Perl | File Handling Introduction

In Perl, file handling is the process of creating, reading, writing, updating, and deleting files. Perl provides a variety of built-in functions and modules that make it easy to work with files. Here's an introduction to file handling in Perl:

File modes:
When opening a file in Perl, you need to specify a file mode, which determines how the file can be accessed. There are three main file modes:

1. Read mode (<): Opens the file for reading. The file must already exist.
2. Write mode (>): Opens the file for writing. If the file does not exist, it will be created. If the file already exists, its contents will be truncated.
3. Append mode (>>): Opens the file for writing, but appends new data to the end of the file instead of overwriting its contents. If the file does not exist, it will be created.

File handling functions:
Here are some of the most commonly used built-in file-handling functions in Perl:

1. open(): Opens a file and returns a file handle.
2. close(): Closes a file handle.
3. print(): Writes data to a file.
4. read(): Reads data from a file.
5. seek(): Moves the file pointer to a specific location in the file.
6. tell(): Returns the current position of the file pointer.
7. stat(): Returns information about a file, such as its size, owner, and permissions.

File handling modules:
In addition to the built-in file-handling functions, Perl also provides a number of modules that can make file handling easier and more efficient. Here are some of the most commonly used file-handling modules in Perl:

1. File::Basename: Provides functions for working with file paths, such as extracting the filename or directory from a path.
2. File::Copy: Provides functions for copying files.
3. File::Find: Provides functions for searching a directory tree for files that match a pattern.
4. File::Path: Provides functions for creating and removing directories.
5. File::Spec: Provides functions for working with file paths that are platform-independent.

Overall, file handling is an important aspect of programming in Perl. Whether you need to read data from a file, write data to a file, or perform more complex file operations, Perl provides a variety of tools and modules to help you get the job done.

In Perl, a FileHandle associates a name to an external file, that can be used until the end of the program or until the FileHandle is closed. In short, a FileHandle is like a connection that can be used to modify the contents of an external file and a name is given to the connection (the FileHandle) for faster access and ease. 
The three basic FileHandles in Perl are STDIN, STDOUT, and STDERR, which represent Standard Input, Standard Output, and Standard Error devices respectively. File Handling is usually done through the open function.

Syntax: open(FileHandle, Mode, FileName); 

Parameters:

• FileHandle- The reference to the file, that can be used within the program or until its closure.
• Mode- Mode in which a file is to be opened.
• FileName- The name of the file to be opened.

Also, Mode and FileName can be clubbed to form a single expression for opening. 

Syntax: open(FileHandle, Expression); 

Parameters:

• FileHandle- The reference to the file, that can be used within the program or until its closure.
• Expression- Mode and FileName clubbed together.

The FileHandle is closed using the close function.  

Syntax: close(FileHandle); 

Parameters:

• FileHandle- The FileHandle to be closed.

  Reading from a FileHandle can be done through the print function.

Syntax: print(<FileHandle>); 

Parameters:

• FileHandle- FileHandle opened in read mode or a similar mode.

Writing to a File can also be done through the print function.  

Syntax: print FileHandle String 

Parameters:

• FileHandle- FileHandle opened in write mode or a similar mode.
• String- The String to be inserted in the file.

Examples: Consider a file Hello.txt containing the string "Welcome to GeeksForGeeks!!!" initially.

1. Mode = "<" This is read-only Mode. This mode is used to Read the content line by line from the file. 

#!/usr/bin/perl

# Opening a File in Read-only mode
open(r, "<", "Hello.txt");

# Printing content of the File
print(<r>);

# Closing the File
close(r);

Output:

Mode = ">" This is write-only Mode. Original contents of the File are cleared once it is opened in this Mode. It creates a new File with the same name, if one is not found. 

#!/usr/bin/perl

# Opening File Hello.txt in Read mode
open(r, "<", "Hello.txt");

# Printing the existing content of the file
print("Existing Content of Hello.txt: " . <r>);

# Opening File in Write mode
open(w, ">", "Hello.txt");

# Set r to the beginning of Hello.txt
seek r, 0, 0;

print "\nWriting to File...";

# Writing to Hello.txt using print
print w "Content of this file is changed";

# Closing the FileHandle
close(w);

# Set r to the beginning of Hello.txt
seek r, 0, 0;

# Print the current contents of Hello.txt
print("\nUpdated Content of Hello.txt: ".<r>);

# Close the FileHandle
close(r);

1. Output:
2. Mode=">>" This is Append Mode. Original content of the File is not cleared when it is opened in this Mode. This Mode cannot be used to overwrite as the String always attaches at the End. It creates a new File with the same name, if one is not found. 

#!/usr/bin/perl

# Opening File Hello.txt in Read mode
open(r, "<", "Hello.txt");

# Printing the existing content of the file
print("Existing Content of Hello.txt: " . <r>);

# Opening the File in Append mode
open(A, ">>", "Hello.txt");

# Set r to the beginning of Hello.txt
seek r, 0, 0;

print "\nAppending to File...";

# Appending to Hello.txt using print
print A " Hello Geeks!!!";

# close the FileHandle
close(A);

# Set r to the beginning of Hello.txt
seek r, 0, 0;

# Print the current contents of Hello.txt
print("\nUpdated Content of Hello.txt: ".<r>);

# Close the FileHandle
close(r);

Output: 

1. Mode = "+<" This is Read-Write Mode. This can be used to overwrite an existing String in File. It cannot create a new File. 

#!/usr/bin/perl

# Open Hello.txt in Read-Write Mode
open(rw, "+<", "Hello.txt");

# Print original contents of the File.
# rw is set to the end.
print("Existing Content of Hello.txt: ".<rw>);

# The string is attached at the end
# of the original contents of the file.
print rw "Added using Read-Write Mode.";

# Set rw to the beginning of the File for reading.
seek rw, 0, 0;

# Printing the Updated content of the File
print("\nUpdated contents of Hello.txt: ".<rw>);

# Close the FileHandle
close(rw);

Output: 

Mode = "+>" This is Read-Write Mode. The difference between "+<" and "+>" is that "+>" can create a new File, if one with the name is not found, but a "+<" cannot. 

#!/usr/bin/perl

# Opening File Hello.txt in Read mode
open(r, "<", "Hello.txt");

# Printing the existing content of the file
print("Existing Content of Hello.txt: " . <r>);

# Closing the File
close(r);

# Open Hello.txt in Read-Write Mode
open(rw, "+>", "Hello.txt");

# Original contents of the File
# are cleared when the File is opened
print("\nContents of Hello.txt gets cleared...");

# The string is written to the File
print rw "Hello!!! This is updated file.";

# Set rw to the beginning of the File for reading.
seek rw, 0, 0;

print("\nUpdated Content of Hello.txt: " .<rw>);

# Closing the File
close(rw);

Output: 

Mode = "+>>" This is Read-Append Mode. This can be used to Read from a File as well as Append to it. A new File with same name is created, if one is not Found. 

# Open Hello.txt in Read-Append Mode
open(ra, "+>>", "Hello.txt");

# Set ra to the beginning of the File for reading.
seek ra, 0, 0;

# Original content of the File
# is NOT cleared when the File is opened
print("Existing Content of the File: " . <ra>);

print "\nAppending to the File....";

# The string is appended to the File
print ra "Added using Read-Append Mode";

# Set ra to the beginning of the File for reading.
seek ra, 0, 0;

# Printing the updated content
print("\nUpdated content of the File: " . <ra>);

# Closing the File
close(rw);

Output: 

• Open a FileHandle to write i.e. ">", ">>", "+<", "+>" or "+>>".
• Select the FileHandle using select function.