If you've read the previous chapters and have executed some of the programs, then you already know that a file is a series of bytes stored on a disk instead of inside the computer's memory. A file is good for long-term storage of information. Information in the computer's memory is lost when the computer is turned off. Information on a disk, however, is persistent. It will be there when the computer is turned back on.
We already know how to create a file using a text editor program. In this chapter, you'll see how to manipulate files with Perl.
There are four basic operations that you can do with files. You can open them, read from them, write to them, and close them. Opening a file creates a connection between your program and the location on the disk where the file is stored. Closing a file shuts down that connection.
Every file has a unique fully qualified name so that it can't be confused with other files. The fully qualified name includes the name of the disk, the directory, and the file name. Files in different directories can have the same name because the operating system considers the directory name to be a part of the file name. Here are some fully qualified file names:
c:/windows/win95.txt c:/windows/command/scandisk.ini c:/a_long_directory_name/a_long_subdirectory_name/a_long_file_name.doc
Caution You may be curious to know if spaces can be used inside file names. Yes, they can. But, if you use spaces, you need to surround the file name with quotes when referring to it from a DOS or UNIX command line.
Note It is very important that you check for errors when dealing with files. To simplify the examples in this chapter, little error checking will be used in the example. Instead, error checking information will be discussed in Chapter 13, "Handling Errors and Signals."