Perl File Handling

In PERL files are given a name, also called handle. All the input and output of the file is achieved by filehandling functions. Filehandles are also a means to communicate from one program to another program.

How to assign handles

A filehandle is nothing but name given for the files which you intend to use in your PERL programs and scripts. A handle is a name which is temporarly assigned to a file. The example below shows how to use a file handle in your PERL program.

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header
$FilePath = "home/html/myhtml.html"
sysopen(HANDLE, $FilePath, O_RDWR);
printf HANDLE "Welcome to Tizag!";
close (HANDLE);

Files with the die Function

The die function also exists in several other programming languages. It is used to kill your scripts and also helps to pinpoint where/if your code is failing. We use this function as as shown below.

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header

$filepath = "htmlpage.html";

sysopen (HTML, '$filepath', O_RDWR|O_EXCL|O_CREAT, 0755) or die "$filepath cannot be opened.";
printf HTML "<html>\n";
printf HTML "<head>\n";
printf HTML "vtitle>My Home Page</title>";
printf HTML "</head>\n";
printf HTML "<body>\n";
printf HTML "<p align='center'>Here we have an HTML
page with a paragraph.</p>";
printf HTML "v/body>\n";
printf HTML "</html>\n";
close (HTML);

If due to some problem PERL is unable to open or create our file, we will be informed. It is good practice to make use of the die function and we will be using it as we go deeper into the file handling.

How to Open the File

Files can be opened using either of open and sysopen function. for either of the function can pass upto 4 arguments, the first argument is always the file handle, then the file name also known as a URL or filepath, flags, and finally any of the permissions that are to be granted to the file. The following program opens up a previously saved HTML document.

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header

$FH = "filehandle";
$FilePath = "htmlpage.html";

open(FH, $FilePath, permissions);
or
sysopen(FH, $FileName, permission);

Files which are having unusual file names or special characters are all best opened by declaring the URL first, as a variable. This method removes the confusion that might occur when PERL tries to interpret the program. However, filenames require a step for a brief character substitution before which they can be placed into the open statements.

Various File Permissions

File permissions are crucial to the file function and security. For instance, in order to function, a PERL file (.pl) must have executable file permissions in order to function on your web server. Also, you may not want all of your HTML files to be set to allow others to write to them or over them. Here's a listing of what to pass to the open function when working with file handles.

Shorthand Flags:

Entities Definition
< or r Read Only Access
> or w Creates, Writes, and Truncates
>>or a Writes, Appends, and Creates
+< or r+ Reads and Writes
+> or w+ Reads, Writes, Creates, and Truncates
+>> or a+ Reads, Writes, Appends, and Creates

O_ Flags:

Value Definition
O_RDWR Read and Write
O_RDONLY Read Only
O_WRONLY Write Only
O_CREAT Create the file
O_APPEND Append the file
O_TRUNC Truncate the file
O_EXCL Stops if file already exists
O_NONBLOCK Non-Blocking usability

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header
use Fcntl; #The Module

sysopen (HTML, '/home/html/htmlpage.html', O_RDWR|O_EXCL|O_CREAT, 0755);
sysopen (HTML, >htmlpage.html');

File Creation

Files are opened and created using the same function "sysopen". Our syntax open(FILEHANDLE, '$filename', permissions, CHMOD); or sysopen(FILEHANDLE, $filename, permissions, CHMOD);

#!/usr/bin/perl
use Fcntl; #The Module

print "content-type: text/html \n\n"; #The header
sysopen (HTML, 'myhtml.html', O_RDWR|O_EXCL|O_CREAT, 0755);
printf HTML "<html>\n";
printf HTML "<head>\n";
printf HTML "<title>My Home Page";
printf HTML "</head>\n";
printf HTML "<bodyv\n";
printf HTML "<p align='center'>Here we have an HTML
page with a paragraph.</p>";
printf HTML "</body>\n";
printf HTML "</html>\n";
close (HTML);

With sysopen you can also set hexadecimal priviledges; CHMOD values. Sysopen needs the declaration of a new module for PERL. We will now be using the Fcntl module.

Reading from a File

It is easy to read lines from files and then input them using the input operator <>. By placing the file handler inside the input operator, then your script will input that line of the file..

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header
$HTML = "htmlpage.html";
open (HTML) or die "Can't open the file!";
print <HTML>;
close (HTML);

Here we have a small PERL script to display several lines of HTML code. Each line is stored into an array and it is automatically printed to the browser in HTML format using the input operator <> .

Copy a File

Using the copy function we can duplicate the file. Copy takes two arguments, the URL of the file which needs to be copied and the URL of the new file/directory to which the file is to be copied.If the same file name is used or the same URL, PERL rewrite over the file if permissions is allowed.

#!/usr/bin/perl
use File::Copy;

print "content-type: text/html \n\n"; #The header
$filetobecopied = "htmlpage.html.";
$newfile = "html/htmlpage.html.";
copy($filetobecopied, $newfile) or die "File cannot be copied.";

Here, we have simply duplicated the "htmlpage.html" file and will be using it in future examples. While using PERL on the web, it is best to use the complete internet URL. We have used a shorthand way in the example, but it is better to hardcode the full URL like: http://www.vyom.co.in/htmlpage.html.

Moving the Files

Moving the file requires the use of the "move" function. This function works similarly to the copy function from above and we send the same module to PERL. The difference is here is, instead of copying we just 'cut' the file and send it to a new location. This functioning is same as cutting and pasting text from office document to another.

#!/usr/bin/perl
use File::Copy;

print "content-type: text/html \n\n"; #The header
$oldlocation = "htmlpage.html";
$newlocation = "html/htmlpage.html";
move($oldlocation, $newlocation);

Our file has now been completly removed from its present location to the new location.

Deleting the Files

To delete specific files from your web server use the "unlink" function. The best way is often to set a variable name equal to URL of the file you wish to delete.

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header
$file = "newtext.txt";
if (unlink($file) == 0) {
print "File deleted successfully.";
} else {
print "File was not deleted.";
}

Removing Multiple Files at Once

To remove multiple files at once, we should first create an array of files that has to be deleted and then loop through each one. There are many other ways to go about this process.

#!/usr/bin/perl

print "content-type: text/html \n\n"; #The header
@files = ("newtext.txt","moretext.txt","yetmoretext.txt");
foreach $file (@files) {
unlink($file);
}

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