combines the electronics and audio modules and which separates the transmit and receive sensors.

### Measure

#### Hardware required

• 1 x JSN-SR04T sensor
• 1k Ohm resister
• 2k Ohm resister
• Jumper cables with Dupont connectors on the end
• Solder
• Heat-shrink

#### Connect

For the sake of keeping the connecting pins together on the Pi’s GPIO block we will be connecting pins 4,6,8 and 10 to the JSN-SR04T electronics module. The following diagram provides a high level overview of the complete chain.

• Pin 4 on the Pi connects directly to the +5V connector on the sensor
• Pin 6 on the Pi connects directly to the GND connector on the sensor, but is also connected to one side of the 2k Ohm resistor in the voltage divider.
• Pin 8 on the Pi connects to the centre of the voltage divider, then to the Echo Pin via the 1k resistor
• Pin 10 on the Pi connects to the Trig connector on the sensor.
• The echo connector on the sensor connects to one end of the 1k Ohm resistor

As we noted earlier, the Raspberry Pi provides general purpose digital input/output pins (called GPIO pins) that you can use for reading digital logic signals. In our case, we will want to apply the output signal from the ‘Echo’ pin to them. Unfortunately the output voltage from the Echo pin is 5V and the GPIO pins on the Raspberry Pi are only able to operate safely at voltage levels of 3.3V.

To safely apply the correct voltage to the Pi we can employ a voltage divider which divides the signal in a ratio corresponding to the resistance of two connected resistors. In the following diagram we can see that when a 5V signal is applied from the JSN-SR04T, 1.7V is dropped across the 1k Ohm resistor and 3.3V is across the 2k Ohm resistor.

#### Basic Operation

A pulse of at least 10us is applied to the trigger (Trig) pin of the electronics module.

This will automatically generate 8 audio pulses at 40kHz from the audio transmitter / receiver module.

The 8 pulses will reflect off a surface and be received back at the audio module.

The electronics module will generate a signal from the Echo pin that corresponds to the time between the audio pulses being transmitted and the reflections being received. If no reflected audio is received, a signal of time, t = 38ms will be generated.

The distance from the audio module to the reflecting surface can be found by multiplying the time that the echo pin was high (represented as 12ms in the diagram)by the speed of sound (340m/s) and dividing by 2 (since the sound travelled out and back). Therefore;

Our object is 2.04 metres away!

#### Test

The following python program can be saved onto our Pi as distance.py and run from the command line using;

This program is available as distance.py with the code samples that can be downloaded with the book from Leanpub.

When executed the sensor will settle and then start displaying the distance every two seconds.

Move the audio module about and confirm that it is operating as expected.

### Record

To record this data we will use a Python program that checks the sensor and writes the distance and the time into our database.

Our Python program will use cron to execute the program at a regular intervals (We used this earlier to automatically reconnect to the network if required.).

#### Code

The following Python code is a combination of our test code and one that simply writes the value for time and distance into our database.

The full code can be found in the code samples bundled with this book (distance-record.py).

This script can be saved in our home directory (/home/pi) and can be run by typing;

The output should look something like this;

Run this script a few times and then we can check the results in our database by starting up SQLite as follows;

From the SQLite prompt we can query the database to return all the records using SELECT * FROM distance; as follows;

There are three records, including the times they were taken and the distances recorded.

#### Recording data on a regular basis with cron

As mentioned earlier, while our code is a thing of beauty, it only records a single entry for the distance every time it is run.

What we need to implement is a schedule so that at a regular time, the program is run. This is achieved using cron via the crontab.

To set up our schedule we need to edit the crontab file. This is is done using the following command;

Once run it will open the crontab in the nano editor. We want to add in an entry at the end of the file that looks like the following;

This instructs the computer that at 0 minutes, every hour of every day of every month we run the command /usr/bin/python /home/pi/distance-record.py (which, if we were at the command line in the pi home directory, we would run as python distance-record.py, but since we can’t guarantee where we will be when running the script, we are supplying the full path to the python command and the distance-record.py script.

Save the file and when the next hour rolls over our program will run on its designated schedule and we will have distance entries written to our database every hour. After a while we can check it out by running a SELECT * FROM distance; on the measurement database.

#### Managing database size

While it’s a great idea to save our local data into a database, we stand the risk of gradually letting that database fill up until it exceeds the capacity of our storage.

In the case of the measurements that we are carrying out, this will only really happen after a long time. Capturing some simple measurements every hour means in the scheme of things that’s about 9000 recordings per year.

However, let’s not let a triviality like practical matters get in the way of writing some more code to address a theoretical problem.

What we’re looking for is a script that will run on a repeating schedule and remove old records. Sound familiar? That’s a very similar process to what we are doing when we record our data. A python script that is executed regularly by cron.

Here’s how we can do it.

The following python script (which we can name db-manage.py) opens our database, deletes any records older than two years, cleans up and exits.

The file is available as db-manage.py and can be found in the code sample extras that can be downloaded with this book.

It’s a pretty simple script and we can schedule its operation by editing the crontab file like so;

We want to add in an entry at the end of the file that looks like the following;

This instructs the computer that at 1 minute past the hour at midnight (hence the 0) on the 1st day of every month we run the command /usr/bin/python /home/pi/db-manage.py (which, if we were at the command line in the pi home directory, we would run as python db-manage.py, but since we can’t guarantee where we will be when running the script, we are supplying the full path to the python command and the db-manage.py script.

Save the file and every month our program will run on its designated schedule and will make sure to delete any records older than two years.

### Explore

#### Simple data point API

The main mechanism for exploring and using our data is going to be via a simple data block returned from a http request.

What does all that actually mean?

That’s a good question. Ultimately we’re measuring something and we want to be able to communicate that measurement to an external service. That service could be another database somewhere or to a web page or to a system that will alert based on the value of the distance being within certain boundaries.

The very simplest way that we can do this is to present the data as the measured values when we ask for them in a web request. This could be thought of as a simplified form of an API (and I plan to make something more complicated in the future).

The data will be presented as JSON as that is one of the most ubiquitous data forms around.

Enough esoterics, what does the magic code look like that will do this?

We can save this file as distance.php and have it in the /var/www/html directory on our Pi (distance.php can be found in the code sample extras that can be downloaded with this book).

Now we can put in the IP address of our Pi to our browser along with our distance php file (http://10.1.1.160/distance.php) and we should get something like the following appear in the browser;

What good will getting this data be? Well……. I’m a bit of a believer that the information that gets captured by the Pi shouldn’t ultimately reside on the device in the long term. In the perfect world I would see it being requested by an external service that was checking a range of data points that would exist around the home (temperature inside / outside, CO2 levels, is the car parked in the garage, that sort of thing) so this is more of an enabling device than a ‘let’s display stuff’ deal. But I hear what you’re saying. “That’s lame. How can I impress people with that?”. Fair point. To deal with that problem let’s make a simple graph.

#### Extracting a Range of Data

Righto… If we’re going to make a graph of our distances we’ll need a variation of our API that will gather and present a range of data that our graph can then display.

This will form a piece of code that our graph will use as a JSON formatted data source.

It will look as follows;

This block of PHP code will connect to our database and instead of returning a single piece of data it will return (‘echo’) a range of values from the past 14 days. We’ll call the file distance-range.php and it will be in the /var/www/html directory. A copy of the file can be found in the code sample extras that can be downloaded with this book.

The following file is our graph which will use our distance-range.php file and display it. It uses the d3.js visualisation library and for a full description of the workings of the code please feel free to consult a copy of ‘D3 Tips and Tricks v4.x’. It’s free and can be downloaded from here.

We will want to place a copy of this file which we will call distance-graph.html in the /var/www/html directory. A copy of it can be found in the code sample extras that can be downloaded with this book.

We can see the end result by putting the web address into our browser. It should look something like http://10.1.1.160/distance-graph.html. The end result should look a bit like the following;

### Wrap Up

There we have it.

We’ve assembled our Raspberry Pi with an ultrasonic sensor, installed an operating system and configured it for use. We’ve set up networking and installed a database and a web server. We’ve written code to record data into our database and an API to pull data out of it. We’ve even installed a graph to display it in a visual form. Nice work.

There is a strong possibility that the information I have laid out here could be rife with evil practices and gross inaccuracies.

But look on the bright side. Irrespective of the nastiness of the way that any of it was accomplished or the inelegance of the code, if the picture drawn on the screen is relatively pretty, you can walk away with a smile. :-)

Those with a smattering of knowledge of any of the topics I have butchered above (or below) are fully justified in feeling a large degree of righteous indignation. To those I say, please feel free to amend where practical and possible, but please bear in mind this was written from the point of view of someone with only a little experience in the topic and therefore try to keep any instructions at a level where a new entrant can step in.

## Linux Concepts

### What is Linux?

In it’s simplest form, the answer to the question “What is Linux?” is that it’s a computer operating system. As such it is the software that forms a base that allows applications that run on that operating system to run.

In the strictest way of speaking, the term ‘Linux’ refers to the Linux kernel. That is to say the central core of the operating system, but the term is often used to describe the set of programs, tools, and services that are bundled together with the Linux kernel to provide a fully functional operating system.

An operating system is software that manages computer hardware and software resources for computer applications. For example Microsoft Windows could be the operating system that will allow the browser application Firefox to run on our desktop computer.

Linux is a computer operating system that is can be distributed as free and open-source software. The defining component of Linux is the Linux kernel, an operating system kernel first released on 5 October 1991 by Linus Torvalds.

Linux was originally developed as a free operating system for Intel x86-based personal computers. It has since been made available to a huge range of computer hardware platforms and is a leading operating system on servers, mainframe computers and supercomputers. Linux also runs on embedded systems, which are devices whose operating system is typically built into the firmware and is highly tailored to the system; this includes mobile phones, tablet computers, network routers, facility automation controls, televisions and video game consoles. Android, the most widely used operating system for tablets and smart-phones, is built on top of the Linux kernel.

The development of Linux is one of the most prominent examples of free and open-source software collaboration. Typically, Linux is packaged in a form known as a Linux distribution, for both desktop and server use. Popular mainstream Linux distributions include Debian, Ubuntu and the commercial Red Hat Enterprise Linux. Linux distributions include the Linux kernel, supporting utilities and libraries and usually a large amount of application software to carry out the distribution’s intended use.

A distribution intended to run as a server may omit all graphical desktop environments from the standard install, and instead include other software to set up and operate a solution stack such as LAMP (Linux, Apache, MySQL and PHP). Because Linux is freely re-distributable, anyone may create a distribution for any intended use.

Linux is not an operating system that people will typically use on their desktop computers at home and as such, regular computer users can find the barrier to entry for using Linux high. This is made easier through the use of Graphical User Interfaces that are included with many Linux distributions, but these graphical overlays are something of a shim to the underlying workings of the computer. There is a greater degree of control and flexibility to be gained by working with Linux at what is called the ‘Command Line’ (or CLI), and the booming field of educational computer elements such as the Raspberry Pi have provided access to a new world of learning opportunities at this more fundamental level.

### Linux Directory Structure

To a new user of Linux, the file structure may feel like something at best arcane and in some cases arbitrary. Of course this isn’t entirely the case and in spite of some distribution specific differences, there is a fairly well laid out hierarchy of directories and files with a good reason for being where they are.

We are frequently comfortable with the concept of navigating this structure using a graphical interface similar to that shown below, but to operate effectively at the command line we need to have a working knowledge of what goes where.

The directories we are going to describe form a hierarchy similar to the following;

For a concise description of the directory functions check out the cheat sheet. Alternatively their function and descriptions are as follows;

#### /

The / or ‘root’ directory contains all other files and directories. It is important to note that this is not the root users home directory (although it used to be many years ago). The root user’s home directory is /root. Only the root user has write privileges for this directory.

#### /bin

The /bin directory contains common essential binary executables / commands for use by all users. For example: the commands cd, cp, ls and ping. These are commands that may be used by both the system administrator and by users, but which are required when no other filesystems are mounted.

#### /boot

The /boot directory contains the files needed to successfully start the computer during the boot process. As such the /boot directory contains information that is accessed before the Linux kernel begins running the programs and process that allow the operating system to function.

#### /dev

The /dev directory holds device files that represent physical devices attached to the computer such as hard drives, sound devices and communication ports as well as ‘logical’ devices such as a random number generator and /dev/null which will essentially discard any information sent to it. This directory holds a range of files that strongly reinforces the Linux precept that Everything is a file.

#### /etc

The /etc directory contains configuration files that control the operation of programs. It also contains scripts used to startup and shutdown individual programs.

#### /etc/cron.d

The /etc/cron.d, /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly directories contain scripts which are executed on a regular schedule by the crontab process.

#### /etc/rc?.d

The /rc0.d, /rc1.d, /rc2.d, /rc3.d, /rc4.d, /rc5.d, /rc6.d, /rcS.d directories contain the files required to control system services and configure the mode of operation (runlevel) for the computer.

#### /home

Because Linux is an operating system that is a ‘multi-user’ environment, each user requires a space to store information specific to them. This is done via the /home directory. For example, the user ‘pi’ would have /home/pi as their home directory.

#### /lib

The /lib directory contains shared library files that supports the executable files located under /bin and /sbin. It also holds the kernel modules (drivers) responsible for giving Linux a great deal of versatility to add or remove functionality as needs dictate.

#### /lost+found

The /lost+found directory will contain potentially recoverable data that might be produced if the file system undergoes an improper shut-down due to a crash or power failure. The data recovered is unlikely to be complete or undamaged, but in some circumstances it may hold useful information or pointers to the reason for the improper shut-down.

#### /media

The /media directory is used as a directory to temporarily mount removable devices (for example, /media/cdrom or /media/cdrecorder). This is a relatively new development for Linux and comes as a result of a degree of historical confusion over where was best to mount these types of devices (/cdrom, /mnt or /mnt/cdrom for example).

#### /mnt

The /mnt directory is used as a generic mount point for filesystems or devices. Recent use of the directory is directing it towards it being used as a temporary mount point for system administrators, but there is a degree of historical variation that has resulted in different distributions doing things different ways (for example, Debian allocates /floppy and /cdrom as mount points while Redhat places them in /mnt/floppy and /mnt/cdrom respectively).

#### /opt

The /opt directory is used for the installation of third party or additional optional software that is not part of the default installation. Any applications installed in this area should be installed in such a way that it conforms to a reasonable structure and should not install files outside the /opt directory.

#### /proc

The /proc directory holds files that contain information about running processes and system resources. It can be described as a pseudo filesystem in the sense that it contains runtime system information, but not ‘real’ files in the normal sense of the word. For example the /proc/cpuinfo file which contains information about the computers cpus is listed as 0 bytes in length and yet if it is listed it will produce a description of the cpus in use.

#### /root

The /root directory is the home directory of the System Administrator, or the ‘root’ user. This could be viewed as slightly confusing as all other users home directories are in the /home directory and there is already a directory referred to as the ‘root’ directory (/). However, rest assured that there is good reason for doing this (sometimes the /home directory could be mounted on a separate file system that has to be accessed as a remote share).

#### /sbin

The /sbin directory is similar to the /bin directory in the sense that it holds binary executables / commands, but the ones in /sbin are essential to the working of the operating system and are identified as being those that the system administrator would use in maintaining the system. Examples of these commands are fdisk, shutdown, ifconfig and modprobe.

#### /srv

The /srv directory is set aside to provide a location for storing data for specific services. The rationale behind using this directory is that processes or services which require a single location and directory hierarchy for data and scripts can have a consistent placement across systems.

#### /tmp

The /tmp directory is set aside as a location where programs or users that require a temporary location for storing files or data can do so on the understanding that when a system is rebooted or shut down, this location is cleared and the contents deleted.

#### /usr

The /usr directory serves as a directory where user programs and data are stored and shared. This potential wide range of files and information can make the /usr directory fairly large and complex, so it contains several subdirectories that mirror those in the root (/) directory to make organisation more consistent.

#### /usr/bin

The /usr/bin directory contains binary executable files for users. The distinction between /bin and /usr/bin is that /bin contains the essential commands required to operate the system even if no other file system is mounted and /usr/bin contains the programs that users will require to do normal tasks. For example; awk, curl, php, python. If you can’t find a user binary under /bin, look under /usr/bin.

#### /usr/lib

The /usr/lib directory is the equivalent of the /lib directory in that it contains shared library files that supports the executable files for users located under /usr/bin and /usr/sbin.

#### /usr/local

The /usr/local directory contains users programs that are installed locally from source code. It is placed here specifically to avoid being inadvertently overwritten if the system software is upgraded.

#### /usr/sbin

The /usr/sbin directory contains non-essential binary executables which are used by the system administrator. For example cron and useradd. If you can’t locate a system binary in /usr/sbin, try /sbin.

#### /var

The /var directory contains variable data files. These are files that are expected to grow under normal circumstances For example, log files or spool directories for printer queues.

#### /var/lib

The /var/lib directory holds dynamic state information that programs typically modify while they run. This can be used to preserve the state of an application between reboots or even to share state information between different instances of the same application.

#### /var/log

The /var/log directory holds log files from a range of programs and services. Files in /var/log can often grow quite large and care should be taken to ensure that the size of the directory is managed appropriately. This can be done with the logrotate program.

#### /var/spool

The /var/spool directory contains what are called ‘spool’ files that contain data stored for later processing. For example, printers which will queue print jobs in a spool file for eventual printing and then deletion when the resource (the printer) becomes available.

#### /var/tmp

The /var/tmp directory is a temporary store for data that needs to be held between reboots (unlike /tmp).

### Everything is a file in Linux

A phrase that will often come up in Linux conversation is that;

Everything is a file

For someone new to Linux this sounds like some sort of ‘in joke’ that is designed to scare off the unwary and it can sometimes act as a barrier to a deeper understanding of the philosophy behind the approach taken in developing Linux.

The explanation behind the statement is that Linux is designed to be a system built of a group of interacting parts and the way that those parts can work together is to communicate using a common method. That method is to use a file as a common building block and the data in a file as the communications mechanism.

The trick to understanding what ‘Everything is a file’ means, is to broaden our understanding of what a file can be.

The traditional concept of a file is an object with a specific name in a specific location with a particular content. For example, we might have a file named foo.txt which is in the directory /home/pi/ and it could contain a couple of lines of text similar to the following;

#### Directories

As unusual as it sounds a directory is also a file. The special aspect of a directory is that is is a file which contains a list of information about which files (and / or subdirectories) it contains. So when we want to list the contents of a directory using the ls command what is actually happening is that the operating system is getting the appropriate information from the file that represents the directory.

#### System Information

However, files can also be conduits of information. The /proc/ directory contains files that represent system and process information. If we want to determine information about the type of CPU that the computer is using, the file cpuinfo in the /proc/ directory can list it. By running the command cat /proc/cpuinfo’ we can list a wealth of information about our CPU (the following is a subset of that information by the way);

Now that might not mean a lot to us at this stage, but if we were writing a program that needed a particular type of CPU in order to run successfully it could check this file to ensure that it could operate successfully. There are a wide range of files in the /proc/ directory that represent a great deal of information about how our system is operating.

#### Devices

When we use different devices in a Linux operating system these are also represented as a file. In the /dev/ directory we have files that represent a range of physical devices that are part of our computer. In larger computer systems with multiple disks they could be represented as /dev/sda1 and /dev/sda2, so that when we wanted to perform an action such as formatting a drive we would use the command mkfs on the /dev/sda1 file.

The /dev/ directory also holds some curious files that are used as tools for generating or managing data. For example /dev/random is an interface to the kernels random number device. /dev/zero represents a file that will constantly stream zeros (while this might sound weird, imagine a situation where you want to write over an area of disk with data to erase it). The most well known of these unusual files is probably /dev/null. This will act as a ‘null device’ that will essentially discard any information sent to it.

## File Editing

Working in Linux is an exercise in understanding the concepts that Linux uses as its foundations such as ‘Everything is a file’ and the use of wildcards, pipes and the directory structure.

While working at the command line there will very quickly come the realisation that there is a need to know how to edit a file. Linux being what it is, there are many ways that files can be edited.

An outstanding illustration of this is via the excellent cartoon work of the xkcd comic strip (Buy his stuff, it’s awesome!).

For a taste of the possible options available Wikipedia has got our back. Inevitably where there is choice there are preferences and where there are preferences there is bias. Everyone will have a preference towards a particular editor and don’t let a particular bias influence you to go down a particular direction without considering your options. Speaking from personal experience I was encouraged to use ‘vi’ as it represented the preference of the group I was in, but because I was a late starter to the command line I struggled for the longest time to try and become familiar with it. I know I should have tried harder, but I failed. For a while I wandered in the editor wilderness trying desperately to cling to the GUI where I could use ‘gedit’ or ‘geany’ and then one day I was introduced to ‘nano’.

This has become my preference and I am therefore biased towards it. Don’t take my word for it. Try alternatives. I’ll describe ‘nano’ below, but take that as a possible path and realise that whatever editor works for you will be the right one. The trick is simply to find one that works for you.

### The nano Editor

The nano editor can be started from the command line using just the command and the /path/name of the file.

If the file requires administrator permissions it can be executed with ‘sudo.

When it opens it presents us with a working space and part of the file and some common shortcuts for use at the bottom of the console;

It includes some simple syntax highlighting for common file formats;

This can be improved if desired (cue Google).

There is a swag of shortcuts to make editing easier, but the simple ones are as follows;

• CTRL-x - Exit the editor. If we are in the middle of editing a file we will be asked if we want to save our work
• CTRL-r - Read a file into our current working file. This enables us to add text from another file while working from within a new file.
• CTRL-k - Cut text.
• CTRL-u - Uncut (or Paste) text.
• CTRL-o - Save file name and continue working.
• CTRL-t - Check the spelling of our text.
• CTRL-w - Search the text.
• CTRL-a - Go to the beginning of the current working line.
• CTRL-e - Go to the end of the current working line.
• CTRL-g - Get help with nano.

## Linux Commands

### Executing Commands in Linux

A command is an instruction given by a user telling the computer to carry out an action. This could be to run a single program or a group of linked programs. Commands are typically initiated by typing them in at the command line (in a terminal) and then pressing the ENTER key, which passes them to the shell.

A terminal refers to a wrapper program which runs a shell. This used to mean a physical device consisting of little more than a monitor and keyboard. As Unix/Linux systems advanced the terminal concept was abstracted into software. Now we have programs such as LXTerminal (on the Raspberry Pi) which will launch a window in a Graphical User Interface (GUI) which will run a shell into which you can enter commands. Alternatively we can dispense with the GUI all together and simply start at the command line when we boot up.

The shell is a program which actually processes commands and returns output. Every Linux operating system has at least one shell, and most have several. The default shell on most Linux systems is bash.

### The Commands

Commands on Linux operating systems are either built-in or external commands. Built-in commands are part of the shell. External commands are either executables (programs written in a programming language and then compiled into an executable binary) or shell scripts.

A command consists of a command name usually followed by one or more sequences of characters that include options and/or arguments. Each of these strings is separated by white space. The general syntax for commands is;

commandname [options] [arguments]

The square brackets indicate that the enclosed items are optional. Commands typically have a few options and utilise arguments. However, there are some commands that do not accept arguments, and a few with no options. As an example we can run the ls command with no options or arguments as follows;

The ls command will list the contents of a directory and in this case the command and the output would be expected to look something like the following;

##### Options

An option (also referred to as a switch or a flag) is a single-letter code, or sometimes a single word or set of words, that modifies the behaviour of a command. When multiple single-letter options are used, all the letters are placed adjacent to each other (not separated by spaces) and can be in any order. The set of options must usually be preceded by a single hyphen, again with no intervening space.

So again using ls if we introduce the option -l we can show the total files in the directory and subdirectories, the names of the files in the current directory, their permissions, the number of subdirectories in directories listed, the size of the file, and the date of last modification.

The command we execute therefore looks like this;

And so the command (with the -l option) and the output would look like the following;

Here we can see quite a radical change in the formatting and content of the returned information.

##### Arguments

An argument (also called a command line argument) is a file name or other data that is provided to a command in order for the command to use it as an input.

Using ls again we can specify that we wish to list the contents of the python_games directory (which we could see when we ran ls) by using the name of the directory as the argument as follows;

The command (with the python_games argument) and the output would look like the following (actually I removed quite a few files to make it a bit more readable);

##### Putting it all together

And as our final example we can combine our command (ls) with both an option (-l) and an argument (python_games) as follows;

Hopefully by this stage, the output shouldn’t come as too much surprise, although again I have pruned some of the files for readabilities sake;

#### apt-get

The apt-get command is a program, that is used with Debian based Linux distributions to install, remove or upgrade software packages. It’s a vital tool for installing and managing software and should be used on a regular basis to ensure that software is up to date and security patching requirements are met.

There are a plethora of uses for apt-get, but we will consider the basics that will allow us to get by. These will include;

• Updating the database of available applications (apt-get update)
• Upgrading the applications on the system (apt-get upgrade)
• Installing an application (apt-get install *package-name*)
• Un-installing an application (apt-get remove *package-name*)
##### The apt-get command

The apt part of apt-get stands for ‘advanced packaging tool’. The program is a process for managing software packages installed on Linux machines, or more specifically Debian based Linux machines (Since those based on ‘redhat’ typically use their rpm (red hat package management (or more lately the recursively named ‘rpm package management’) system). As Raspbian is based on Debian, so the examples we will be using are based on apt-get.

APT simplifies the process of managing software on Unix-like computer systems by automating the retrieval, configuration and installation of software packages. This was historically a process best described as ‘dependency hell’ where the requirements for different packages could mean a manual installation of a simple software application could lead a user into a sink-hole of despair.

In common apt-get usage we will be prefixing the command with sudo to give ourselves the appropriate permissions;

##### apt-get update

This will resynchronize our local list of packages files, updating information about new and recently changed packages. If an apt-get upgrade (see below) is planned, an apt-get update should always be performed first.

Once the command is executed, the computer will delve into the internet to source the lists of current packages and download them so that we will see a list of software sources similar to the following appear;

##### apt-get upgrade

The apt-get upgrade command will install the newest versions of all packages currently installed on the system. If a package is currently installed and a new version is available, it will be retrieved and upgraded. Any new versions of current packages that cannot be upgraded without changing the install status of another package will be left as they are.

As mentioned above, an apt-get update should always be performed first so that apt-get upgrade knows which new versions of packages are available.

Once the command is executed, the computer will consider its installed applications against the databases list of the most up to date packages and it will prompt us with a message that will let us know how many packages are available for upgrade, how much data will need to be downloaded and what impact this will have on our local storage. At this point we get to decide whether or not we want to continue;

Once we say yes (‘Y’) the upgrade kicks off and we will see a list of the packages as they are downloaded unpacked and installed (what follows is an edited example);

There can often be alerts as the process identifies different issues that it thinks the system might strike (different aliases, runtime levels or missing fully qualified domain names). This is not necessarily a sign of problems so much as an indication that the process had to take certain configurations into account when upgrading and these are worth noting. Whenever there is any doubt about what has occurred, Google will be your friend :-).

##### apt-get install

The apt-get install command installs or upgrades one (or more) packages. All additional (dependency) packages required will also be retrieved and installed.

If we want to install multiple packages we can simply list each package separated by a space after the command as follows;

##### apt-get remove

The apt-get remove command removes one (or more) packages.

#### cat

The cat command is a really versatile command that is typically used to carry out three different functions. It can display a file on the screen, combine different files together (concatenate them) or create new files. This is another core command that is immensely useful to learn in when working with Linux from the command line. It’s simple, flexible and versatile.

• cat [options] filename filename : Display, combine or create new files.

For example: To display the file foo.txt on the screen we would use;

To display the files foo.txt and bar.txt on the screen one after the other we would use;

Or to combine the files foo.txt and bar.txt into a new file called foobar.txt using the redirection symbol >;

##### The cat command

The cat command is a vital tool to use on the Linux command line because ultimately Linux is an operating system that is file driven. Without a graphical user interface there needs to be a mechanism whereby creating and manipulating text files can be accomplished easily. The cat command is one of the commands that makes this possible. The name ‘cat’ is short for ‘catenate’ or ‘concatenate’ (either appears to be acceptable, but ‘concatenate’ appears to be more widely used), which is to say to connect things in a series. This is certainly one of it’s common uses, but a better overview would be to say that the cat command is used to;

• Display text files at the command line
• Join one text file to the end of another text file, combining them
• Copy text files into a new document
##### Options

The only option that gets any degree of use with cat is the -n option that numbers the output lines.

##### Arguments and Examples

To display text

For example, to display a text file (foo.txt) on the screen we can use the following command;

The output would be;

As we can see, the contents of the file ‘foo.txt’ is sent to the screen (be aware, if the file is sufficiently large, it will simple dump the contents in a long scrolling waterfall of text).

To join more than one file together

We could just as easily display two files one after the other (concatenated) as follows;

The output would be;

To create a new file

Instead of having the file sent to the screen, we can specify that cat send our file to a new (renamed) file as follows;

This could be thought of an a equivalent to a file copy action and uses the redirection symbol >.

Taking the process one step further we can take our original two files and combine them into a single file with;

We can then check the results of our combination using cat on the new file as follows;

And the output would be;

Then we could use cat to append a file to an already existing file by using the redirection operator >>;

Here we use the redirection operator >> to add the contents of the file newfoo.txt to the already existing file foobar.txt.

The resulting file content would be;

Finally, we can use cat to create a file from scratch. In this scenario if we use cat without a source file and redirect to a new file (here called newfile.txt. It will take the input from the command line to add to the file until CONTROL-d is pressed.

The resulting file content would be;

##### Test yourself
1. Which is the safest redirector to use?
2. Create a new file using cat and enter a few lines of text to give it some content
3. Copy that file using cat to a new file.
4. Combine the original file and the copy into a new file.
5. Display that new file on the screen.

#### cd

The cd command is used to move around in the directory structure of the file system (change directory). It is one of the fundamental commands for navigating the Linux directory structure.

cd [options] directory : Used to change the current directory.

For example, when we first log into the Raspberry Pi as the ‘pi’ user we will find ourselves in the /home/pi directory. If we wanted to change into the /home directory (go up a level) we could use the command;

Take some time to get familiar with the concept of moving around the directory structure from the command line as it is an important skill to establish early in Linux.

##### The cd command

The cd command will be one of the first commands that someone starting with Linux will use. It is used to move around in the directory structure of the file system (hence cd = change directory). It only has two options and these are seldom used. The arguments consist of pointing to the directory that we want to go to and these can be absolute or relative paths.

The cd command can be used without options or arguments. In this case it returns us to our home directory as specified in the /etc/passwd file.

If we cd into any random directory (try cd /var) we can then run cd by itself;

… and in the case of a vanilla installation of Raspbian, we will change to the /home/pi directory;

In the example above, we changed to /var and then ran the cd command by itself and then we ran the pwd command which showed us that the present working directory is /home/pi. This is the Raspbian default home directory for the pi user.

##### Options

As mentioned, there are only two options available to use with the cd command. This is -P which instructs cd to use the physical directory structure instead of following symbolic links and the -L option which forces symbolic links to be followed.

For those beginning Linux, there is little likelihood of using either of these two options in the immediate future and I suggest that you use your valuable memory to remember other Linux stuff.

##### Arguments

As mentioned earlier, the default argument (if none is included) is to return to the users home directory as specified in the /etc/passwd file.

When specifying a directory we can do this by absolute or relative addressing. So if we started in the /home/pi directory, we could go the /home directory by executing;

… or using relative addressing and we can use the .. symbols to designate the parent directory;

Once in the /home directory, we can change into the /home/pi/Desktop directory using relative addressing as follows;

We can also use the - argument to navigate to the previous directory we were in.

##### Examples

Change into the root (/) directory;

##### Test yourself
1. Having just changed from the /home/pi directory to the /home directory, what are the five variations of using the cd command that will take the pi user to the /home/pi directory
2. Starting in the /home/pi directory and using only relative addressing, use cd to change into the /var directory.

#### chmod

The chmod command allows us to set or modify a file’s permissions. Because Linux is built as a multi-user system there are typically multiple different users with differing permissions for which files they can read / write or execute. chmod allows us to limit access to authorised users to do things like editing web files while general users can only read the files.

• chmod [options] mode files : Change access permissions of one or more files & directories

For example, the following command (which would most likely be prefixed with sudo) sets the permissions for the /var/www directory so that the user can read from, write to and change into the directory. Group owners can also read from, write to and change into the directory. All others can read from and change into the directory, but they cannot create or delete a file within it;

This might allow normal users to browse web pages on a server, but prevent them from editing those pages (which is probably a good thing).

##### The chmod command

The chmod command allows us to change the permissions for which user is allowed to do what (read, write or execute) to files and directories. It does this by changing the ‘mode’ (hence chmod = change file mode) of the file where we can make the assumption that ‘mode’ = permissions.

Every file on the computer has an associated set of permissions. Permissions tell the operating system what can be done with that file and by whom. There are three things you can (or can’t) do with a given file:

• write (modify) it and
• execute it.

Linux permissions specify what the owning user can do, what the members of the owning group can do and what other users can do with the file. For any given user, we need three bits to specify access permissions: the first to denote read (r) access, the second to denote (w) access and the third to denote execute (x) access.

We also have three levels of ownership: ‘user’, ‘group’ and ‘others’ so we need a triplet (three sets of three) for each, resulting in nine bits.

The following diagram shows how this grouping of permissions can be represented on a Linux system where the user, group and others had full read, write and execute permissions;

If we had a file with more complex permissions where the user could read, write and execute, the group could read and write, but all other users could only read it would look as follows;

This description of permissions is workable, but we will need to be aware that the permissions are also represented as 3 bit values (where each bit is a ‘1’ or a ‘0’ (where a ‘1’ is yes you can, or ‘0’ is no you can’t)) or as the equivalent octal value.

The full range of possible values for these permission combinations is as follows;

Another interesting thing to note is that permissions take a different slant for directories.

• read determines if a user can view the directory’s contents, i.e. execute ls in it.
• write determines if a user can create new files or delete file in the directory. (Note here that this essentially means that a user with write access to a directory can delete files in the directory even if he/she doesn’t have write permissions for the file! So be careful.)
• execute determines if the user can cd into the directory.

We can check the check the permissions of files using the ls -l command which will list files in a long format as follows;

This command will list the details of the file foo.txt that is in the /tmp directory as follows

The permissions on the file, the user and the group owner can be found as follows;

From this information we can see that the file’s user (‘pi’) has permissions to read, write and execute the file. The group owner (‘pi-group’) can read and write to the file and all other users can read the file.

##### Options

The main option that is worth remembering is the -R option that will Recursively apply permissions on the files in the specified directory and its sub-directories.

The following command will change the permissions for all the files in the /srv/foo directory and in all the directories that are under it;

##### Arguments

Simplistically (in other words it can be more complicated, but we’re simplifying it) there are two main ways that chmod is used. In either symbolic mode where the permissions are changed using symbols associated with read, write and execute as well as symbols for the user (u), the group owner (g), others (o) and all users (a). Or in numeric mode where we use the octal values for permission combinations.

Symbolic Mode

In symbolic mode we can change the permissions of a file with the following syntax:

• chmod [who][op][permissions] filename

Where who can be the user (u), the group owner (g) and / or others (o). The operator (op) is either + to add a permission, - to remove a permission or = to explicitly set permissions. The permissions themselves are either readable (r), writeable (w), or executable (x).

For example the following command adds executable permissions (x) to the user (u) for the file /tmp/foo.txt;

This command removes writing (w) and executing (x) permissions from the group owner (g) and all others (o) for the same file;

Note that removing the execute permission from a directory will prevent you from being able to list its contents (although root will override this). If you accidentally remove the execute permission from a directory, you can use the +X argument to instruct chmod to only apply the execute permission to directories.

Numeric Mode

In numeric mode we can explicitly state the permissions using the octal values, so this form of the command is fairly common.

For example, the following command will change the permissions on the file foo.txt so that the user can read, write and execute it, the group owner can read and write it and all others can read it;

##### Examples

To change the permissions in your home directory to remove reading and executing permissions from the group owner and all other users;

To make a script executable by the user;

Windows marks all files as executable by default. If you copy a file or directory from a Windows system (or even a Windows-formatted disk) to your Linux system, you should ideally strip the unnecessary execute permissions from all copied files unless you specifically need to retain it. Note of course we still need it on all //directories// so that we can access their contents! Here’s how we can achieve this in one command:

This instructs chmod to remove the execute permission for each file and directory, and then immediately set execute again if working on a directory.

#### crontab

The crontab command give the user the ability to schedule tasks to be run at a specific time or with a specific interval. If you want to move beyond using Linux from a graphical user interface, you will most likely want to schedule a task to run at a particular time or interval. Even just learning about it might give you ideas of what you might do.

• crontab [-u user] [-l | -r | -e] : Schedule a task to run at a particular time or interval

For example, you could schedule a script to run every day to carry our a backup process in the middle of the night. or capture some data every hour to store in a database.

##### The crontab command

The command crontab is a concatenation of ‘cron table’ because it uses the job scheduler cron to execute tasks which are stored in a ‘table’ of sorts in the users crontab file. cron is named after ‘Khronos’, the Greek personification of time.

While each user who sets up a job to run using the crontab creates a crontab file, the file is not intended to be edited by hand. It is in different locations in different flavour of Linux distributions and the most reliable mechanism for editing it is by running the crontab -e command. Each user has their own crontab file and the root user can edit another users crontab file. This would be the situation where we would use the -u option, but honestly once we get to that stage it can probably be assumed that we know a fair bit about Linux.

There are only three main options that are used with crontab.

##### Options

The first option that we should examine is the -l option which allows us to list the crontab file;

Once run it will list the contents of the crontab file directly to the screen. The output will look something like;

Here we can see that the main part of the file (in fact everything except the final line) is comments that explain how to include an entry into the crontab file.

The entry in this case is specified to run every 10 minutes and when it does, it will run the PHP script scrape-books.php (we’ll explain how this is encoded later in the examples section).

If we want to remove the current crontab we can use the -r option. Probably not something that we would do an a regular basis, as it would be more likely to be editing the content rather than just removing it wholesale.

Lastly there is the option to edit the crontab file which is initiated using -e. This is the main option that would be used and the one we will cover in detail in the examples below.

##### Examples

As an example, consider that we wish to run a Python script every day at 6am. The following command will let us edit the crontab;

Once run it will open the crontab in the default editor on your system (most likely ‘vi’, ‘vim’ or ‘nano’). The file will look as follows;

As stated earlier, the default file obviously includes some explanation of how to format an entry in the crontab. In our case we wish to add in an entry that tells the script to start at 6 hours and 0 minutes each day. The crontab accepts six pieces of information that will allow that action to be performed. each of those pieces is separated by a space.

1. A number (or range of numbers), m, that represents the minute of the hour (valid values 0-59);
2. A number (or range of numbers), h, that represents the hour of the day (valid values 0-23);
3. A number (or range of numbers), dom, that represents the day of the month (valid values 0-31);
4. A number (or list, or range), or name (or list of names), mon, that represents the month of the year (valid values 1-12 or Jan-Dec);
5. A number (or list, or range), or name (or list of names), dow, that represents the day of the week (valid values 0-6 or Sun-Sat); and
6. command, which is the command to be run, exactly as it would appear on the command line.

The layout is therefore as follows;

Assuming that we want to run a Python script called ‘m_temp.py which was in the ‘pi’ home directory the line that we would want to add would be as follows;

So at minute 0, hour 6, every day of the month (where the asterisk denotes ‘everything’), every month, every day of the week we run the command /usr/bin/python /home/pi/m_temp.py (which, if we were at the command line in the pi home directory we would run as python m_temp.py, but since we can’t guarantee where we will be when running the script, we are supplying the full path to the python command and the m_temp.py script.

If we wanted to run the command twice a day (6am and 6pm (1800hrs)) we can supply a comma separated value in the hours (h) field as follows;

If we wanted to run the command at 6am but only on weekdays (Monday through Friday) we can supply a range in the dow field as follows (remembering that 0 = Sunday);

If we want to run the same command every 2 hours we can use the */2 notation, so that our line in the crontab would look like the following;

It’s important to note that we need to include the 0 at the start (instead of the *) so that it doesn’t run every minute every 2 hours (every minute in other words)

##### Test yourself
1. How could you set up a schedule job in crontab that ran every second?
2. Create a crontab line to run a command on the 20th of July every year at 2 minutes past midnight.

#### ifconfig

The ifconfig command can be used to view the configuration of, or to configure a network interface. Networking is a fundamental function of modern computers. ifconfig allows us to configure the network interfaces to allow that connection.

• ifconfig [arguments] [interface]

or

• ifconfig [arguments] interface [options]

Used with no ‘interface’ declared ifconfig will display information about all the operational network interfaces. For example running;

… produces something similar to the following on a simple Raspberry Pi.

The output above is broken into three sections; eth0, lo and wlan0.

• eth0 is the first Ethernet interface and in our case represents the RJ45 network port on the Raspberry Pi (in this specific case on a B+ model). If we had more than one Ethernet interface, they would be named eth1, eth2, etc.
• lo is the loopback interface. This is a special network interface that the system uses to communicate with itself. You can notice that it has the IP address 127.0.0.1 assigned to it. This is described as designating the ‘localhost’.
• wlan0 is the name of the first wireless network interface on the computer. This reflects a wireless USB adapter (if installed). Any additional wireless interfaces would be named wlan1, wlan2, etc.
##### The ifconfig command

The ifconfig command is used to read and manage a servers network interface configuration (hence ifconfig = interface configuration).

We can use the ifconfig command to display the current network configuration information, set up an ip address, netmask or broadcast address on an network interface, create an alias for network interface, set up hardware addresses and enable or disable network interfaces.

To view the details of a specific interface we can specify that interface as an argument;

Which will produce something similar to the following;

The configuration details being displayed above can be interpreted as follows;

• Link encap:Ethernet - This tells us that the interface is an Ethernet related device.
• HWaddr b8:27:eb:2c:bc:62 - This is the hardware address or Media Access Control (MAC) address which is unique to each Ethernet card. Kind of like a serial number.
• inet addr:10.1.1.8 - indicates the interfaces IP address.
• Bcast:10.1.1.255 - denotes the interfaces broadcast address
• Mask:255.255.255.0 - is the network mask for that interface.
• UP - Indicates that the kernel modules for the Ethernet interface have been loaded.
• BROADCAST - Tells us that the Ethernet device supports broadcasting (used to obtain IP address via DHCP).
• RUNNING - Lets us know that the interface is ready to accept data.
• MULTICAST - Indicates that the Ethernet interface supports multicasting.
• MTU:1500 - Short for for Maximum Transmission Unit is the size of each packet received by the Ethernet card.
• Metric:1 - The value for the Metric of an interface decides the priority of the device (to designate which of more than one devices should be used for routing packets).
• RX packets:119833 errors:0 dropped:0 overruns:0 frame:0 and TX packets:8279 errors:0 dropped:0 overruns:0 carrier:0 - Show the total number of packets received and transmitted with their respective errors, number of dropped packets and overruns respectively.
• collisions:0 - Shows the number of packets which are colliding while traversing the network.
• txqueuelen:1000 - Tells us the length of the transmit queue of the device.
• RX bytes:8895891 (8.4 MiB) and TX bytes:879127 (858.5 KiB) - Indicates the total amount of data that has passed through the Ethernet interface in transmit and receive.
##### Options

The main option that would be used with ifconfig is -a which will will display all of the interfaces on the interfaces available (ones that are ‘up’ (active) and ‘down’ (shut down). The default use of the ifconfig command without any arguments or options will display only the active interfaces.

##### Arguments

We can disable an interface (turn it down) by specifying the interface name and using the suffix ‘down’ as follows;

Or we can make it active (bring it up) by specifying the interface name and using the suffix ‘up’ as follows;

To assign a IP address to a specific interface we can specify the interface name and use the IP address as the suffix;

To add a netmask to a a specific interface we can specify the interface name and use the netmask argument followed by the netmask value;

To assign an IP address and a netmask at the same time we can combine the arguments into the same command;

##### Test yourself
1. List all the network interfaces on your server.
2. Why might it be a bad idea to turn down a network interface while working on a server remotely?
3. Display the information about a specific interface, turn it down, display the information about it again then turn it up. What differences do you see?

#### ls

The ls command lists the contents of a directory and can show the properties of those objects it lists. It is one of the fundamental commands for knowing what files are where and the properties of those files.

• ls [options] directory : List the files in a particular directory

For example: If we execute the ls command with the -l option to show the properties of the listings in long format and with the argument /var so that it lists the content of the /var directory…

… we should see the following;

##### The ls command

The ls command will be one of the first commands that someone starting with Linux will use. It is used to list the contents of a directory (hence ls = list). It has a large number of options for displaying listings and their properties in different ways. The arguments used are normally the name of the directory or file that we want to show the contents of.

By default the ls command will show the contents of the current directory that the user is in and just the names of the files that it sees in the directory. So if we execute the ls command on its own from the pi users home directory (where we would be after booting up the Raspberry Pi), this is the command we would use;

… and we should see the following;

This shows two directories (Desktop and python_games) that are in pi’s home directory, but there are no details about the directories themselves. To get more information we need to include some options.

##### Options

There are a very large number of options available to use with the ls command. For a full listing type man ls on the command line. Some of the most commonly used are;

• -l gives us a long listing (as explained above)
• -a shows us aLL the files in the directory, including hidden files
• -s shows us the size of the files (in blocks, not bytes)
• -h shows the size in “human readable format” (ie: 4K, 16M, 1G etc). (must be used in conjunction with the -s option).
• -S sorts by file Size
• -t sorts by modification time
• -r reverses order while sorting

A useful combination of options could be a long listing (-l) that shows all (-a) the files with the file size being reported in human readable (-h) block size (-s).

… will produce something like the following;

##### Arguments

The default argument (if none is included) is to list the contents of the directory that the user is currently in. Otherwise we can specify the directory to list. This might seem like a simple task, but there are a few tricks that can make using ls really versatile.

The simplest example of using a specific directory for an argument is to specify the location with the full address. For example, if we wanted to list the contents of the /var directory (and it doesn’t matter which directory we run this command from) we simply type;

… will produce the following;

We can also use some of the relative addressing characters to shortcut our listing. We can list the home directory by using the tilde (ls ~) and the parent directory by using two full stops (ls ..).

The asterisk (*) can be used as a wildcard to list files with similar names. E.g. to list all the png file in a directory we can use ls *.png.

If we just want to know the details of a specific file we can use its name explicitly. For example if we wanted to know the details of the swap file in /var we would use the following command;

… which will produce the following;

##### Examples

List all the configuration (.conf) files in the /etc directory;

… which will produce the following;

#### ping

The ping command allows us to check the network connection between the local computer and a remote server. It does this by sending a request to the remote server to reply to a message (kind of like a read-request in email). This allows us to test network connectivity to the remote server and to see if the server is operating. The ping command is a simple and commonly used network troubleshooting tool.

• ping [options] remote server : checks the connection to a remote server.

To check the connection to the server at CNN for example we can simple execute the following command (assuming that we have a connection to the internet);

Which will return something like the following;

The first thing to note is that by default the ping command will just keep running. When we want to stop it we need to press CTRL-c to get it to stop.

The information presented is extremely useful and tells us that www.cnn.com’s IP address is 157.166.226.25 and that the time taken for a ping send and return message took about 250 milliseconds.

##### The ping command

The ping command is a very simple network / connectivity checking tool that is one of the default ‘go-to’ commands for system administrators. You might be wondering about how the name has come about. It is reminiscent of the echo-location technique used by dolphins, whales and bats to send out a sound and to judge their surroundings by the returned echo. In the dramatised world of the submariner, a ping is the sound emitted by a submarine in the same way to judge the distance and direction to an object. It was illustrated to best effect in the book by Tom Clancy and the subsequent movie “The Hunt for Red October” where the submarine commander makes the request for “One Ping Only”.

It works by sending message called an ‘Echo Request’ to a specific network location (which we specify as part of the command). When (or if) the server receives the request it sends an ‘Echo Reply’ to the originator that includes the exact payload received in the request. The command will continue to send and (hopefully) receive these echoes until the command completes its requisit number of attempts or the command is stopped by the user (with a CTRL-c). Once complete, the command summarises the effort.

From the example used above we can see the output as follows;

We can see from the returned pings that the IP address of the server that is designated as ‘www.cnn.com’ is ‘157.166.226.25’ The resolution of the IP address would be made possible by DNS, but using a straight IP address is perfectly fine). The icmp_seq= column tells us the sequence of the returned replies and ttl indicates how many IP routers the packet can go through before being thrown away. The time provides the measured return trip of the request and reply.

The summary at completion tells us how many packets were sent and how many received back. This forms a percentage of lost packets which is established over the specified time. The final line provides a minimum, average maximum and standard deviation from the mean.

##### Options

There are a few different options for use, but the more useful are as follows;

• -c only ping the connection a certain number (count) of times
• -i change the time interval between pings

It’s really useful to have ping running continuously so that we can make changes to networking while watching the results, but it’s also useful to run the command for a limited amount of time. This is where the -c option comes in. This will simply restrict the number of pings that are sent out and will then cease and summarise the effort. This can be used as follows;

Which will return something like the following;

Sometimes it can be convenient to set our own time interval between pings. This can be accomplished with the -i option which will let us vary the repeat time. The default is 1 second, however the value cannot be set below 0.2 seconds without doing so as the superuser. Interestingly there is an option to flood the network with pings (flood mode) to test the network infrastructure. However, this would be something typically left to research carefully when you really need it.

##### Test yourself
1. How does the ping command to a server name know how to return an IP address?
2. What does ‘ttl’ stand for?

#### sudo

The sudo command allows a user to execute a command as the ‘superuser’ (or as another user). It is a vital tool for system administration and management.

• sudo [options] [command] : Execute a command as the superuser

For example, if we want to update and upgrade our software packages, we will need to do so as the super user. All we need to do is prefix the command apt-get with sudo as follows;

One of the best illustrations of this is via the excellent cartoon work of the xkcd comic strip (Buy his stuff, it’s awesome!).

##### The sudo command

The sudo command is shorthand for ‘superuser do’.

When we use sudo an authorised user is determined by the contents of the file /etc/sudoers.

As an example of usage we should check out the file /etc/sudoers. If we use the cat command to list the file like so;

We get the following response;

That’s correct, the ‘pi’ user does not have permissions to view the file

Let’s confirm that with ls;

Which will result in the following;

It would appear that only the root user can read the file!

So let’s use sudo to cat](#cat) the file as follows;

That will result in the following output;

There’s a lot of information in the file, but there, right at the bottom is the line that determines the privileges for the ‘pi’ user;

We can break down what each section means;

pi

pi ALL=(ALL) NOPASSWD: ALL

The pi portion is the user that this particular rule will apply to.

ALL

pi ALL=(ALL) NOPASSWD: ALL

The first ALL portion tells us that the rule applies to all hosts.

ALL

pi ALL=(ALL) NOPASSWD: ALL

The second ALL tells us that the user ‘pi’ can run commands as all users and all groups.

NOPASSWD

pi ALL=(ALL) NOPASSWD: ALL

The NOPASSWD tells us that the user ‘pi’ won’t be asked for their password when executing a command with sudo.

All

pi ALL=(ALL) NOPASSWD: ALL

The last ALL tells us that the rules on the line apply to all commands.

Under normal situations the use of sudo would require a user to be authorised and then enter their password. By default the Raspbian operating system has the ‘pi’ user configured in the /etc/sudoers file to avoid entering the password every time.

If your curious about what privileges (if any) a user has, we can execute sudo with the -l option to list them;

This will result in output that looks similar to the following;

##### The ‘sudoers’ file

As mentioned above, the file that determines permissions for users is /etc/sudoers. DO NOT EDIT THIS BY HAND. Use the visudo command to edit. Of course you will be required to run the command using sudo;

##### sudo vs su

There is a degree of confusion about the roles of the sudo command vs the su command. While both can be used to gain root privileges, the su command actually switches the user to another user, while sudo only runs the specified command with different privileges. While there will be a degree of debate about their use, it is widely agreed that for simple on-off elevation, sudo is ideal.

##### Test yourself
1. Write an entry for the sudoers file that provides sudo priviledges to a user for only the cat command.
2. Under what circumstances can you edit the sudoers file with a standard text editor.

## Directory Structure Cheat Sheet

• / : The ‘root’ directory which contains all other files and directories
• /bin : Common commands / programs, shared by all users
• /boot : Contains the files needed to successfully start the computer during the boot process
• /dev : Holds device files that represent physical and ‘logical’ devices
• /etc : Contains configuration files that control the operation of programs
• /etc/cron.d: One of the directories that allow programs to be run on a regular schedule
• /etc/rc?.d : Directories containing files that control the mode of operation of a computer
• /home : A directory that holds subdirectories for each user to store user specific files
• /lib : Contains shared library files and kernel modules
• /lost+found : Will hold recoverable data in the event of an an improper shut-down
• /media : Used to temporarily mount removable devices
• /mnt : A mount point for filesystems or temporary mount point for system administrators
• /opt : Contains third party or additional software that is not part of the default installation
• /proc : Holds files that contain information about running processes and system resources
• /root : The home directory of the System Administrator, or the ‘root’ user
• /sbin : Contains binary executables / commands used by the system administrator
• /srv : Provides a consistent location for storing data for specific services
• /tmp : A temporary location for storing files or data
• /usr : Is the directory where user programs and data are stored and shared
• /usr/bin : Contains binary executable files for users
• /usr/lib : Holds shared library files to support executables in /usr/bin and /usr/sbin
• /usr/local : Contains users programs that are installed locally from source code
• /usr/sbin : The directory for non-essential system administration binary executables
• /var : Holds variable data files which are expected to grow under normal circumstances
• /var/lib : Contains dynamic state information that programs modify while they run
• /var/log : Stores log files from a range of programs and services
• /var/spool : Contains files that are held (spooled) for later processing
• /var/tmp : A temporary store for data that needs to be held between reboots (unlike /tmp)