linux, Mariadb, mysql, Ubuntu

How I screwed MySQL install with MariaDB and recovered

Broken packages are mess! I recently installed MariaDB to give it a try. After testing, I decided to uninstall it for now and maybe come back later. I was careless while removing MariaDB and ended up messing MySQL configs (MariaDB runs on top of MySQL). Things were only to get worse from here. A high priority task pops up that requires me to access MySQL. Unable to access it, I decide to completely remove MySQL and do a fresh install – basically a hard reset. If only things were that simple in real world. By this time my MariaDB was half removed, MySQL configs were a mess and I was now about to remove MySQL completely. Here’s what happened:

Alright, maybe I did not uninstall MariaDB correctly. Lets try removing that once more:

Same error! Ok, I clearly screwed MySQL. I should try installing it back. Lets see how that goes:

Unmet dependencies? Ok! I know what to do in such cases:

Nothing! That’s scary. There’s no way forward (installing) and there’s no way backward (uninstalling). I am basically stuck!

Maybe autoclean is a good idea? Sadly no difference.

I noticed both MySQL and MariaDB remove commands were exiting because of ‘mysql-server-5.6’, so fixing it was my best bet. I also noticed the error raised while removing mysql-server was from file `/var/lib/dpkg/info/mysql-server-5.6.postrm` as evident in following line:

/var/lib/dpkg/info/mysql-server-5.6.postrm: line 53: /usr/share/mysql-common/configure-symlinks: No such file or directory

dtdm

So what if I comment the line that’s raising this error? Risky, but desperate times – desperate measures. After creating a backup of the file, I edited it to remove the line that raised the error and retried. Here’s what happened :

Finally! Editing the file worked and I was able to remove both packages. Next I fresh installed mysql-server and it worked perfectly. What a relief! 🙂

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linux

Creating, Exploding and Defusing – The Fork Bomb

A fork bomb is a simple, Japanese emoticon looking, notorious, malicious, destructive, yet beautiful code. The motive is simple – keep on eating the system resources until the system runs out of resources and crashes.

In unix and likewise systems fork is an operation where process creates a copy of itself. Basically fork is a primary source of process creating in unix(and likewise OS).

 

The BOMB

:(){ :|:& };:

If you try this on your terminal, your system may stall and eventually crash, proving the bomb did its work (i.e. exploded) successfully. This kind of attack is called Denial-of-Service attack. As the name suggests, the attack intends to make all the resources busy or unavailable to the authorized user, hence interrupting or suspending the service.

 

Logic_bombs_are_harmful_to_your_computer.

Understanding the Bomb

This bomb is essentially a function calling itself recursively and the pipeline is made background. That said, lets visualize our code in this format:

:()          # Define a function
{ :|:& };    # The body of the function
:            # Invoking the function

If its still not clear to you, here’s the thing, our function name is ‘:’. Lets replace it with something else, say bomb, to make our code more readable.

bomb() {
bomb | bomb&
};
bomb

The trick here lies in invoking the process recursively and putting it in background. The ampersand operator is doing that for us. Since the function is being called recursively it will keep on creating new processes  and background them which will eventually eat all the resources and crash the system. If we do not put the process in background and just keep calling it recursively, it will still mess up the system but all the processes will immediately end after a stack overflow, which any system can deal with easily. Making the process background, however makes it immortal.

 

Defusing the bomb

The problem with this program is that even if you get all the PIDs and run a kill command, by the time the command executes many new child process will get created, which makes it difficult to stop.

One way is to freeze all the processes and then running a kill command. This can be done as follows:

killall -STOP processWithBombName   # Freezes all the notorious processes
killall -KILL processWithBombName    # Kills all freezes processes

However, there’s obviously a time constraint after which nothing can be done except a reboot!

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wget pause resume
linux

Pause/Resume Downloads With ‘wget’

You have to admit when it comes to downloading, nothing beats wget. However almost every time I say this I find someone complaining about a pause option. So here’s a little less known trick for beginners.

press 'Ctrl + C' to pause the download which you started normally using something like this:
wget some_random_archive_file.zip

Now to resume it go to the same location where you were downloading and wget the download file with -c option.
wget -c some_random_archive_file.zip

And your download will resume from where it was paused. Enjoy! 🙂

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