Perl | File Handling Introduction

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 open.

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 and Writing to a File using FileHandle

 
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.

Different Modes in File Handling

Mode Explanation
“<“ Read Only Mode
“>” Creates file (if necessary), Clears the contents of the File and Writes to it
“>>” Creates file (if necessary), Appends to the File
“+<“ Reads and Writes but does NOT Create
“+>” Creates file (if necessary), Clears, Reads and Writes
“+>>” Creates file (if necessary), Reads and Appends

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.

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    #!/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);

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    Output:

  2. 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.

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    #!/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); 

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    Output:

  3. 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.

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    #!/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); 

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    Output:

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

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    #!/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); 

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    Output:

  5. 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.

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    #!/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);

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    Output:

  6. 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.

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    # 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);

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    Output:

  7.  

    Redirecting Output

     
    Output can be redirected away from the Console and into a file using the select function.

    Syntax: select FileHandle;
    Parameters:


    • FileHandle – FileHandle of the File to be selected.

    Steps:

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

    Now, anything that is printed using the print function is redirected to the File.

    Example:

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    # Open a FileHandle in Write Mode.
    open(File, ">", "Hello.txt"); 
      
    # This sets File as the default FileHandle
    select File; 
       
    # Writes to File
    print("This goes to the File."); 
      
    # Writes to File
    print File "\nThis goes to the File too."
      
    # This sets STDOUT as default FileHandle
    select STDOUT; 
    print("This goes to the console.");
      
    # Close the FileHandle.
    close(File); 

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    Output in the Console:


    Contents of Hello.txt:
    Original File:

    Updated File:



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