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SSH trick to improve your system security

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This ASCII-art graph represents your host’s public key, which uniquely identifies (or at least, it better damn should uniquely identify!) it. This representation was added mainly because it is way easier to be able to visually record the shape of your most frequently used hosts.

Enable visualization for hostkeys:

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id@acer:~$ cat .ssh/config 
VisualHostKey   yes
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id@acer:~$ ssh root@192.168.1.3
Host key fingerprint is b6:dc:cb:ff:33:83:aa:03:56:94:0c:bc:7f:38:cf:b7
+--[ RSA 2048]----+
|     ..o .       |
|      . +        |
|       o         |
|      . .        |
|       oS.       |
|      oo+o.      |
|     . .o=.  .   |
|        ..o.o +  |
|        .o++Eo.+ |
+-----------------+
root@192.168.1.3's password: 
Linux piper 2.6.26-2-xen-686 #1 SMP Mon Jun 21 10:37:05 UTC 2010 i686

Star Wars anthem – pc speaker

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Really imperial march through the motherboard speaker:

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aptitude update ; aptitude install beep

And now execute this fun:

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beep -l 350 -f 392 -D 100 --new -l 350 -f 392 -D 100 --new -l 350 -f 392 -D 100 --new -l 250 -f 311.1 -D 100 --new -l 25 -f 466.2 -D 100 --new -l 350 -f 392 -D 100 --new -l 250 -f 311.1 -D 100 --new -l 25 -f 466.2 -D 100 --new -l 700 -f 392 -D 100 --new -l 350 -f 587.32 -D 100 --new -l 350 -f 587.32 -D 100 --new -l 350 -f 587.32 -D 100 --new -l 250 -f 622.26 -D 100 --new -l 25 -f 466.2 -D 100 --new -l 350 -f 369.99 -D 100 --new -l 250 -f 311.1 -D 100 --new -l 25 -f 466.2 -D 100 --new -l 700 -f 392 -D 100 --new -l 350 -f 784 -D 100 --new -l 250 -f 392 -D 100 --new -l 25 -f 392 -D 100 --new -l 350 -f 784 -D 100 --new -l 250 -f 739.98 -D 100 --new -l 25 -f 698.46 -D 100 --new -l 25 -f 659.26 -D 100 --new -l 25 -f 622.26 -D 100 --new -l 50 -f 659.26 -D 400 --new -l 25 -f 415.3 -D 200 --new -l 350 -f 554.36 -D 100 --new -l 250 -f 523.25 -D 100 --new -l 25 -f 493.88 -D 100 --new -l 25 -f 466.16 -D 100 --new -l 25 -f 440 -D 100 --new -l 50 -f 466.16 -D 400 --new -l 25 -f 311.13 -D 200 --new -l 350 -f 369.99 -D 100 --new -l 250 -f 311.13 -D 100 --new -l 25 -f 392 -D 100 --new -l 350 -f 466.16 -D 100 --new -l 250 -f 392 -D 100 --new -l 25 -f 466.16 -D 100 --new -l 700 -f 587.32 -D 100 --new -l 350 -f 784 -D 100 --new -l 250 -f 392 -D 100 --new -l 25 -f 392 -D 100 --new -l 350 -f 784 -D 100 --new -l 250 -f 739.98 -D 100 --new -l 25 -f 698.46 -D 100 --new -l 25 -f 659.26 -D 100 --new -l 25 -f 622.26 -D 100 --new -l 50 -f 659.26 -D 400 --new -l 25 -f 415.3 -D 200 --new -l 350 -f 554.36 -D 100 --new -l 250 -f 523.25 -D 100 --new -l 25 -f 493.88 -D 100 --new -l 25 -f 466.16 -D 100 --new -l 25 -f 440 -D 100 --new -l 50 -f 466.16 -D 400 --new -l 25 -f 311.13 -D 200 --new -l 350 -f 392 -D 100 --new -l 250 -f 311.13 -D 100 --new -l 25 -f 466.16 -D 100 --new -l 300 -f 392.00 -D 150 --new -l 250 -f 311.13 -D 100 --new -l 25 -f 466.16 -D 100 --new -l 700 -f 392

What does this “> /dev/null 2>&1″ mean?

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I remember being very confused for a very long time about the trailing garbage in commands that I saw in Linux systems.

Now I can explain it to you with a very easy example.

Standard in, out, and error

There are 3 standard sources of input and output for a program. Standard input usually comes from the keyboard if it’s an interactive program, or from another program if it’s processing the other program’s output. The program usually prints to standard output, and sometimes prints to standard error. These three file descriptors (you can think of them as “data pipes”) are often called STDIN, STDOUT and STDERR

Sometimes they’re not named, they’re numbered! The built-in numberings for them are 0, 1, and 2, in that order. By default, if you don’t name or number one explicitly, you’re talking about STDOUT.

Given that context, you can see the command above is redirecting standard output into /dev/null, which is a place you can dump anything you don’t want (often called the bit-bucket), then redirecting standard error into standard output (you have to put an & in front of the destination when you do this).

Will create a new directory

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mkdir test

if you execute it second time you will get a error.
With this redirection you will suppress the error.

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mkdir test > /dev/null 2>&1

The short explanation, therefore, is “all output from this command should be shoved into a black hole.” That’s one good way to make a program be really quiet! That means, if you have some critical errors in your code, you will never see them.

MySQL – Database size

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As I know, there are two ways to calculate the MySQL database size:

First one:

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du -shc /var/lib/mysql/*

Second one:

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SELECT table_schema ,SUM( data_length + index_length) / 1024 / 1024 "Data Base Size in MB" FROM information_schema.TABLES GROUP BY table_schema;