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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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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System administrator

Reading File Permissions in Linux

Linux is based on the idea that the same system is going to be used by different users. With different users accessing the same system, there arises the need of permission. Think of it like this, you are sharing a room with different person/people in a hostel. There are a few things that are common to all, say table, chair, etc. But you still need some personal space (most likely your cupboard in this analogy) where you keep things that are accessible only by you.

Same concept can be applied in case of files present in your linux system. There are different users who can locate these files, but depending upon the type of permission those users can read them or modify them.

To check the permissions of files in a particular folder, run this:
ls -l
You will get something like this as output:
-rw-rw-r-- 1 shivam users 4004 May 8 20:19 CONTRIBUTING.md

Lets understand it backwards i.e right to left.
CONTRIBUTING.md: is the name of the file.
May 8 20:19: represents the time this file was last modified/created.
4004: is the size of the file in bytes.
users: is the group of users to which this file belongs.
shivam: is the owner of the file.
1: is the number of files.

Now comes the file permissions.
-rw-rw-r--
starting ‘-‘ means its a file. Had it been a directory, you would have found a ‘d’.
Now, ‘rw’ represents the permissions of the owner. In this case the owner has both read and write permissions.
Second ‘rw’ represents the permission of group of users. Here as you can see even the group users have both read and write permissions(not quite secure, eh?).
Finally the ‘r’ is the permission for everyone else, i.e. they can only read the file and cannot edit it.

Changing the permissions
You can change the permissions of the file using the chmod command which changes the permission for the site.
For example.
sudo chmod 0600 CONTRIBUTING.md

Now if you do ls -l, you will get
-rw------- 1 shivam shivam 4005 May 24 22:47 CONTRIBUTING.md
meaning only the owner had read and write permissions. Other cant even read it.

Different file permissions and there meaning are as follows:

Value Meaning
777 (rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.
755 (rwxr-xr-x) The file’s owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.
700 (rwx——) The file’s owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.
666 (rw-rw-rw-) All users may read and write the file.
644 (rw-r–r–) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.
600 (rw——-) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.
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mysql

Moving complete mysql from one server to another without mysqldump

Whenever it comes to transfer database (mysql) from one server to other, the first obvious choice that comes to one’s mind is mysqldump. All you need to do is run the command:

mysqldump -u root -p --opt [database name] > [database name].sql

It will output a sql file for your database. Scp the sql file to your new servers, create a database with the same name in the new machine and run this:

mysql -u root -p newdatabase < /path/to/newdatabase.sql

All is well unless your database is of limited size. But suppose your database is in GBs meaning you’re f’ckd!! This method will still work but it will take years (not literally) to generate all the tables and fields in the new database using the dumped sql file. Also it will eat up a lot of resources on your new server.

The alternative is rather way too simple. All you need to do is go to your mysql data directory. If you are using ubuntu, most probably you are looking for this location:

/var/lib/mysql/

Before proceeding ahead, you may want to stop your mysql server. Just run this:

sudo service mysql stop

Also as we are going to access files that need root permissions, better do:

sudo bash

We will now zip all the contents of the mysql data directory.

zip -r completemysql.zip /var/lib/mysql/*

This is will create a zip file containing all you mysql data. Now scp this to the new servers (or machine).
Once you ssh to your new server with the zip file already t transferred in its home. Unzip it in a directory(we name it ‘database’)

unzip completemysql.zip -d database

Now delete all the log files from the unzip location ( ~/database in our case).
It should look something like:

rm ib_logfile0 ib_logfile1

Before proceeding ahead we need to stop the mysql server. Run:

sudo service mysql stop

to confirm the mysql server stopped do:

mysqladmin -u root -p status

If this gives an error, it means the mysql server is not running, which is what we want in our case.
Now copy all the files from ~/database directory to the mysql data directory of this machine.

cp -R ~/database/* /var/lib/mysql/

We are almost there, if at this moment you start your mysql server and try show databases it will list all your databases from the previous server. However when you try to access them, you may not be able to do so and get following permission error :

ERROR 1018 (HY000): Can't read dir of './<your_database_dir>/' (errno: 13)

This is because the owner of the copied files is the ‘root’ and we need to make it ‘mysql’. We can resolve this by:

chown -R mysql:mysql /var/lib/mysql/
chmod -R 755 /var/lib/mysql/

Now start the mysql server and its done.

sudo service mysql start

cheers!!

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