Perl File Operations

Perl uses a variable type called a file handle to manipulate files.

To read from or write to a file, a file handle is required.

A file handle (file handle) is the name of an I/O connection.

Perl provides three file handles: STDIN, STDOUT, STDERR, which represent standard input, standard output, and standard error output, respectively.

Files can be opened in Perl using the following methods:

open FILEHANDLE, EXPR
open FILEHANDLE

sysopen FILEHANDLE, FILENAME, MODE, PERMS
sysopen FILEHANDLE, FILENAME, MODE

Parameter descriptions:

• FILEHANDLE: The file handle, used to store a unique identifier for a file.
• EXPR: An expression consisting of the file name and file access type.
• MODE: The file access type.
• PERMS: The permission bits.

The open Function

The following code uses the open function to open the file file.txt in read-only mode <:

open(DATA, "<file.txt");

< indicates read-only mode.

In the code, DATA is the file handle used to read the file. The following example will open the file and output its contents:

Example

#!/usr/bin/perl

open(DATA, "<file.txt") or die "Cannot open file.txt, $!";

while(<DATA>){
   print "$_";
}

The following code opens the file file.txt in write mode >:

open(DATA, ">file.txt") or die "Cannot open file.txt, $!";

> indicates write mode.

If you need to open the file in read-write mode, you can add a + sign before the > or < characters:

open(DATA, "+<file.txt") or die "Cannot open file.txt, $!";

This method will not delete the original contents of the file. To delete the contents, the format is as follows:

open DATA, "+>file.txt" or die "Cannot open file.txt, $!";

To append data to a file, simply open the file in append mode before appending:

open(DATA, ">>file.txt") || die "Cannot open file.txt, $!";

>> indicates appending data to the end of an existing file. If you need to read the contents of the file to be appended, you can add a + sign:

open(DATA, "+>>file.txt") || die "Cannot open file.txt, $!";

The following table lists different access modes:

The sysopen Function

The sysopen function is similar to the open function, but with a different parameter format.

The following example opens the file in read-write mode (+<filename):

sysopen(DATA, "file.txt", O_RDWR);

If you need to clear the file before updating it, the format is as follows:

sysopen(DATA, "file.txt", O_RDWR|O_TRUNC);

You can use O_CREAT to create a new file, O_WRONLY for write-only mode, and O_RDONLY for read-only mode.

The PERMS parameter is an octal attribute value representing the permissions of the file after creation, with a default of 0x666.

The following table lists possible mode values:

The Close Function

After using a file, it should be closed to flush the input/output buffer associated with the file handle. The syntax for closing a file is as follows:

close FILEHANDLE
close

FILEHANDLE is the specified file handle. If the file is successfully closed, it returns true.

close(DATA) || die "Cannot close file";

Reading and Writing Files

There are several different ways to read from and write to a file:

The <FILEHANDLE> Operator

The primary method for reading information from an open file handle is the <FILEHANDLE> operator. In scalar context, it returns a single line from the file handle. For example:

Example

#!/usr/bin/perl

print "URL of tutorialpro.org?\n";
$name = <STDIN>;
print "URL: $name\n";

When the above program is executed, it displays the following information, and the print statement outputs the URL after input:

When using the <FILEHANDLE> operator, it returns a list of every line in the file handle, for example, we can import all lines into an array.

Create the import.txt file with the following content:

$ cat import.txt
1
2
3

Read import.txt and place each line into the @lines array:

Example

#!/usr/bin/perl

open(DATA, "<import.txt") or die "Cannot open data";
@lines = <DATA>;
print @lines;    # Output the array content
close(DATA);

Executing the above program will output:

1
2
3

The getc Function

The getc function returns a single character from the specified FILEHANDLE, or from STDIN if none is specified:

getc FILEHANDLE
getc

If an error occurs or the file handle is at the end of the file, it returns undef.

The read Function

The read function is used to read information from the buffer of a file handle.

This function is used to read binary data from a file.

read FILEHANDLE, SCALAR, LENGTH, OFFSET
read FILEHANDLE, SCALAR, LENGTH

Parameter descriptions:

• FILEHANDLE: The file handle, used to store a unique identifier for a file.
• SCALAR: Stores the result. If OFFSET is not specified, the data is placed at the beginning of SCALAR. Otherwise, the data is placed OFFSET bytes after the start of SCALAR.
• LENGTH: The length of the content to be read.
• OFFSET: The offset.

If the read is successful, it returns the number of bytes read. If it reaches the end of the file, it returns 0. If an error occurs, it returns undef.

The print Function

For all functions that read information from a file handle, the primary writing function in the backend is print:

print FILEHANDLE LIST
print LIST
print

Using the file handle and the print function, you can send the results of the program to the output device (STDOUT: standard output), for example:

print "Hello World!\n";

File Copy

The following example opens an existing file file1.txt and reads each line of it to write into the file file2.txt:

Example

#!/usr/bin/perl

# Open the file in read-only mode
open(DATA1, "<file1.txt");

# Open the new file in write mode
open(DATA2, ">file2.txt");

# Copy data
while(<DATA1>)
{
   print DATA2 $_;
}
close(DATA1);
close(DATA2);

File Renaming

The following example renames the existing file file1.txt to file2.txt, with the specified directory being /usr/tutorialpro/test/:

#!/usr/bin/perl

rename ("/usr/tutorialpro/test/file1.txt", "/usr/tutorialpro/test/file2.txt");

The rename function only accepts two parameters and only renames existing files.

Deleting a File

The following example demonstrates how to use the unlink function to delete a file:

Example

#!/usr/bin/perl

unlink ("/usr/tutorialpro/test/file1.txt");

Specifying File Position

You can use the tell function to get the current position in a file and the seek function to set the position within the file:

The tell Function

The tell function is used to get the current position in a file:

tell FILEHANDLE
tell

If a FILEHANDLE is specified, it returns the position of the file pointer in bytes. If none is specified, it returns the position of the default selected file handle.

The seek Function

The seek() function moves the file read/write pointer within the file handle in bytes:

seek FILEHANDLE, POSITION, WHENCE

Parameter descriptions:

• FILEHANDLE: The file handle, used to store a unique identifier for a file.
• POSITION: The number of bytes to move the file handle (read/write pointer).
• WHENCE: The starting position for the file handle (read/write pointer) movement, with possible values of 0, 1, 2; representing the beginning of the file, the current position, and the end of the file, respectively.

The following example reads 256 bytes from the beginning of the file:

seek DATA, 256, 0;

File Information

Perl's file operations can also test whether a file exists, is readable, or writable.

First, create the file1.txt file with the following content:

$ cat file1.txt
www.tutorialpro.org

Example

#!/usr/bin/perl

my $file = "/usr/test/tutorialpro/file1.txt";
my (@description, $size);
if (-e $file)
{
    push @description, 'is a binary file' if (-B _);
    push @description, 'is a socket' if (-S _);
    push @description, 'is a text file' if (-T _);
    push @description, 'is a special block file' if (-b _);
    push @description, 'is a special character file' if (-c _);
    push @description, 'is a directory' if (-d _);
    push @description, 'file exists' if (-x _);
    push @description, (($size = -s _)) ? "$size bytes" : 'empty';
    print "$file information: ", join(', ', @description), "\n";
}

Executing the above program will output:

file1.txt information: is a text file, 15 bytes

The file test operators are as shown in the table below: