Perl File Management
Opening and closing files
- One of the main resources that your server provides is a
file management system. CGI scripts, for
example, use a multitude of supporting files in the server's
file system such as temporary files, counter files, user
files, data files, setup files, and libraries.
Perl includes
several excellent tools for working with these files.
- First, Perl gives your scripts the ability to open files using
the open function. The open function allows you to create
a "filehandle" with which to manipulate a file. A filehandle
is another name for a connection between the script and
the server. Often, filehandles manage connections
between the script and standard input, output, or error,
however, in the case of open, any file can be read into a
filehandle using the syntax:
open ([FILE_HANDLE_NAME], "[filename]");
- For example, we might open a data file
for reading using
open (DATA_FILE, "inventory.dat");
- In this case, all of the lines of inventory.dat will be read
into the filehandle "DATA_FILE" that Perl can then
use within the program. However, you must also close a
file once you are done with it. The syntax for closing a file
is as follows:
close ([FILE_HANDLE_NAME]);
- Finally, Perl gives you the ability to execute an error
routine if there is a problem opening a file. The "or"
logical operator is sometimes discussed in terms of a
short circuit. For instance, the logic of the "or" operator is
such that if the first expression evaluates to true, there is
no need to evaluate the next. On the other hand, if the
first expression evaluates to false, the second expression
is executed.
- Thus, using the double pipe (||) operator,
you can specify the default action to perform if an "open"
fails. In CGI applications, the alternate action executed is
usually something like the subroutine, CgiDie located in
cgi-lib.pl (discussed later today). For example, the
following routine would
execute the CgiDie subroutine if there was a problem
opening "address.dat".
open (ADDRESS_BOOK, "address.dat") ||
&CgiDie("Cannot open address.dat");
- Thus, if the script has a problem opening a needed file,
the double pipe (||) operator provides a convenient and
elegant way to quit the program and report the problem.
Additional Resources:
Formatting Control Structures
Table of Contents
Reading a File Line by Line
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