Handling Files with Filehandles

Since my last article was the completion of a marathon-sized project, I thought I'd keep the article short and targeted this week. I have been getting a lot of questions lately about Perl programs that modify data and many of them were simply a file handling problems. So this week we'll take a look at files and filehandles.

The filehandle 

Perl uses filehandles when opening, manipulating and closing files and their contents. There are several other uses for filehandles besides this, but in this column, we will cover just the basics of handling files.

In Perl, there are three simple ways to open a file that allow you to do different operations. All of these use Perl's open function. The three basic file operations in Perl are read, append, and write.

Read mode 

open (HANDLE,"filename.txt");

When we open a file in read mode, the only thing we can do to the file is read data from it, we cannot manipulate it.

To read the data from the file, we use the angle brackets or diamond operator (<>):

open (HANDLE,"filename.txt");
while (<HANDLE>) {
  # do something in here...
  # This loop will read data from the file, one line
  # at-a-time and place the contents in $_
}

Write mode 

open (HANDLE,">filename.txt");

The greater-than sign tells Perl to take this file and if it already exists, wipe out the contents and start fresh. If the file does not exist, Perl will create a new file with the file name specified.

When we open a file in write mode, we cannot read from the file because whatever was there is now gone. We can, however, write data to the file.

To write the data to the file, we simply print to the filehandle:

open (HANDLE,">filename.txt");
print HANDLE "Write this text to the file...";

Append mode 

open (HANDLE,">>filename.txt");

The double greater-than signs tell Perl to take this file and open it so that we can write more data to it, keeping whatever was in there before and adding the new data after it. If the file does not exist, Perl will create a new file with the file name specified.

When we open a file in append mode, we cannot read from the file because with append mode, Perl's internal file pointer is at the end of the file. Any attempted reads would immediately fail because we are already at the end of the file.

Writing data to the file is the same as it was in write mode, so we simply print to the filehandle in the same way as above.

open (HANDLE,">>filename.txt");
print HANDLE "Append this text to the file.";

Debugging 

On the end of the open function, we add a die function, using the || (logical or) operator.

open (HANDLE,">>filename.txt") ||
        die "Error appending to file filename.txt $!\n";

Closing filehandles 

close (HANDLE);

Win32 differences 

When working with files on a Windows system, you need to specify the entire path name and drive letter of the file you want to work with. 

open (HANDLE,"c:/scripts/newfile.txt") ||
        die "Error opening c:/scripts/newfile.txt $!\n";
...
close (HANDLE);

Assign file names to variables 

Using a variable to open a filehandle is simple:

$myfile = "/usr/local/testing.txt";
open (THIS_HANDLE, ">$myfile") ||
        die "Error opening $myfile for writing $!\n";
close THIS_HANDLE;
...
open (THIS_HANDLE, "<$myfile") ||
        die "Error opening $myfile for reading $!\n";;
close THIS_HANDLE;

This helps tremendously when you are opening files more than once in the script. If you put all your file names into variables at the top of your script, it makes it a breeze to modify the program for different installations or directory structures.

As you can see, working with files and filehandles is very simple in Perl. Now that you have the basics of handling files mastered, you are ready to move on and start writing programs that manipulate files.

Next: Revisiting the Database Application