Category: Arch Linux

running rails + ruby + arch linux arm on raspberry pi 3

whbeebe

I’ve been asked to create a web monitoring application running on a Raspberry Pi (in this case the 3). The requirements are very modest; run a low-traffic web page to monitor another machine next to it, and provide a simple web report on demand across local WiFi. For this particular project I’ve elected to go back in time, as it were, and install Ruby and Rails as the web foundation. And when I say back in time, I’m talking 2007, when I created another embedded system using a milliwatt CMOS 486 clone chip running a custom built Linux kernel with µclib and BusyBox running Ruby and Rails.

This small post documents the steps, in order, for installing Ruby and Rails peculiar to Arch Linux ARM and the Raspberry Pi 3.

First, make sure that Ruby is installed. That’s as simple as ‘sudo pacman -S ruby’, and for the documentation, ‘sudo pacman -S ruby-docs’. Ruby Gems are a part of Ruby, and you can look to see if they’ve been installed with either a ‘gem -v’ or by looking down /usr/lib/ruby.

Second, add the following line to your .bashrc:

  • export PATH=”$(ruby -e ‘print Gem.user_dir’)/bin:$PATH”

Log out, then log back in. Make sure to do all this before you install Rails, or else Very Bad Things will happen when you do install Rails. It took two attempts to install Rails, all because I failed to add that line to my bashrc file before the first attempt. You can read all about RubyGems setup here.

Third, install Rails with ‘gem install rails’. This will install an account local copy of Rails under ~/.gem, specifically ~/.gem/ruby/2.3.0/gems with this version of Gems. Installing a local account copy is no different than what happens with Node.js installations. There’s always a debate about local vs global installation; due to security and my personal paranoia I always prefer local (non-root) account installation to minimize any unintended consequences a global installation might engender.

Fourth, to be able to reach the Ruby instance outside the Raspberry Pi, I modified the file ~/[project]/config/boot.rb and added the following lines of code at the end of the file:

require 'rails/commands/server'module Railsclass Serverdef default_optionssuper.merge({Port: 8081, Host: '0.0.0.0'})endendend

The merge binds the web server to whatever IP address DNS assigns to the Raspberry Pi (‘0.0.0.0’) instead of using localhost as the defult, and changes the port from the default of 3000 to 8081.

Finally, and this is just extra, I made a tiny modification to the default index file at ~/.gem/ruby/2.3.0/gems/railties-5.0.0.1/lib/rails/templates/rails/welcome/index.html.erb to add the text “Raspberry Pi 3” in order to drive home that the default web page was in fact coming from the Raspberry Pi 3.

Now on to doing something more useful…

surprise! django runs on arch linux arm on a rasperry pi 3 after all!

whbeebe

tl;dr: It was permissions issues that kept Django from executing properly on Arch ARM and the Raspberry Pi 3. Read on for the details.

Well, looks like I have to walk back that snide comment about Arch running on a Raspberry Pi 3. While I’m certainly no hero, I was irritated considerably by Django’s flagrant failure, enough to find out why. But first, some background.

  1. Django, along with a lot of other Python packages, is located in the system’s site-packages directory. On the RPi 3 running Arch ARM it’s /usr/lib/python3.5/site-packages. The site-packages directory is owned by root (root:root) with permissions 755. Permissions 755 means root can read/write, while group and the world can only read and execute from that directory.
  2. With that kind of ownership I had to install Django with ‘sudo pip install django’ (without the quotes, obviously). After installation every directory had 750 permissions, and every file had 640 permissions, with root:root ownership. That means, using the non-root account alarm, I had no permissions to read anything in the django installation.
  3. And thus any attempt to use django-admin was bound to fail

I don’t have copious amounts of time to work on these kinds of issues. So while I was out working on my honey-do list, while at Trader Joes in the organic produce section, the light went on as to what the probable problem and solution was. Sure enough, when I got home and found a minute to look, that’s when I found the permission settings all wrong. A quick ‘sudo find . -type d -exec chmod 755 {} \+’ and ‘sudo find . -type f -exec chmod 644{}\+’ fixed things up, and I was able to get a quick and dirty Django web server up and running. In fact, to test a capability I’ll need, I tarred up the sample I built on the MBP, secure copied it over to the RPi 3, untarred it there, and got it up and running. Here’s a couple of screen shots of it in action on the RPi 3:

Looks like I’m back in business. The only thing I edited on the RPi 3 was the polls/views.py with vi to change the text output by views.py. Other than that, I simply started it up.

This is the first time I’ve ever had a problem with permissions using pip and Python. In the future I’m going to have to be careful with Arch ARM. It’s a small issue, considering everything else works just fine.