Rusty

Over the years, I've been working on several different products, all of which has allowed me to practice building products and shipping code. However, I haven't had much time to experiment and play. I feel I've lost many things that I've learnt from my CS degree.

So starting from today, I will start practising and learning again.

To start off, I'm going to work my way through Code Kata. I'll be posting my solutions on Gist.

I also plan on going through my Introduction to Algorithms book again. I'll try to summarize what I learn as I go.

Well, time to get started!

Ubuntu: Out of Disk Space

Problem

When I first installed Ubuntu on my desktop, I had decided I would like to partition my /home directory from the rest of my filesystem. This way, if an Ubuntu upgrade fails, I can always perform a clean install without having to backup my data. Or, theoretically, I could install another OS and my data would all still be there. When I partitioned the / directory, I decided that 10 GB should be enough.

Well, recently, I've been hitting up against that 10 GB limit. Every day when I boot up my computer, I get a warning that I only have a few hundred MB of space left. Well, today, I'm at 100% usage. I couldn't even send an email out from Thunderbird.

So, I tried clearing space. I tried running

sudo apt-get autoremove

Unfortunately, there was nothing left to autoremove.

I then browsed through /usr/local as that's where I store any applications I need to compile and run. Again, nothing extra to remove.

My last resort was heading to /tmp to clear anything there. However, it didn't make much of a difference.

I was still stuck.

Solution

Suddenly, a thought came to me. I've gone through several Ubuntu updates where it installed Linux kernel headers. Maybe that's what was taking up space. After a quick Google search, I came upon a post that would delete all unused Linux headers.

Just to be safe, before running this, restart your machine.

dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge

This ran in the terminal for some time while it cleaned out all my unused Linux kernel headers. I ended up getting rid of 3 GB of data.

I was free again!

So next time you're running out of room, make sure it isn't some old Linux headers taking up space.

References

http://ubuntugenius.wordpress.com/2011/01/08/ubuntu-cleanup-how-to-remove-all-unused-linux-kernel-headers-images-and-modules/

Ubuntu 12.04: No Root File System Is Defined

Problem

I got this error when trying to do a fresh install of Ubuntu 12.04 from USB. I get past the screen that asks if you want to download updates and install third party software, then I get stuck on the "Installation Type" screen.

The installer only sees /dev/sdb which is my USB and shows nothing in the partitions table. All the buttons are disabled. Clicking continue gives me the error "No root file system is defined".

Instead of installing, I went to "Try Ubuntu". This loaded up the Ubuntu desktop. Checking gparted and fdisk, I was able to find /dev/sda. Great! Unfortunately, trying to install gives me the exact same problem.

Solution

When you "Try Ubuntu", open up a terminal and type

sudo apt-get remove dmraid -y

It gave me some errors, but I ignored them. I ran the installer from the desktop and voila! The installer sees /dev/sda and I could install Ubuntu 12.04

References

http://askubuntu.com/questions/111926/installation-stuck-on-installation-type-screen

Ubuntu: Automating Locking and Suspending

Locking and Suspending from the Terminal

The first thing to note is that suspending the computer does not lock the computer. They are separate actions.

To lock the screen.

gnome-screensaver-command --lock

To suspend the computer, check out this article on How To Suspend Ubuntu from the Terminal. If you're simply running this from the terminal either by typing or using a script, I would suggest method 3 as it does not require sudo.

gnome-screensaver-command --lock
dbus-send --print-reply --system --dest=org.freedesktop.UPower /org/freedesktop/UPower org.freedesktop.UPower.Suspend

Locking and Suspending from cron (Problems)

Note: If you don't care about the problems and just want the solution, skip down to the next section.

If you had the above in a script that you could run from the terminal, it follows that you could run this script from a cron. Perhaps the scenario is that you wanted to lock down and suspend a particular computer at work every day at 5pm.

The script would look like this.

#!/bin/sh
gnome-screensaver-command --lock
dbus-send --print-reply --system --dest=org.freedesktop.UPower /org/freedesktop/UPower org.freedesktop.UPower.Suspend

Let's say it was stored in ~/bin/lock-suspend.sh. Remember to make sure that it's executable.

chmod +x ~/bin/lock-suspend.sh

When you run this in the terminal, your computer locks and suspends. Everything is fine.

Next, you would edit your crontab.

crontab -e

And place the following in it. The output from standard out and error is logged to a log file.

0 5 * * * /home/user/bin/lock-suspend.sh > /home/user/logs/lock-suspend.log 2>&1

You then wait for 5pm to come around and lo-and-behold, nothing happens.

Knowing that cron logs to /var/syslog/log, you go and check it and find that the cron job ran, but an error occurred.

Since the script was logged to /home/user/log/lock-suspend.log, you check it out and find the following errors.

** Message: Failed to get session bus: Command line `dbus-launch --autolaunch=364dc0936e296ffa0881a3e90000000a --binary-syntax --close-stderr' exited with non-zero exit status 1: Autolaunch error: X11 initialization failed.\n
Error org.freedesktop.UPower.GeneralError: not authorized

What does this mean?

After much Googling, I came across this thread. It explained that the first message came about because cron does not have access to the same environment as the terminal. More specifically, it is missing the environment variable DBUS_SESSION_BUS_ADDRESS.

I did not look much further into the second error though. I assume it is because cron does not have access to the dbus. This was easily solvable though as there are several methods to suspend, for instance method 2.

sudo pm-suspend

Locking and Suspend from cron (Solution)

In order to solve the first problem, we need to figure out what should go into the environment variable DBUS_SESSION_BUS_ADDRESS.

When you first login, DBUS_SESSION_BUS_ADDRESS is set for you. We can grab this and write it to file. Add the following to your .bashrc file.

set | grep DBUS_SESSION_BUS_ADDRESS > ~/.DBUS_temp

Now we need to modify the lock-suspend.sh script. However, before we do that, we need to think about the solution to the second problem. Since we'll be using sudo to run pm-suspend, the cron job must be run as root instead of as the current user. We will need to modify the script expecting root to be running the script.

Note that in order to set the DBUS_SESSION_BUS_ADDRESS environment variable, we use source. However, in order to use source, we must use bash instead of sh.

The full script.

#!/bin/bash
source /home/user/.DBUS_temp
export DBUS_SESSION_BUS_ADDRESS
su -c "gnome-screensaver-command --lock" user
/usr/sbin/pm-suspend

Now modify the root's crontab.

sudo crontab -e

And add the following.

0 5 * * * /home/user/bin/lock-suspend.sh > /home/user/log/lock-suspend.log 2>&1

Come 5pm, the computer locks and suspends gracefully and you can relax knowing that your data is safe and you're saving the planet by reducing power usage.

SSH Key Fingerprinting

After GitHub got hacked, I received an email informing me that my SSH keys may have been compromised. I needed to login to GitHub and confirm each SSH key.

The thing is, the SSH keys were being shown using their fingerprints. I've seen this before when logging into a server through SSH, but I've never actually had to generate one before.

Thank goodness for Google. Here's how.

ssh-keygen -lf 

References

• SSH fingerprinting

Ubuntu: Install from USB

Apparently, you can install Ubuntu through a USB stick if your BIOS can boot from USB. I though this was pretty neat and wouldn't have to deal with throwaway media like CDs and DVDs. Unfortunately, each time I tried, the computer would not boot from the USB. It just kept booting from the harddrive no matter what I did in the BIOS.

I finally managed to figure this one out. The first 2 computers just didn't boot from USB. No hints. Nothing. Googling this problem brought up nothing. The solutions were to re-download Ubuntu, check your USB, use a CD, try on another computer, etc.

Fortunately, the 3rd computer I used gave me "Missing Operating System". Bingo! I found something searchable. This brought up a nice blog article. A solution! And it works! Basically, the problem was in the creation of the bootable USB. I was using Ubuntu's Startup Disk Creator. Apparently, it doesn't properly wipe out the USB and repartition appropriately. This caused havoc. The idea is to wipe out the USB stick and repartition before using Startup Disk Creator. This solved my problem.

Check out the solution here: http://ubuntuliving.blogspot.com/2008/11/missing-operating-system-step-by-step.html

Ubuntu + iPhone: Upgrade/Restore

It's no secret. Ubuntu + iPhone is a pain in the ass! However, it just got a little less painful for me.

You see, the way I've been approaching this is to never plug my iPhone into my computer. If the two don't meet, there can't be any problems. Don't solve your problems. Get rid of them!

Unfortunately, I've been losing reception the last two weeks and it's been annoying me to no end. I called my service provider and they determined it's probably because I'm still on 3.1.3. Great. I need to upgrade my iPhone. iPhone, meet Ubuntu. Ubuntu, meet iPhone. Now PLEASE play nice.

Of course, that was too much to ask for. I installed VirtualBox and got Windows XP installed. I then added the iPhone as a USB connection in the settings. I then proceeded to plug in my iPhone. iTunes detects my iPhone. Yay! A small triumph. I decided to restore instead of upgrade. I wanted to start from scratch anyways.

After iTunes wiped out my data, it started to prepare for the restore. At which it hung. After a few minutes, it gave me a 1602 error. I tried everything I could think of: reboot iPhone, reboot VirtualBox, reboot Ubuntu, re-plug in iPhone, etc. The best I got out of all this was a different error: 1604. At this point, my iPhone is permanently in Recovery Mode and now, I couldn't even select the iPhone USB connection in VirtualBox.

I thought to myself, "Screw this. I'm fixing you even if it takes all night!" And so, I did. Thanks to this post.

Follow it. It works.

Here are the keypoints and some added tips from the comments.

• Add yourself to both the vboxusers and audio groups.
• Change the USB filter settings so that only Name and Vendor ID have values. DELETE EVERYTHING ELSE. It's not recommended. It's mandatory!
  

After all this, I was able to restore my iPhone without problems. Booyah!