6 Perl File Handle Examples to Open, Read, and Write File - TheGeekStuff

6 Perl File Handle Examples to Open, Read, and Write File:

6 Perl File Handle Examples to Open, Read, and Write File

by BALAKRISHNAN MARIYAPPAN on SEPTEMBER 10, 2010

In this article, let us discuss how to manipulate the file handlers in Perl.

1. Typical Way of Opening a Perl File Handlers

The perl example below opens a file with a bareword. This is a typical perl file open scenario.

#!/usr/bin/perl

open FH,"

Read Operation with Bareword file handle:
#!/usr/bin/perl

open FH,";
print $line;


Write Operation with the Bareword file handle:
#!/usr/bin/perl

open FH,">/tmp/msg";
print FH "Perl - Practical Extraction Report Language\n";


If you want to pass this handler to a perl function, you would use typeglob as shown below.
#!/usr/bin/perl

open FH,";
        print @lines;
}


2. Opening a Perl File Handle reference in Normal Scalar Variable

You can use a scalar variables to store the file handle reference as shown below.
#!/usr/bin/perl

# $log_fh declared to store the file handle.
my $log_fh;
open $log_fh,";
        print @lines;
}


3. Use Perl IO::File to Open a File Handle

IO::File is a perl standard CPAN module which is used for opening a file handle in other colourful conventions. Use cpan command to install perl modules.










#!/usr/bin/perl

use IO::File;

$read_fh = IO::File->new("/tmp/msg",'r');

read_text($read_fh);

sub read_text
{
        local $read_fh = shift;
        my @lines;
        @lines = <$read_fh>;
        print @lines;
}


Following perl code snippet explains perl write operation with IO::File module.
$write_fh = IO::File->new("/tmp/msg",'w');


To open the file handler in append mode, do the following.
$fh = IO::File->new("/tmp/msg",O_WRONLY|O_APPEND);


4. Open Perl File Handler in Both Read and Write mode

When you want to open both in read and write mode, Perl allows you to do it. The below perl mode symbols are used to open the file handle in respective modes.

MODE
DESCRIPTION
+<
READ,WRITE
+>
READ,WRITE,TRUNCATE,CREATE
+>>
READ,WRITE,CREATE,APPEND


Let us write an example perl program to open a sample text file in both read and write mode.
$ cat /tmp/text
one
two
three
four
five


The below code reads first line from the /tmp/text file and immediately does the write operation.
#!/usr/bin/perl
open(FH,"+;
        print $line;
}

sub write_line
{
        local *FH = shift;
        print FH @_;
}

close(FH);


The output of the above code is shown below.
$ perl ./read_and_write.pl
one

$ cat /tmp/text
one
222
three
four
five


Note: Use perl debugger to debug your perl scripts.

5. Open the Standard Input and Standard Output

Perl allows you to open the standard input and standard output with other file handle names.

Perl standard output example:
#!/usr/bin/perl

open(OUT,">-");

print OUT "STDOUT opened with the name as OUT";


Perl standard input example:
#!/usr/bin/perl

open(IN,"-");

print "STDIN opened with the name as IN";

$input = ;


6. Use sysopen() to Open the File

sysopen() function requires three arguments such as file handle, filename and mode.

Read Operation Example:
#!/usr/bin/perl

sysopen(FH,"/tmp/text",O_RDONLY);

$line = ;

print $line;


Write Operation Example :
#!/usr/bin/perl

sysopen(FH,"/tmp/text",O_WRONLY);

print FH "write operation";


Different types of modes are shown in the table below.

MODE
DESCRIPTION
O_RDONLY
READ
O_WRONLY
WRITE
O_RDWR
READ and WRITE
O_CREAT
CREATE
O_APPEND
APPEND
O_TRUNC
TRUNCATE
O_NONBLOCK
NON BLOCK MODE


Note : You would need to have the habit of validating opened file handlers. The most common way of handling the file handler open failure with the die function is shown below.
open(FH,">/tmp/text") or die "Could not open /tmp/text file : $!\n";


If the above code is unable to open the file “/tmp/text”, it returns failure, and die gets executed. And the “$!” Buildin variable contains the reason for open function failure.

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