mkdir and rmdir

You can create and remove directories using the mkdir and rmdir system calls.

#include <sys/types.h>
#include <sys/stat.h>
int mkdir(const char *path, mode_t mode);


The mkdir system call is used for creating directories and is the equivalent of the mkdir program. mkdir
makes a new directory with path as its name. The directory permissions are passed in the parameter
mode and are given as in the O_CREAT option of the open system call and, again, subject to umask.

#include <unistd.h>
int rmdir(const char *path);

The rmdir system call removes directories, but only if they are empty. The rmdir program uses this
system call to do its job.

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The Shell as a Programming Language

Now that you’ve seen some basic shell operations, it’s time to move on to some actual shell programs. There
are two ways of writing shell programs. You can type a sequence of commands and allow the shell to execute
them interactively, or you can store those commands in a file that you can then invoke as a program.
Interactive Programs
Just typing the shell script on the command line is a quick and easy way of trying out small code fragments,
and is very useful while you are learning or just testing things.

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What Is a Shell?

Before jumping in and discussing how to program using a shell, let’s review the shell’s function and the
different shells available for Linux. A shell is a program that acts as the interface between you and the Linux
system, enabling you to enter commands for the operating system to execute. In that respect, it resembles the
Windows command prompt, but as mentioned earlier, Linux shells are much more powerful. For example,
input and output can be redirected using < and >, data piped between simultaneously executing programs
using |, and output from a subprocess grabbed by using $(...). On Linux it’s quite feasible to have multiple
shells installed, with different users able to pick the one they prefer.

You can check the version of bash you have with the following command:
$ /bin/bash --version
GNU bash, version 3.2.9(1)-release (i686-pc-linux-gnu)
Copyright (C) 2005 Free Software Foundation, Inc.

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RSVP

Resource Reservation protocol is used to reserve resources across a network. It is used for requesting a specific Quality of Service (QoS) from the network.

RSVP is a signalling protocol for the internet. RSVP stands for bandwidth 
reservation protocol. This protocol allows application to reserve bandwidth for their 
data flows.

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What is the use of PEAR in php?

PEAR is known as PHP Extension and Application Repository. It provides structured library to the PHP users and also gives provision for package maintenance.

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Create the Other Pages

Now that I have my basic page structure down, it’s time to start looking at the other
pages in my site. Working with the styles I’ve already created (and creating new ones
as I need to), I’ll create the following pages in my layout:
• Blog listing, with the sidebars as used in my two-sidebar layout
• Category page, based on the blog listing page
• Project page, with associated images and text
• Project listing, with images and a brief project description
• Contact page
• A 404 (page not found) and 403 (access denied) page
• The home page, with associated blocks and callout areas
This should cover most of the pages that I will be setting up in the Drupal implementation,
and give me more than enough to work with. Many of the pages will feed each
other—for example, my blog listing page will start with my two-sidebar layout, and
change the listing, and then the category page will follow from that. However, the
project pages, being highly image/case study focused, will require special treatment,
including putting some thought into how I’m going to organize the projects,

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Single Node Pages with Sidebars

The point of starting off your template with a node page that doesn’t have sidebars is
this: you will inevitably have a page like this somewhere on your site. And many designers,
well-meaning as they are, end up forgetting this and assume there will be 1–2 sidebars
on the page. As Drupal’s default behavior reflows the text to fill the entire page when
there are no sidebars, this results in these pages having long and drastically uncomfortable
line lengths.
That said, it’s safe to assume that most pages will have at least one sidebar, and that
the sidebars will contain different types of blocks, for example:
• A list of node titles or categories
• Static text or images
• A tag cloud or something similar
• Callout boxes, like a contact form or customer testimonial
Therefore, while I’m working on my node pages, I should also take a look at how these
different types of sidebar blocks will be styled, and how I’ll set up both one- and twosidebar
layouts.

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Storing Complex Data Types

You can use sessions to store complex data types such as objects and arrays simply by treating them as standard variables, as this code shows:

    $myarr["0"] = "Sunday";
    $myarr["1"] = "Monday";
    $myarr["2"] = "Tuesday";
    $myarr["3"] = "Wednesday";
    $myarr["4"] = "Thursday";
    $myarr["5"] = "Friday";
    $myarr["6"] = "Saturday";

    $_SESSION["myarr"] = $myarr;

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Cookies Versus Sessions?

Cookies The setcookie( ) call needs to be before the HTML form because of the way the web works. HTTP operates by sending all "header" information before it sends "body" information. In the header, it sends things like server type (e.g., "Apache"), page size (e.g., "29019 bytes"), and other important data. In the body, it sends the actual HTML you see on the screen. HTTP works in such a way that header data cannot come after body datayou must send all your header data before you send any body data at all. Cookies come into the category of header data. When you place a cookie using setcookie( ), your web server adds a line in your header data for that cookie. If you try and send a cookie after you have started sending HTML, PHP will flag serious errors and the cookie will not get placed. There are two ways to correct this: Put your cookies at the top of your page. By sending them before you send anybody data, you avoid the problem entirely. Enable output buffering in PHP. This allows you to send header information such as cookies wherever you likeeven after (or in the middle of) body data. Output buffering is covered in depth in the following chapter. The setcookie( ) function itself takes three main parameters: the name of the cookie, the value of the cookie, and the date the cookie should expire. For example:   setcookie("Name", $_POST['Name'], time( ) + 31536000);   In the example code, setcookie( ) sets a cookie called Name to the value set in a form element called Name. It uses time( ) + 31536000 as its third parameter, which is equal to the current time in seconds plus the number of seconds in a year, so that the cookie is set to expire one year from the time it was set. Once set, the Name cookie will be sent with every subsequent page request, and PHP will make it available in $_COOKIE. Users can clear their cookies manually, either by using a special option in their web browser or just by deleting files. print $_COOKIE["Name"];  Sessions Sessions store temporary data about your visitors and are particularly good when you don't want that data to be accessible from outside of your server. They are an alternative to cookies if the client has disabled cookie access on her machine, because PHP can automatically rewrite URLs to pass a session ID around for you.  Starting a Session A session is a combination of a server-side file containing all the data you wish to store, and a client-side cookie containing a reference to the server data. The file and the client-side cookie are created using the function session_start( ) it has no parameters but informs the server that sessions are going to be used. When you call session_start( ), PHP will check to see whether the visitor sent a session cookie. If it did, PHP will load the session data. Otherwise, PHP will create a new session file on the server, and send an ID back to the visitor to associate the visitor with the new file. Because each visitor has his own data locked away in his unique session file, you need to call session_start( ) before you try to read session variablesfailing to do so will mean that you simply will not have access to his data. Furthermore, as session_start( ) needs to send the reference cookie to the user's computer, you need to have it before the body of your web pageeven before any spaces.  Adding Session Data All your session data is stored in the session superglobal array, $_SESSION, which means that each session variable is one element in that array, combined with its value. Adding variables to this array is done in the same way as adding variables to any array, with the added bonus that session variables will still be there when your user browses to another page. To set a session variable, use syntax like this: $_SESSION['var'] = $val; $_SESSION['FirstName'] = "Jim"; Older versions of PHP used the function session_register( ); however, use of this function is strongly discouraged, as it will not work properly in default installations of PHP 5. If you have scripts that use session_register( ), you should switch them over to using the $_SESSION superglobal, as it is more portable and easier to read. Before you can add any variables to a session, you need to have already called the session_start( ) functiondon't forget!

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$_ENV and $_SERVER ?

PHP sets several variables for you containing information about the server, the environment, and your visitor's request. These are stored in the superglobal arrays $_ENV and $_SERVER, but their availability depends on whether the script is being run through a web server or on the command line.

Useful preset variables in the $_SERVER superglobal

Name	Value
HTTP_REFERER	If the user clicked a link to get the current page, this will contain the URL of the previous page, or it will be empty if the user entered the URL directly.
HTTP_USER_AGENT	The name reported by the visitor's web browser.
PATH_INFO	Any data passed in the URL after the script name.
PHP_SELF	The name of the current script.
REQUEST_METHOD	Either GET or POST.
QUERY_STRING	Includes everything after the question mark in a GET request. Not available on the command line.

Of those, HTTP_REFERER and HTTP_USER_AGENT are the most important, as you can use these two to find out a lot about your visitor and then take the appropriate action. For example:

<?php
            if (isset($_SERVER['HTTP_REFERER'])) {
          print "previously was {$_SERVER['HTTP_REFERER']}

      <br />";
            } else {
              print "You didn't click any links to get here<br />";
            }
    ?>

    <a href="refer.php">Click me!</a>

If you load that page in your browser by typing the URL in by hand, the "You didn't click any links to get here" text is shown because HTTP_REFERER has not been set. However, if once the page is loaded you follow the "Click me!" link, the page will reload itself; this time, HTTP_REFERER will be set and the other message should appear. Although it can be easily spoofed, HTTP_REFERER is generally a good way to make sure a visitor came from a certain pagewhether you want to use that to say, "You can't download my files because you came from another site" or "Welcome, Google users!" is up to you.

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