Leveraging PRoot and qemu, it’s easy to configure Raspberry Pi’s, build and install packages, without the need to do so on physical hardware. It’s especially nice if you have to work with many disk images at once, create specialized distributions, reset passwords, or install/customize applications that aren’t yet in the official repositories.

I’ve recently dug in to building apps and doing fun things with the Raspberry Pi. With the recent release of the Raspberry Pi 2, its an even more exciting platform. I’ve documented what I’ve been using to make my workflow more productive.

Table Of Contents

Setup

We’ll use a Linux machine. Below are setup instructions for Ubuntu and Arch. I prefer Arch for desktop and personal work, I use Debian or Ubuntu for production deployments.

Arch Linux is a great “tinkerer’s” distribution – if you haven’t used it before it’s worth checking out. It’s great on the Raspberry Pi.

Debian and Ubuntu have some differences, but share the same base and use the same package management system (apt). I’ve included instructions for Ubuntu in particular, since it’s the most similar to Raspbian, the default Raspberry Pi operating system, and folks may be more familiar with that environment.

Generally speaking, you’ll need the following things:

A physical computer or virtual machine running some version of Linux (setup instructions are provided for the latest Arch and Ubuntu, but any Linux should work).

Installation files for the Raspberry Pi.

SD cards suitable for whatever Raspberry Pi you have. We’ll learn how to work with raw disk images and how to copy disk images to SD cards.

QEMU, an emulator system, and it’s ARM processor support (the Raspberry Pi uses an ARM processor).

PRoot – a convenience tool that makes it easy to mount a “foreign” filesystem and run commands inside of it without booting.

A way to create disk images, and mount them like physical devices.

Once the packages are installed, the commands and processes for building and working with Raspberry Pi boot disks are the same.

NOTE: we assume you have sudo installed and configured.

Virtual Machine Notes

If you’re using an Apple (Mac OS X) computer or Windows, the easiest way to work with Linux systems is via virtualization. VirtualBox is available for most platforms and is easy to work with.

The virtualbox documentation can walk you through the installation of VirtualBox and creating your first virtual machine.

When working with an SD card, you’ll might want to follow instructions for “Access to entire physical hard disk” to make the card accessible to the virtual machine. As an alternative, you could use a USB SD card reader, and usb pass-thru to present not the disk to the virtual machine, but the entire USB device, and let the virtual machine deal with mounting it.

Both of these approaches can be (very) error prone, but provide the most “native” way of working.

Instead I’d recommend installing guest additions. With guest additions installed in your virtual machine, you can use the shared folders feature of VirtualBox. This makes it easy to copy disk images created in your virtual machine to your host machine, and then you can use the standard instructions for Windows and Mac OS to copy the disks images to your SD cards.

Advanced Usage Note: Personally, my usual method of operations with VirtualBox VMs is to set up Samba in my virtual machine and share a folder over a host-only network (or I’ll use bridged networking so I can connect to it from any machine on my LAN) – I’d consider this a more “advanced” approach but I’ve had more consistent results for day-to-day work than using guest additions or mounting host disks. However, for the simple task of just copying disk images back and forth to the virtual machine, the shared folders feature should suffice.

Arch Linux

We’ll use pacman and wget to procure and install most of the tools we need:

$ sudo pacman -S dosfstools wget qemu unzip pv $ wget http://static.proot.me/proot-x86_64 $ chmod +x proot-x86_64 $ sudo mv proot-x86_64 /usr/local/bin/proot

First, we install the following packages:

dosfstools Gives us the ability to create FAT filesystems, required for making a disk bootable on the RaspberryPi. wget General purpose file grabber – used for downloading installation files and PRoot qemu QEMU emulator – allows us to run RaspberryPi executables unzip Decompresses ZIP archives. pv Pipeline middleware that shows a progress bar (we’ll be using it to make copying disk images with dd a little easier for the impatient)

Then we download PRoot, make the file executable, and copy it to a common location for global executable that everyone on a machine can access, /usr/local/bin . This location is just a suggestion – to follow along with the examples in this article, you just need to put the proot executable somewhere on your $PATH .

Finally, we’ll use an AUR package to obtain the kpartx tool.

kpartx wraps a handful of tasks required for creating loopback devices into a single action.

If you haven’t used the AUR before, check out the documentation first for an overview of the process, and to install prerequisites.

$ wget https://aur.archlinux.org/packages/mu/multipath-tools/multipath-tools.tar.gz $ tar -zxvf multipath-tools.tar.gz $ cd multipath-tools $ makepkg $ sudo pacman -U sudo pacman -U multipath-tools-*.pkg.tar.xz

Ubuntu

Ubuntu Desktop comes with most of the tools we need (in particular, wget , the ability to mount dos file systems, and unzip ). As such, the process of getting set up for using PRoot is a bit simpler, compared to Arch.

Ubuntu uses apt-get for package installation.

$ sudo apt-get install qemu kpartx pv $ wget http://static.proot.me/proot-x86_64 $ chmod +x proot-x86_64 $ sudo mv proot-x86_64 /usr/local/bin/proot

First, we install the following packages:

qemu QEMU emulator – allows us to run RaspberryPi executables kpartx Helper tool that wraps a handful of tasks required for creating loopback devices into a single action. pv Pipeline middleware that shows a progress bar (we’ll be using it to make copying disk images with dd a little easier for the impatient)

Then, we install PRoot by downloading the binary from proot.me, making it executable, and putting it somewhere on our $PATH , /usr/local/bin , making it available to all users on the system. This location is merely a suggestion, but putting the proot executable somewhere on your $PATH will make it easier to follow along with the examples below.

Working With A Disk Image

A disk (in the Raspberry Pi’s case, we’re talking about an SD card) is just an arrangement of blocks for data storage. On top of those blocks is a description of how files are represented in those blocks, or a filesystem (for more detail, see the Wikipedia articles on Disk Storage and File System).

Disks can exist in the physical world, or can be represented by a special file, called a disk image. We can download pre-made images with Raspbian already installed from the official Raspberry Pi downloads page.

$ wget http://downloads.raspberrypi.org/raspbian_latest -O rasbian_latest.img.zip $ unzip rasbian_latest.img.zip Archive: raspbian_latest.zip inflating: 2015-02-16-raspbian-wheezy.img

Take note of the name of the img file – it will vary depending on the current release of Raspbian at the time.

At this point we have a disk image we can mount by creating a loopback device. Once we have it mounted, we can use QEMU and PRoot to run commands within it without fully booting it.

We’ll use kpartx to set up a loopback device for each partition in the disk image:

$ sudo kpartx -a -v 2015-02-16-raspbian-wheezy.img add map loop0p1 (254:0): 0 114688 linear /dev/loop0 8192 add map loop0p2 (254:1): 0 6277120 linear /dev/loop0 122880

The -a command line switch tells kpartx to create new loopback devices. The -v switch asks kpartx to be more verbose and print out what it’s doing.

We can do a dry-run and inspect the disk image using the -l switch:

$ sudo kpartx -l 2015-02-16-raspbian-wheezy.img loop0p1 : 0 114688 /dev/loop0 8192 loop0p2 : 0 6277120 /dev/loop0 122880 loop deleted : /dev/loop0

We can see the partitions to be sure, using fdisk -l

$ sudo fdisk -l /dev/loop0 Disk /dev/loop0: 3.1 GiB, 3276800000 bytes, 6400000 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x0009bf4f Device Boot Start End Sectors Size Id Type /dev/loop0p1 8192 122879 114688 56M c W95 FAT32 (LBA) /dev/loop0p2 122880 6399999 6277120 3G 83 Linux

We can also see them using lsblk :

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH loop0 7:0 0 3.1G 0 loop ├─loop0p1 254:0 0 56M 0 part └─loop0p2 254:1 0 3G 0 part

Generally speaking, the first, smaller partition will be the boot partition, and the others will hold data. It’s typical with RaspberryPi distributions to use a simple 2-partition scheme like this.

The new partitions will end up in /dev/mapper :

$ ls /dev/mapper control loop0p1 loop0p2

Now we can mount our partitions. We’ll first make a couple of descriptive directories for mount points:

$ mkdir raspbian-boot raspbian-root $ sudo mount /dev/mapper/loop0p1 raspbian-boot $ sudo mount /dev/mapper/loop0p2 raspbian-root

At this point we can go to the next section where we will run PRoot and start doing things “inside” the disk image.

Working With An Existing Disk

We can use PRoot with an existing disk (SD card) as well. The first step is to insert the disk into your computer. Your operating system will likely automatically boot it. We also need to find out which device the disk is registered as.

lsblk can answer both questions for us:

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdb 8:16 1 14.9G 0 disk ├─sdb1 8:17 1 56M 0 part /run/media/jj/boot └─sdb2 8:18 1 3G 0 part /run/media/jj/f24a4949-f4b2-4cad-a780-a138695079ec sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

On my system, the SD card I inserted (a Raspbian disk I pulled out of a Raspberry Pi) came up as /dev/sdb . It has two paritions, sdb1 and sdb2 . Both partitions were automatically mounted, to /run/media/jj/boot and /run/media/jj/f24a4949-f4b2-4cad-a780-a138695079ec , respectively.

Typically, the first, smaller partition will be the boot partition. To verify this, we’ll again use fdisk -l :

$ sudo fdisk -l /dev/sdb Disk /dev/sdb: 14.9 GiB, 16021192704 bytes, 31291392 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x0009bf4f Device Boot Start End Sectors Size Id Type /dev/sdb1 8192 122879 114688 56M c W95 FAT32 (LBA) /dev/sdb2 122880 6399999 6277120 3G 83 Linux

Here we see that /dev/sdb1 is 56 megabytes in size, and is of type “W95 FAT32 (LBA)”. This is typically indicative of a RasbperryPi boot partition, so /dev/sdb1 is our boot partition, and /dev/sdb2 is our root partition.

We can use the existing mounts that the operating system set up automatically for us, if we want, but it’s a bit easier to un-mount the partitions and mount them somewhere more descriptive, like raspbian-boot and raspbian-boot :

$ sudo umount /dev/sdb1 /dev/sdb2 $ mkdir -p raspbian-boot raspbian-root $ sudo mount /dev/sdb1 raspbian-boot $ sudo mount /dev/sdb2 raspbian-root

Note: The -p switch to mkdir causes mkdir to ignore already-exsiting directories. We’ve added it here in case you were following along in the previous section and already have these directories handy.

A call to lsblk will confirm that we’ve mounted things as we expected:

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdb 8:16 1 14.9G 0 disk ├─sdb1 8:17 1 56M 0 part /run/media/jj/STEALTH/raspbian-boot └─sdb2 8:18 1 3G 0 part /run/media/jj/STEALTH/raspbian-root sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

Now we can proceed to the next section, and run the same PRoot command to configure, compile and/or install things – but this time we’ll be working directly on the SD card instead of inside of a disk image.

Basic Configuration/Package Installation

Now that we’ve got either a disk image or a physical disk mounted, we can run commands within those filesystems using PRoot.

NOTE: The following command line switches worked for me, but took some experimentation to figure out. Please take some time to read the PRoot documentation so you understand exactly what the switches mean.

We can run any command directly (like say, apt-get ) but it’s useful to be able to “log in” to the disk image (run a shell), and then perform our tasks:

$ sudo proot -q qemu-arm -S raspbian-root -b raspbian-boot:/boot /bin/bash

This mode of PRoot forces the root user inside of the disk image. The -q switch wraps every command in the qemu-arm emulator program, making it possible to run code compiled for the RaspberryPi’s ARM processor. The -S parameter sets the directory that will be the “root” – essentially that means that raspbian-root will map to / . -S also fakes the root user (id 0), and adds some protections for us in the event we’ve mixed in files from our host system that we don’t want the disk image code to modify. -b splices in additional directories – we add the /boot partition, since that’s where new kernel images and other boot-related stuff gets installed. This isn’t entirely necessary, but its useful for system upgrades and making changes to boot settings. Finally, we tell PRoot which command to run, in this case, /bin/bash , the BASH shell.

Now that we’re “in” the disk image, we can update and install new packages.

Since root is not a “normal” user in the default Rasbian installation, the path needs to be adjusted:

# export PATH=$PATH:/usr/sbin:/sbin:/bin:/usr/local/sbin

Now we can do the update/upgrade, and install any additional packages we might want (for example, the samba file sharing server):

# apt-get update # apt-get upgrade # apt-get install samba

Check out the man page for apt-get for full details (type man apt-get at a shell prompt).

You will likely see a lot of warnings and possibly errors when installing packages – these can usually be ignored, but make note of them – there may be some environmental tweaks that need to be made.

We can do almost anything in the PRoot environment that we could do logged into a running Raspberry Pi.

We can edit config.txt and change settings (for an explanation of the settings, see the documentation):

# vi /boot/config.txt

We can add a new user:

# adduser jj Adding user `jj' ... Adding new group `jj' (1004) ... Adding new user `jj' (1001) with group `jj' ... Creating home directory `/home/jj' ... Copying files from `/etc/skel' ... Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully Changing the user information for jj Enter the new value, or press ENTER for the default Full Name []: Josh Johnson Room Number []: Work Phone []: Home Phone []: Other []:

We can grant a user sudo privileges (the default sudo configuration allows anyone in the sudo group to run commands as root via sudo):

# usermod -a -G sudo jj # groups jj jj : jj sudo

You can reset someone’s password, or change the password of the default pi user:

# passwd pi Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully

The possibilities here are endless, with a few exceptions:

Running code that relies on the GPIO pins or drivers loaded into the kernel will not work.

Configuring devices (like, say, a wifi adapter) may work, but device information will likely be wrong.

Testing startup/shutdown scripts – since we’re not booting the disk image, these scripts aren’t run.

Compiling For The RPi

Raspbian comes with most of the tools we’ll need (in particular, the build-essential package). Lets build and install the nginx web server – a relatively easy to build package.

If you’ve never compiled software on Linux before, most (but not all!) source code packages are provided as tarballs, and include some scripts that help you build the software in what’s known as the “configure, make, make install” (or CMMI) procedure.

Note: For a great explanation (with examples you can follow to build your own CMMI package), George Brocklehurst wrote an excellent article explaining the details behind CMMI called “The magic behind configure, make, make install“.

First we’ll need to obtain the nginx tarball:

# wget http://nginx.org/download/nginx-1.7.12.tar.gz # tar -zxvf nginx-1.7.12.tar.gz

Next we’ll look for a README or INSTALL file, to check for any extra build dependencies:

# cd nginx-1.7.12 # ls -l total 660 -rw-r--r-- 1 jj indiecity 249016 Apr 7 15:35 CHANGES -rw-r--r-- 1 jj indiecity 378885 Apr 7 15:35 CHANGES.ru -rw-r--r-- 1 jj indiecity 1397 Apr 7 15:35 LICENSE -rw-r--r-- 1 root root 46 Apr 18 10:21 Makefile -rw-r--r-- 1 jj indiecity 49 Apr 7 15:35 README drwxr-xr-x 6 jj indiecity 4096 Apr 18 10:21 auto drwxr-xr-x 2 jj indiecity 4096 Apr 18 10:21 conf -rwxr-xr-x 1 jj indiecity 2478 Apr 7 15:35 configure drwxr-xr-x 4 jj indiecity 4096 Apr 18 10:21 contrib drwxr-xr-x 2 jj indiecity 4096 Apr 18 10:21 html drwxr-xr-x 2 jj indiecity 4096 Apr 18 10:21 man drwxr-xr-x 2 root root 4096 Apr 18 10:23 objs drwxr-xr-x 8 jj indiecity 4096 Apr 18 10:21 src # view README

We’ll note that, helpfully (cue eye roll) that nginx has put into the README:

Documentation is available at http://nginx.org

A more direct link gives us a little more useful information. Scanning this, there aren’t any obvious dependencies or features we want to add/enable, so we can proceed.

We can also find out which options are available by running ./configure --help .

Note: There are several configuration options that control where files are put when the compiled code is installed – they may be of use, in particular the standard --PREFIX . This can help segregate multiple versions of the same application on a system, for example if you need to install a newer/older version and already have one installed via the apt package. It is also useful to build self-contained directory structures that you can easily copy from one system to another.

Run ./configure , note any warnings or errors. There may be some modules or other things not found – that’s typically OK, but can help explain why an eventual error happened toward the end of the configure script or during compilation:

# cd nginx-1.7.12 # ./configure ... checking for PCRE library ... not found checking for PCRE library in /usr/local/ ... not found checking for PCRE library in /usr/include/pcre/ ... not found checking for PCRE library in /usr/pkg/ ... not found checking for PCRE library in /opt/local/ ... not found ... ./configure: error: the HTTP rewrite module requires the PCRE library. You can either disable the module by using --without-http_rewrite_module option, or install the PCRE library into the system, or build the PCRE library statically from the source with nginx by using --with-pcre=<path> option.

Whoa, we ran into a problem! For our use case (just showing off how to do a CMMI build in a PRoot environment) we probably don’t need the rewrite module, so we can re-run ./configure with the --without-http_rewrite_module switch.

However, it’s useful to understand how to track down dependencies like this, and rewriting is a pretty killer feature of any http server, so lets install the dependency.

The configure script mentions the “PCRE library”. PCRE stands for “Perl Compatible Regular Expressions”. Perl is a classical systems language that has hard-core text processing capabilities. It’s particularly known for its regular expression support and syntax. The Perl regular expression syntax is so useful in fact, that some folks built a library allowing other programmers to use it without having to use Perl itself.

Note: This information can be found by using your favorite search engine!

There are two ways libraries like PCRE are installed. The first, and easiest, is that a system package will be available with the library pre-compiled and ready to go. The second will require the same steps we’re following to install nginx – download a tarball, extract, and configure, make, make install.

To find a package, you can use apt-cache search or aptitude search .

I prefer aptitude , since it will tell us what packages are already installed:

# aptitude search pcre v apertium-pcre2 - p cl-ppcre - Portable Regular Express Library for Common Lisp p clisp-module-pcre - clisp module that adds libpcre support p gambas3-gb-pcre - Gambas regexp component p haskell-pcre-light-doc - transitional dummy package p libghc-pcre-light-dev - Haskell library for Perl 5-compatible regular expressions v libghc-pcre-light-dev-0.4-4f534 - p libghc-pcre-light-doc - library documentation for pcre-light p libghc-pcre-light-prof - pcre-light library with profiling enabled v libghc-pcre-light-prof-0.4-4f534 - p libghc-regex-pcre-dev - Perl-compatible regular expressions v libghc-regex-pcre-dev-0.94.2-49128 - p libghc-regex-pcre-doc - Perl-compatible regular expressions; documentation p libghc-regex-pcre-prof - Perl-compatible regular expressions; profiling libraries v libghc-regex-pcre-prof-0.94.2-49128 - p libghc6-pcre-light-dev - transitional dummy package p libghc6-pcre-light-doc - transitional dummy package p libghc6-pcre-light-prof - transitional dummy package p liblua5.1-rex-pcre-dev - Transitional package for lua-rex-pcre-dev p liblua5.1-rex-pcre0 - Transitional package for lua-rex-pcre p libpcre++-dev - C++ wrapper class for pcre (development) p libpcre++0 - C++ wrapper class for pcre (runtime) p libpcre-ocaml - OCaml bindings for PCRE (runtime) p libpcre-ocaml-dev - OCaml bindings for PCRE (Perl Compatible Regular Expression) v libpcre-ocaml-dev-werc3 - v libpcre-ocaml-werc3 - i libpcre3 - Perl 5 Compatible Regular Expression Library - runtime files p libpcre3-dbg - Perl 5 Compatible Regular Expression Library - debug symbols p libpcre3-dev - Perl 5 Compatible Regular Expression Library - development f p libpcrecpp0 - Perl 5 Compatible Regular Expression Library - C++ runtime f p lua-rex-pcre - Perl regular expressions library for the Lua language p lua-rex-pcre-dev - PCRE development files for the Lua language v lua5.1-rex-pcre - v lua5.1-rex-pcre-dev - v lua5.2-rex-pcre - v lua5.2-rex-pcre-dev - p pcregrep - grep utility that uses perl 5 compatible regexes. p pike7.8-pcre - PCRE module for Pike p postfix-pcre - PCRE map support for Postfix

See man aptitude for full details, but the gist is that p means the package is available but not installed, v is a virtual package that points to other packages, and i means the package is installed.

What we want is a package with header files and modules we can compile against – these are usually named lib[SOMETHING]-dev .

Scanning the list, we see a package named libpcre3-dev – this is probably what we want, we can find out by installing it:

# apt-get install libpcre3-dev

Now we can re-run ./configure and see if it works:

# ./configure ... checking for PCRE library ... found ... Configuration summary + using system PCRE library + OpenSSL library is not used + using builtin md5 code + sha1 library is not found + using system zlib library nginx path prefix: "/usr/local/nginx" nginx binary file: "/usr/local/nginx/sbin/nginx" nginx configuration prefix: "/usr/local/nginx/conf" nginx configuration file: "/usr/local/nginx/conf/nginx.conf" nginx pid file: "/usr/local/nginx/logs/nginx.pid" nginx error log file: "/usr/local/nginx/logs/error.log" nginx http access log file: "/usr/local/nginx/logs/access.log" nginx http client request body temporary files: "client_body_temp" nginx http proxy temporary files: "proxy_temp" nginx http fastcgi temporary files: "fastcgi_temp" nginx http uwsgi temporary files: "uwsgi_temp" nginx http scgi temporary files: "scgi_temp"

The library was found, the error is gone, and so now we can proceed with compilation.

To build nginx, we simply run make :

# make

If all goes well, then you can isntall it:

# make install

This same basic process can be used to build custom applications written in C/C++, to build applications that aren’t yet in the package repository, or build applications with specific features or optimizations enabled that the standard packages might not have.

Using Apt To Install Build Dependencies

One more useful thing that apt-get can do for us: it can install the build dependencies for any given package in the repository. This allows us to get most, if not all, potentially missing dependencies to build a known application.

We could have started off with our nginx exploration by first installing it’s build dependencies:

# apt-get build-dep nginx

This won’t solve every dependency issue, but it’s a useful tool in getting all of your ducks in a row for building, especially for more complex things like desktop applications.

Be careful with build-dep – it can bring in a lot of things, some you may not really need. In our case it’s not really a problem, but be aware of space limitations.

Umount and Clean Up

Once we’ve gotten our disk image configured as we like, we need to un-mount it.

First, we need to exit the bash shell we started with PRoot, then we’ll call sync to ensure all data is flushed to any disks:

# exit $ sync

Now we can un-mount the partitions (the command is the same whether we’re using a disk image or an SD card):

$ sudo umount raspbian-root rasbian-boot

We can double-check that the disk is no longer mounted by calling mount without any additional parameters, or using lsblk

$ mount ...

With lsblk , we’ll still see the disks (or loopback devices) present, but not mounted:

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH loop0 7:0 0 3.1G 0 loop ├─loop0p1 254:0 0 56M 0 part └─loop0p2 254:1 0 3G 0 part

If we’re using a disk image, we’ll want to destroy the loopback devices. This is accomplished with kpartx -d :

$ sudo kpartx -d 2015-02-16-raspbian-wheezy.img

We can verify that it’s gone using lsblk again:

$ lsblk sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

At this point we can write the disk image to an SD card, or eject the SD card and insert it into a Raspberry Pi.

Writing a Disk Image to an SD Card

We’ll use the dd command, which writes raw blocks of data from one block device to another, to copy the disk image we made into an SD card.

NOTE: The SD card you use will be COMPLETELY erased. Proceed with caution.

First, insert the SD card into your computer (or card reader, etc). Depending on your system, it may be automatically mounted. We can find out the device name and if its mounted using lsblk :

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdb 8:16 1 14.9G 0 disk ├─sdb1 8:17 1 114.3M 0 part ├─sdb2 8:18 1 1K 0 part └─sdb3 8:19 1 32M 0 part /run/media/jj/SETTINGS sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

We can see the new disk came up as sdb . It has three partitions, sdb1 , sdb2 , and sdb3 . Looking at the MOUNTPOINT column, we can tell that my operating system auto-mounted sdb3 into the /run/media/jj/SETTINGS directory.

Note: The partition layout may vary depending on what was on the SD card before you inserted it. My SD card had a fresh copy of NOOBS that hadn’t yet installed an OS.

We can double-check that sdb is the right disk with fdisk :

$ sudo fdisk -l /dev/sdb Disk /dev/sdb: 14.9 GiB, 16021192704 bytes, 31291392 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x000cb53d Device Boot Start End Sectors Size Id Type /dev/sdb1 8192 242187 233996 114.3M e W95 FAT16 (LBA) /dev/sdb2 245760 31225855 30980096 14.8G 85 Linux extended /dev/sdb3 31225856 31291391 65536 32M 83 Linux

fdisk tells us that this is a 16GB drive. The exact amount cited by some drive manufacturers is not in “real” gigabytes, an exponent of 2[*] but in billions of bytes – note the byte count: 16,021,192,704.

We can see the three partitions, and what format they are in. The small FAT filesystem is a good indication that this is a bootable Raspberry Pi disk.

With a fresh SD card, the call to fdisk may look more like this:

Disk /dev/sdb: 14.9 GiB, 16021192704 bytes, 31291392 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x00000000 Device Boot Start End Sectors Size Id Type /dev/sdb1 8192 31291391 31283200 14.9G c W95 FAT32 (LBA)

Most SD cards are pre-formatted with a single partition containing a FAT32 filesystem.

It’s important to be able to differentiate between your system drives and the target for copying over your disk image – if you point dd at the wrong place, you can destroy important things, like your operating system!

Now that we’re sure that /dev/sdb is our SD card, we can proceed.

Since lsblk indicated that at least one of the partitions was mounted ( sdb3 ), we will fist need to un-mount it:

$ sudo umount /dev/sdb3

Now we can verify it’s indeed not mounted:

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdb 8:16 1 14.9G 0 disk ├─sdb1 8:17 1 114.3M 0 part ├─sdb2 8:18 1 1K 0 part └─sdb3 8:19 1 32M 0 part sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

And copy the disk image:

$ sudo dd if=2015-02-16-raspbian-wheezy.img of=/dev/sdb bs=4M 781+1 records in 781+1 records out 3276800000 bytes (3.3 GB) copied, 318.934 s, 10.3 MB/s

This will take some time, and dd gives no output until it’s finished. Be patient.

dd has a fairly simple interface. The if option indicates the in file, or the disk (or disk image in our case) that is being copied. The of option sets the out file, or the disk to write to. bs sets the block size, which indicates how big of a piece of data to write at a time.

The bs value can be tweaked to get faster or more reliable performance in various situations – we’re using 4M (four megabytes) as recommended by raspberrypi.org. The larger the value, the faster dd will run, but there are physical limits to what your system can handle, so it’s best to stick with the recommended value.

So dd gives us no output until it’s completed. This is kind of an annoying thing about dd but it can be remedied. The easiest way is to install a tool called pv , and split the command – pv acts as an intermediary between two commands and displays a progress bar as it moves along. dd can read and write data to a pipe (details). So we can use two dd commands, put pv in the middle, and get a nice progress bar.

Here’s the same copy as before, but using pv :

Note: Here we’re using sh -c to wrap the command pipeline in quotes. This allows us to provide the entire pipeline as a single unit. If we didn’t, the shell would interpret the first pipe in the pipeline as part of the call to sudo, and not what we want to run as root.

$ ls -l 2015-02-16-raspbian-wheezy.img -rw-r--r-- 1 jj jj 3276800000 Apr 18 07:58 2015-02-16-raspbian-wheezy.img $ sudo sh -c "dd if=2015-02-16-raspbian-wheezy.img bs=4M | pv --size=3276800000 | dd of=/dev/sdb" 613MiB 0:02:31 [4.22MiB/s] [===========> ] 19% ETA 0:10:04 # exit

We pass pv a --size argument to give it an idea of how big the file is, so it can provide accurate progress. We found out the size of our disk image using ls -l. , which shows the size of the file in bytes.

If we run lsblk again, we’ll see the different partition arrangement now on sdb :

$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 0 14.9G 0 disk └─sda1 8:1 0 14.9G 0 part / sdb 8:16 1 14.9G 0 disk ├─sdb1 8:17 1 56M 0 part └─sdb2 8:18 1 3G 0 part sdc 8:32 0 29.8G 0 disk └─sdc1 8:33 0 29.8G 0 part /run/media/jj/STEALTH

fdisk -l gives a bit more detail:

$ sudo fdisk -l /dev/sdb Disk /dev/sdb: 14.9 GiB, 16021192704 bytes, 31291392 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x0009bf4f Device Boot Start End Sectors Size Id Type /dev/sdb1 8192 122879 114688 56M c W95 FAT32 (LBA) /dev/sdb2 122880 6399999 6277120 3G 83 Linux

Now we can sync the disks:

$ sync

At this point we have an SD card we can put into a Raspberry Pi and boot.

[*] (1GB = 1 byte * 1024 (kilobyte) * 1024 (megabyte) * 1024, or 1,073,741,824 bytes)

Extra Credit: Making our own disk image

Some distributions, such as Arch, don’t distribute disk images, but instead distribute tarballs of files. They let you set up the disk however you want, then copy the files over to install the operating system.

We can create our own disk images using fallocate , and then use fdisk or parted (or if you prefer a GUI, gparted) to partition the disk.

We’ll create a disk image for the latest Arch Linux ARM distribution for the Raspberry Pi 2.

Note: You must create the disk image file on a compatible filesystem, such as ext4, for this to work. This is the default system disk filesystem for most modern Linux distributions, including Arch and Ubuntu, so for most people this isn’t a problem. The implication is that this will not work on, say, an external hard drive formatted in an incompatible format, such as FAT32.

First we’ll create an 8 gigabyte empty disk image:

$ fallocate -l 8G arch-latest-rpi2.img

We’ll use fdisk to partition the disk. We need two partitions. The first will be 100 megabytes, formatted as FAT32. We’ll need to set the partition’s system id to correspond to FAT32 with LBA so that the Raspberry Pi’s BIOS knows how to read it.

Note: I’ve had trouble finding documentation as to exactly why FAT + LBA is required, the assumption is it has something to do with how the ARM processor loads the operating system in the earliest boot stages; if anyone knows more detail or can point me to the documentation about this, it would be greatly appreciated!

The offset for the partition will be 2048 blocks – this is the default that fdisk will suggest (and what the Arch installation instructions tell us to do).

Note: This seems to work well- however, there is some confusion about partition alignment. The Raspbian disk images use a 8192 block offset, and there is a lot of information available explaining how a bad alignment can cause quicker SD card degradation and hurt write performance. I’m still trying to figure out the best way to address this, this is another area where community help would be appreciated :) Here are a few links that dig into the issue: http://wiki.laptop.org/go/How_to_Damage_a_FLASH_Storage_Device, http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/, http://3gfp.com/wp/2014/07/formatting-sd-cards-for-speed-and-lifetime/.

The second partition will be ext4, and use the rest of the the available disk space.

We’ll start fdisk and get the initial prompt. No changes will be saved until we instruct fdisk to do so:

$ fdisk arch-latest-rpi2.img Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel Building a new DOS disklabel with disk identifier 0x152a22d4. Changes will remain in memory only, until you decide to write them. After that, of course, the previous content won't be recoverable. Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite) Command (m for help):

Most of the information here is just telling us that this is a block device with no partitions. If you need help, as indicated, you can type m :

Command (m for help): m Command action a toggle a bootable flag b edit bsd disklabel c toggle the dos compatibility flag d delete a partition l list known partition types m print this menu n add a new partition o create a new empty DOS partition table p print the partition table q quit without saving changes s create a new empty Sun disklabel t change a partition's system id u change display/entry units v verify the partition table w write table to disk and exit x extra functionality (experts only)

First, we need to create a new disk partition table. This is done by entering o :

Command (m for help): o Building a new DOS disklabel with disk identifier 0xa8e8538a. Changes will remain in memory only, until you decide to write them. After that, of course, the previous content won't be recoverable. Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Next, we’ll create our first primary partition, the boot partition, at 2048 blocks offset, 100MB in size.

Command (m for help): n Partition type: p primary (0 primary, 0 extended, 4 free) e extended Select (default p): p Partition number (1-4, default 1): 1 First sector (2048-16777215, default 2048): 2048 Last sector, +sectors or +size{K,M,G} (2048-16777215, default 16777215): +100M

By using the relative number +100M , we save ourselves some trouble of having to do math to figure out how many sectors we need.

We can see what we have so far, by using the p command:

Command (m for help): p Disk arch-latest-rpi2.img: 8589 MB, 8589934592 bytes 255 heads, 63 sectors/track, 1044 cylinders, total 16777216 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0xa8e8538a Device Boot Start End Blocks Id System arch-latest-rpi2.img1 2048 206847 102400 83 Linux

Next, we need to set the partition type (system id) by entering t :

Command (m for help): t Selected partition 1 Hex code (type L to list codes): L 0 Empty 24 NEC DOS 81 Minix / old Lin bf Solaris 1 FAT12 27 Hidden NTFS Win 82 Linux swap / So c1 DRDOS/sec (FAT- 2 XENIX root 39 Plan 9 83 Linux c4 DRDOS/sec (FAT- 3 XENIX usr 3c PartitionMagic 84 OS/2 hidden C: c6 DRDOS/sec (FAT- 4 FAT16 <32M 40 Venix 80286 85 Linux extended c7 Syrinx 5 Extended 41 PPC PReP Boot 86 NTFS volume set da Non-FS data 6 FAT16 42 SFS 87 NTFS volume set db CP/M / CTOS / . 7 HPFS/NTFS/exFAT 4d QNX4.x 88 Linux plaintext de Dell Utility 8 AIX 4e QNX4.x 2nd part 8e Linux LVM df BootIt 9 AIX bootable 4f QNX4.x 3rd part 93 Amoeba e1 DOS access a OS/2 Boot Manag 50 OnTrack DM 94 Amoeba BBT e3 DOS R/O b W95 FAT32 51 OnTrack DM6 Aux 9f BSD/OS e4 SpeedStor c W95 FAT32 (LBA) 52 CP/M a0 IBM Thinkpad hi eb BeOS fs e W95 FAT16 (LBA) 53 OnTrack DM6 Aux a5 FreeBSD ee GPT f W95 Ext'd (LBA) 54 OnTrackDM6 a6 OpenBSD ef EFI (FAT-12/16/ 10 OPUS 55 EZ-Drive a7 NeXTSTEP f0 Linux/PA-RISC b 11 Hidden FAT12 56 Golden Bow a8 Darwin UFS f1 SpeedStor 12 Compaq diagnost 5c Priam Edisk a9 NetBSD f4 SpeedStor 14 Hidden FAT16 <3 61 SpeedStor ab Darwin boot f2 DOS secondary 16 Hidden FAT16 63 GNU HURD or Sys af HFS / HFS+ fb VMware VMFS 17 Hidden HPFS/NTF 64 Novell Netware b7 BSDI fs fc VMware VMKCORE 18 AST SmartSleep 65 Novell Netware b8 BSDI swap fd Linux raid auto 1b Hidden W95 FAT3 70 DiskSecure Mult bb Boot Wizard hid fe LANstep 1c Hidden W95 FAT3 75 PC/IX be Solaris boot ff BBT 1e Hidden W95 FAT1 80 Old Minix Hex code (type L to list codes): c Changed system type of partition 1 to c (W95 FAT32 (LBA))

After the t command, we opted to enter L to see the list of possible codes. We then see that W95 FAT32 (LBA) corresponds to the code c .

Now we can make our second primary partition for data storage, utilizing the rest of the disk. We again use the n command:

Command (m for help): n Partition type: p primary (1 primary, 0 extended, 3 free) e extended Select (default p): p Partition number (1-4, default 2): 2 First sector (206848-16777215, default 206848): Using default value 206848 Last sector, +sectors or +size{K,M,G} (206848-16777215, default 16777215): Using default value 16777215

We accepted the defaults for all of the prompts.

Now, entering p again, we can see the state of the partition table:

Command (m for help): p Disk arch-latest-rpi2.img: 8589 MB, 8589934592 bytes 255 heads, 63 sectors/track, 1044 cylinders, total 16777216 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0xa8e8538a Device Boot Start End Blocks Id System arch-latest-rpi2.img1 2048 206847 102400 c W95 FAT32 (LBA) arch-latest-rpi2.img2 206848 16777215 8285184 83 Linux

Now we can write out the table ( w ), which will exit fdisk :

Command (m for help): w The partition table has been altered! WARNING: If you have created or modified any DOS 6.x partitions, please see the fdisk manual page for additional information. Syncing disks.

Now we need to format the partitions. We’ll use kpartx to create block devices for us that we can format:

$ sudo kpartx -av arch-latest-rpi2.img add map loop0p1 (252:0): 0 204800 linear /dev/loop0 2048 add map loop0p2 (252:1): 0 16570368 linear /dev/loop0 206848

As we saw earilier, the devices will show up in /dev/mapper , as /dev/mapper/loop0p1 and /dev/mapper/loop0p2 .

First we’ll format the boot partition loop0p1 , as :

$ sudo mkfs.vfat /dev/mapper/loop0p1 mkfs.fat 3.0.26 (2014-03-07) unable to get drive geometry, using default 255/63

Next the data partition, in ext4 format:

$ sudo mkfs.ext4 /dev/mapper/loop0p2 mke2fs 1.42.9 (4-Feb-2014) Discarding device blocks: done Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) Stride=0 blocks, Stripe width=0 blocks 518144 inodes, 2071296 blocks 103564 blocks (5.00%) reserved for the super user First data block=0 Maximum filesystem blocks=2122317824 64 block groups 32768 blocks per group, 32768 fragments per group 8096 inodes per group Superblock backups stored on blocks: 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632 Allocating group tables: done Writing inode tables: done Creating journal (32768 blocks): done Writing superblocks and filesystem accounting information: done

At this point we just need to mount the new filesystems, download the installation tarball and use tar to extract and copy the files:

First we’ll grab the installation files:

$ wget http://archlinuxarm.org/os/ArchLinuxARM-rpi-2-latest.tar.gz

Next we’ll mount the new filesystems:

$ mkdir arch-root arch-boot $ sudo mount /dev/mapper/loop0p1 arch-boot $ sudo mount /dev/mapper/loop0p2 arch-root

And finally populate the disk image with the system files, and move the boot directory to the boot partition:

$ sudo tar -xpf ArchLinuxARM-rpi-2-latest.tar.gz -C arch-root $ sync $ sudo mv arch-root/boot/* arch-boot/

We’re using a few somewhat less common parameters for tar . Typically we’ll use -xvf to tell tar to extract ( -x ), be verbose ( -v ) and specify the file ( -f ). We’ve added the -p switch to preserve permissions. This is especially important with system files.

The -C switch tells tar to change to the arch-root directory before extraction, effectively extracting the files directly to the root filesystem.

You may see some warnings about extended header keywords, these can be ignored.

Now we just need to clean up (unmount, remove the loopback devs):

$ sudo umount arch-root arch-boot $ sudo kpartx -d arch-latest-rpi2.img

Now we’ve got our own Arch disk image we can distribute, or copy onto SD cards. We can also mount it on the loopback and use PRoot to further configure it, as we did above with Raspbian.

Where To Go From Here

With this basic workflow, we can do all sorts of interesting things. A few ideas:

Distribute disk images pre-configured with applications we created.

Pre-configure images and SD cards for use in classrooms, meetups, demos, etc.

Set up a cron job that runs nightly and creates a disk image with the latest packages.

Build our own packages (either just create tarballs or use a tool like FPM and build deb packages) for drivers and other software and save other folks the hassle of doing this themselves.

Create rudimentary disk duplication setups for putting one image on a bunch of SD cards.

Fix broken installs.

Construct build and testing systems; integrate with tools like Jenkins.

So there we go – now you can customize the Raspberry Pi operating system with impunity, on your favorite workstation or laptop machine. If you have any questions, corrections, or suggestions for ways to streamline the process, please leave a comment!