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    Accessing Files and Directories in Perl

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    To work with files and directories in Perl, you have to understand filehandles. A filehandle is a named internal structure which is associated with a file name. It is used to perform read/write/update operations on files.

    The basic filehandles are STDIN for standard input, STDOUT for standard output, and STDERR for standard error devices.

    Open Function

    To open a file in read-only mode, < symbol is used with file name. In the following example, DATA is the file handle.

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    open(DATA, "<sample.txt");
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    In this context to print the content of the file on terminal, a loop can be used to iterate over DATA as shown below.


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    #!/usr/bin/perl
    open(DATA, "<sample.txt") or die "Unable to open $!";
    while(<DATA>) {
    print "$_";
    }
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    To open sample.txt in writing mode, > symbol is used before the file name as shown below. Note that, in this mode the file is truncated (previous content is deleted).


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    open(DATA, ">file.txt") or die "Unable to open file $!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    If a file needs to be opened for both reading and writing, the plus sign (+) needs to be added before the > or <.

    To open a file for updation without truncation, open the file as


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    open(DATA, "+<sample.txt"); or die "Unable to open file $!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    To truncate the file, you can use


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    open DATA, "+>sample.txt" or die "Unable to open file $!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    To open a file in append mode (file pointer is set at the end), you can use the follwing.




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    open(DATA,">>sample.txt") || die "Unable to open file $!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    But in this case, read operation can not be performed.

    To perform append along with read operation, you can use




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    open(DATA,"+>>sample.txt") || die "Unable to open file $!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Sysopen Function

    In this function, the parameters are passed to it as the parameters for the system function. To open a file for updation, instead of using +< , the file is opened as




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    sysopen(DATA, "sample.txt", O_RDWR);
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    or to truncate the file before updation




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

    To create a new file O_CREAT can be used. Similarly, O_WRONLY and O_RDONLY can be used to open file in write only mode and read only mode respectively.

    Close Function

    To close a filehandle, and therefore disassociate the filehandle from the corresponding file, you use the close function. This flushes the filehandle's buffers and closes the system's file descriptor.

    The function close(FILEHANDLE) is used to close a specific filehandle, by flushing buffer and closing filedescriptor.

    If filehandle is not specified, the currently selected filehandle is closed.




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    close(DATA) || die "Unable to close file!";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Reading and Writing Files

    The content of a file can be read by using a filehandle operator as shown below.




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    #!/usr/bin/perl
    print "Where do you live?\n";
    $name = <STDIN>;
    print "$name\n";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    To import content of a file having many lines, we can try the following program.




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    #!/usr/bin/perl
    open(DATA,"<sample.txt") or die "Unable to olpen file!";
    @lines = <DATA>;
    close(DATA);
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Copying Files

    The following program can be used to open an existing file, read the content line by line and copy the content in another file.




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    #!/usr/bin/perl
    open(fhold, "<old.txt");
    open(fhnew, ">new.txt");
    while(<fhold>) {
    print fhnew $_;
    }
    close(fhold);
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Renaming a file

    The function rename can be used to rename a file as shown below.




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    #!/usr/bin/perl
    rename ("/usr/test/fold.txt", "/usr/test/fnew.txt" );
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Deleting an Existing File

    The unlink function can be used to delete an existing file.




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    #!/usr/bin/perl
    unlink ("/usr/test/myfile.txt");
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Positioning inside a File

    tell Function

    The tell function, returns current position (in Bytes) of the cursor within a file.




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    tell FILEHANDLE
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    seek Function

    The seek function is used to move the cursor to a specific location within the file




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    seek DATA, 256, 0;
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Here zero (0) sets the positioning relative to the start of the file.

    File Information

    To get file permission related information, we can run the following code.




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    #/usr/bin/perl
    my $file = "/usr/test/file1.txt";
    my (@description, $size);
    if (-e $file) {
    push @description, 'binary' if (-B _);
    push @description, 'a socket' if (-S _);
    push @description, 'a text file' if (-T _);
    push @description, 'a block special file' if (-b _);
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Following are the standard functions used to play with directories.




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    opendir DIRHANDLE, EXPR # To open a directory
    readdir DIRHANDLE # To read a directory
    rewinddir DIRHANDLE # Positioning pointer to the begining
    telldir DIRHANDLE # Returns current position of the dir
    seekdir DIRHANDLE, POS # Pointing pointer to POS inside dir
    closedir DIRHANDLE # Closing a directory.
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Display all the Files

    To get list of files available in a particular directory, the glob operator is used as shown below.




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    #!/usr/bin/perl
    $dir = "/tmp/*";
    my @files = glob( $dir );
    foreach (@files ) {
    print $_ . "\n";
    }
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    The following program shows the list of files available inside a directory.




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    #!/usr/bin/perl
    opendir (DIR, '.') or die "Unable to open directory, $!";
    while ($file = readdir DIR) {
    print "$file\n";
    }
    closedir DIR;
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Create Directory

    The mkdir function is used to create a new directory.




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    #!/usr/bin/perl
    $dir = "/tmp/perl";
    mkdir( $dir ) or die "Unable to create directory $!";
    print "Successful\n";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    Remove a directory

    The rmdir function is used to remove a directory from file system. The user should have proper permission, moreover the directory should be empty.




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    #!/usr/bin/perl
    $dir = "/tmp/perl";
    rmdir( $dir ) or die "Unable to remove directory $!";
    print "Successful\n";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX





    Change a Directory

    To move to a new directory location, chdir function is used. Note that the user should have proper permission.




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    #!/usr/bin/perl
    $dir = "/home/test/demo";
    chdir( $dir ) or die "Unable to perform change directory $!";
    print "Your new location is $dir\n";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX


    Happy Exploring!

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