In this howto, we program a simple database application. It reads data from a database, then prints it on a web page.
Techniques teached here are used for building guestbooks, web shops, web forums and group calendars.
You only need to have CentOS installed. During the tutorial, we install Apache web server, MySQL database and PHP scripting. Finally, we write a simple php database application.
(c) Tero
Karvinen
Updated for CMU 95-799 Linux class by Dennis Moul
First, take snapshot of the CentOS VM so you can revert back to a known good state if you run into problems.
For these steps, you first have to become root. Use su - (with the dash) to "switch user" to root:
su -
systemctl disable firewalld
vi /etc/sysconfig/selinux
SELINUX=enforcing" to read "SELINUX=disabled". Make sure to get this exactly right, some students have had their CentOS fail to boot properly if they messed this up.
Now you have to restart your VM to pick up the changes. You can do that via the upper-right-corner menu Power button, or via the command line with "shutdown -r now".
To install servers, you first have to become root. Use su
- (with the dash):
su -
Install the actual web server program (may be preinstalled, depending on OS version)
yum install httpd
Start web server, make it start automatically on boot
systemctl start httpd
systemctl enable httpd
Now your web server should be running. When starting Apache, you may get an error like "apr_sockaddr_info_get() failed, could not reliably determine the server's fully qualified domain name". This happens because we never set up a proper real domain name for our VM, and is nothing to worry about.
Now start Firefox in your VM and access
http://localhost. You should see an Apache test page. If
you do not
see your test page right away, click shift-reload (ctrl-shift-R or
shift-F5) to bypass the cache.
Create a simple initial page instead of the system default:
echo "powered by linux" >
/var/www/html/index.html
access
http://localhost with Firefox again, If you still get the old page, try clicking shift-reload. If you get a permissions error, examine the file permissions on the index.html file you created.
Now, to access the web server running in your CentOS virtual machine from your host OS, we need to look up your virtual machine's IP address on the virtual network it shares with VirtualBox:
/sbin/ifconfig
Look for the "inet" value for the enp0s8
network interface (which you created during the install as Network Adapter 2, type Host-Only) and note the IP address, which is usually 192.168.56.X (your X may vary). If there's no IP address listed, make sure the enp0s8 interface is active - click on the battery/sound icon in the upper right corner of the CentOS desktop and look to see if enp0s8 is "connected" or "off". If it's off, select that and choose "Connect."
If you still can't get it to connect and show an IP address in the /sbin/ifconfig output, see the troubleshooting steps listed in section 8.a. of Lab 3 part 2 (samba). Also consult the forums and TA(s) for assistance in getting your Virtualbox Host Only Network working properly.
In your host OS, open your browser
and enter that address, for example http://192.168.56.101.
If you see your test page, congratulations, you just remotely accessed your
web server!
If it's working locally in the virtual machine but not remotely from your host OS, check the following:
getenforce; it should return "permissive" or "disabled".systemctl status firewalld should return "Active: inactive".
Also check iptables -L -v | grep -vi vir and verify that the rule chains are all empty. If they aren't, clear them with iptables -F and check again.Web server content directory /var/www/html/ is not
writable for normal users, and you usually don't want to edit web pages as
root. That's why users create homepages
in /home/user/public_html instead. Homepages are shown in
http://localhost/~user
Edit web server configuration with vi.
vi /etc/httpd/conf.d/userdir.conf
UserDir disable
and remove the comment char "#" from the line
UserDir public_html
Activate your changes by restarting the web server.
systemctl restart httpd
Your users can now create homepages.
Creating the homepage should be done as a normal non-root user. The easiest way is to just open a 2nd Terminal window, and run that as your normal user, then you can switch back and forth between the root Terminal window and the normal user Terminal as needed. You can tell them apart by the prompt, "#" for root and "$" for normal user.
If you logged in originally as root (usually not recommended), you can “su” (set user) to the non-root account you created during CentOS installation by using “su – userid”.Create a directory for homepages. Home directory, public_html and all directories under public_html must be executable (x) by all, so that web server can access files under them if it knows their name. Files must be readable by the web server.
cd ~
mkdir public_html
echo "my homepage" > public_html/index.html
chmod a+x ~ ~/public_html
chmod a+r ~/public_html/index.html
http://localhost/~user. If
you can see "my homepage", you are now a happy owner of a homepage.
PHP is a powerful scripting language with a C++ like syntax, many readymade classes, good examples and great documentation.
yum install php
Because php is an Apache module, you must restart the web server to load it.
systemctl restart httpd
As a normal user, write a sample php page
cd ~/public_html
vi hello.php
Write this sample code to hello.php
<?php echo "Hello PHP World, 2+2 is " . (2+2); ?> <br>
Text outside code block is printed normally to web page.<p>
<?php phpinfo(); ?>
Usually there is a normal html web page outside the <?php
code ?>.
Browse to http://localhost/~user/hello.php. Put your own
login name instead of user. If you see "Hello PHP World, 2+2
is 4" and a big list of PHP configuration data, you have installed PHP and written your first program.
yum install mariadb-server
systemctl start mariadb
systemctl enable mariadb
You may get some warnings when MariaDB starts up. For the purposes of this lab you can ignore those, but if you were building a production system you should tighten up the security configuration.
Try your new SQL server
mysqlshow
+--------------------+ | Databases | +--------------------+ | information_schema | | mysql | | performance_schema | +--------------------+
Three databases. mysql and the *_schema tables contain internal data, so don't mess with them. Let's create one for ourselves:
mysql -e 'create database test'
mysqlshow test
Database: test
+--------+
| Tables |
+--------+
+--------+
No tables exist in database test yet. Let's CREATE
some. In your root terminal, start the command-line mysql tool in which to run SQL commands:
mysql
On the prompt mysql>, you can type mysql commands
(USE, SHOW) or sql queries (CREATE, INSERT, SELECT).
USE test;
SHOW tables;
CREATE TABLE persons( name VARCHAR(50), email VARCHAR(50) );
SHOW tables;
DESC persons;
INSERT INTO persons VALUES('Tero Karvinen', 'karvinen at-sign iki.fi');
SELECT * FROM persons;
INSERT INTO persons VALUES('Sample Person', 'recycle@nosuch.invalid');
SELECT * FROM persons;
QUIT;
After the two INSERTs, the last SELECT should
return a table with two records
+---------------+---------------------------+
| name | email |
+---------------+---------------------------+
| Tero Karvinen | karvinen <at-sign> iki.fi |
| Sample Person | recycle@nosuch.invalid |
+---------------+---------------------------+
2 rows in set (0.01 sec)
You have now installed a database management system, and you also know some SQL.
A database by itself is not very useful - it needs a user interface. To create a web interface to database, we use the PHP scripting language and its module (which we installed in Lab 2) to communicate to the database.
yum install php-mysqlnd
systemctl restart httpd
Let's write a simple program to display our database. The database.php script shown here can be cut-and-pasted, or downloaded as http://linuxclass.heinz.cmu.edu/labs/database.php to reduce the likelihood of typing errors. Use right click->”Save Link As” to download the file, otherwise the tags may get garbled. You may need to change the permissions on the downloaded file for it to be accessible via browser. Put the file in your public_html directory and name it database.php.
PHP database example<br>
<?php
/* database.php - Use mysql database from php */
/* Connect to database */
$conn = new mysqli("localhost", "root", "", "test")
or die("Could not connect : " . $conn->connect_error);
print "Connected successfully";
/* Perform SQL query */
$result= $conn->query("SELECT * FROM persons")
or die("Query error : " . $conn->error);
/* Print results in HTML */
print "<table>\n";
while ($line = mysqli_fetch_array($result, MYSQLI_ASSOC)) {
print "\t<tr>\n";
foreach ($line as $col_value) {
print "\t\t<td>$col_value</td>\n";
}
print "\t</tr>\n";
}
print "</table>\n";
/* Clean up */
$result->close();
$conn->close();
?>
Browse to http://localhost/~user/database.php. Use your
own login name instead of user. You should see:
PHP database example
Connected successfully
Tero Karvinen karvinen iki.fi
Sample Person recycle@nosuch.invalid
If you see the two email addreses from the database, congratulations! You have successfully completed every part of this tutorial.
Copyright 2003-09-25, 2003-10-02, 2003-11-12 (Minor consistency edits, links), 2003-11-22 (minor copyedit), 2004-05-17 (updated links) 2005-02-11 (tested on Fedora Core 3, SELinux) Tero Karvinen. All rights reserved.
Updated for CMU 95-799 Linux class and recent CentOS by Dennis Moul.