Ubuntu 9.04 is Coming Soon! On April 23rd, 2009, the newest member of the Ubuntu family will be released and made available for download.
For specific details, check out Ubuntu 9.04
The Ubuntu 9.04 pre-release version is available for download and testing.
Databases are all about saving data. With DBMS and RDBMS, the entire data became relational and all the records became related to each other as in the real world. So came into existence the concepts of primary keys, foreign keys, foreign key constraints and whole bunch of other terms like composite keys, referential integrity, indexes and what not.
So coming back to the objective of pinning down this post, some days ago I came across the requirement of deleting some user records from the user table. As soon as I tried deleting the records, I came across the error of referential integrity where in I was greeted with the error that child records were there and hence the delete operation was not allowed.
So I searched for a shortcut which could let me do my task. And I came across this …
SET foreign_key_checks = 0;
DELETE FROM users where id > 45;
SET foreign_key_checks = 1;
By setting the foreign key check to 0, I was able to update / delete my users table. Once I was done with my operations on the user table, I reset the key check to 1 again and everything is back in place now.
You can always drop a FOREIGN KEY the usual way ..
ALTER TABLE users DROP FOREIGN KEY <foreign_key_name>
As correctly pointed out by tboehm in the comments, I disabled the foreign key checks for some requirement of mine. It is absolutely important to think about the repercussions of this. It is always advisable to start with the child entries and then proceed for the parents. The sole purpose of this article is to make you aware of the option which disables the foreign key constraints.
I guess other available options and arguments for ALTERING TABLES can be found here ..
Ciao!!!
It has been a long time since I posted my last blog post. So here I am ‘populating’ my post with some information that could be of some worth to me and others in need. So without much delay, let me put down the ad hoc article on setting up svn repository. It also talks about creating a structured hierarchy for code management and also gives information on adding projects to SVN repository.
To create svn repository svnadmin command line tool is used.
$ svnadmin /home/gaurav/new_repo
To add directories to the new repository …
$ svn mkdir file:///home/gaurav/new_repo/main_project
$ svn mkdir file:///home/gaurav/new_repo/main_project/branches
$ svn mkdir file:///home/gaurav/new_repo/main_project/tags
$ svn mkdir file:///home/gaurav/new_repo/main_project/trunk
When directories are added, it asks for the commit comments.
Once the repository structure is ready, import the code like this:
$ svn import /home/gaurav/my_new_app/ file:///home/gaurav/new_repo/main_project/trunk
Following command runs svn server @ port 4444 for localhost
-r is for the repository and should be passed the repository path
$ svnserve -d -r /home/gaurav/repository/ --listen-port 4444 --listen-host localhost
Once svn server starts as a daemon, checkout can be done as:
$ svn co svn://localhost:4444/momsworld/trunk mw_0718
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Add users to your server
In order to authenticate our intended users, some changes are required to be made. Inside the new repo (c:\svn-repo or /home/gaurav svn-repo), there is a directory called conf. There you will find svnserve.conf.
You need to enable these lines in the file:
[general]
anon-access = read
auth-access = write
password-db = passwd
All other comments can be left untouched. For easy readability, I have only put the required section above.
With this setting, anyone would be able to read the files but only an authenticated user would be able to checkin. Also password-db tells server that a file named ‘passwd’ would be storing usernames and passwords.
Let’s create that file now. It should also be in the conf directory and look like this:
[users]
david = secret
Similarly many users could be added in the format NAME = PASSWORD. There are many other advanced ways to add password protection to Subversion but this one is the easiest I suppose. Subversion has good integration with Apache’s web server authentication so that could be utilized as well.
Cron task is used to automate tasks in Unix/Linux. It is something that you need to trigger commands / scripts @ specific interval of time. It is sort of task scheduler that windows has. But it is much more customizable and is command line based. So lets see how can we add cron tasks.
Lets say we have a shell script that displays nothing but todayz date. It is named date_today.sh and for some reason you want to run it every day @ 00.00 hrs.
The first thing you should know about cron task is the format in which you specify the time for automating your tasks. There are five fields to set the time.
* * * * * <command to execute>
You need to set the five stars as per your requirement. These 5 *s are five fields for
1. minute (0-59)
2. hour (0-23)
3. day (1-31)
4. month (1-12)
5. day of week (0-6, 6 being Sunday)
6. command to execute
So now to execute our date_today.sh file, we need to add the cron task to the system. To do that issue this command:
$ crontab -e
Depending on your login, your cron tasks will differ. So once you are presented with your editor to edit the file, enter the following line:
0 * * * * /home/user_name/date_today.sh
This will make sure that your shell script runs everyday @ midnight. If you want to run it every 5 minutes, use this instead:
*/5 * * * * /home/user_name/date_today.sh
or
0 1 29 1 1 /home/user_name/date_today.sh
will help you setup the execution on 1.00 AM, 29th January, Monday. You could also use this for same result….
0 1 29 jan mon /home/user_name/date_today.sh
Once you have entered the time, exit and wait for it to be executed. 
Cron tasks can be used for several purposes like backing up data, deleting logfiles / temporary files, copying data etc. So unless you have a lot of time to spend in front of your computer for the right moment to trigger commands on your own, I think you shud be using cron tasks.
