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,"</tmp/msg";

Read Operation with Bareword file handle:

#!/usr/bin/perl

open FH,"</tmp/msg";
$line  = <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,"</tmp/msg";

read_text(*FH);

sub read_text
{
        local *FH = shift;
        my @lines;
        @lines = <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 $log_fh,"</tmp/msg";
read_text($log_fh);

sub read_text
{
        local $log_fh = shift;
        my @lines;
        @lines = <$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,"+</tmp/text");

read_line(*FH);

write_line(*FH,"222\n");

sub read_line
{
        local *FH = shift;
        my $lines;
        $line = <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 = <IN>;

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 = <FH>;

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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{ 4 comments… read them below or add one }

1 Ashwin August 3, 2012 at 1:09 am

I have a csv file like this:

TranID,Date,AcNo,Type,Amount,ChequeNo,DDNo,Bank,Branch
132520,01-01-2011,51321342,Dr,5000,,,,
132524,01-01-2011,51321342,Dr,1000,,4126123,SB,Ashoknagar
132538,08-01-2011,51321342,Cr,1620,192101,,,
132548,17-01-2011,51321342,Cr,3500,192102,,,
132519,01-01-2011,55212341,Dr,2000,142514,,SBM,Hampankatte
132523,01-01-2011,55212341,Cr,500,192121,,,
132529,02-01-2011,55212341,Dr,5000,131211,,SB,Ashoknagar
132539,09-01-2011,55212341,Cr,500,192122,,,
132541,10-01-2011,55212341,Cr,2000,192123,,,
132525,02-01-2011,55212342,Dr,5000,,,,
132533,04-01-2011,55212342,Cr,2100,192201,,,
132526,02-01-2011,55212343,Dr,3000,126200,,ICICI,Hampankatte
132531,03-01-2011,55212343,Cr,500,192221,,,
132537,07-01-2011,55212343,Dr,5456,135123,,CB,Kankanady
132544,15-01-2011,55212343,Cr,3500,192222,,,
132546,15-01-2011,55212343,Cr,1342,192223,,,
132527,02-01-2011,55212344,Dr,5000,127821,,SB,MGRoad
132536,06-01-2011,55212344,Cr,1500,192251,,,
132528,02-01-2011,55212345,Dr,2000,162423,,CB,MGRoad
132530,03-01-2011,55212345,Cr,1000,192271,,,
132542,11-01-2011,55212345,Dr,3500,,4251234,ICICI,Hampankatte
132543,14-01-2011,55212345,Cr,1500,192272,,,
132521,01-01-2011,55212346,Dr,6000,125324,,CB,Kankanady
132532,03-01-2011,55212346,Cr,1000,192341,,,
132547,16-01-2011,55212346,Dr,2300,,,,
132522,01-01-2011,55212415,Dr,1200,162341,,SBI,Hampankatte
132534,05-01-2011,55212415,Dr,5000,162450,,SB,Kuloor
132535,06-01-2011,55212415,Cr,2000,192361,,,
132540,09-01-2011,55212415,Dr,2000,,4521349,CB,MGRoad
132545,15-01-2011,55212415,Cr,1245,192362,,,

How to Calculate and display total balance in each account using hash in perl. Without using parse function

2 Suhail November 9, 2012 at 2:46 am

Good one…

3 jayaram prasad April 19, 2013 at 9:37 pm

Excellent Documentation with examples on File handling in perl…

Really useful…

4 Sai Madhuri October 21, 2013 at 3:50 pm

here is my function

open (MYFILE1, ‘>>/home/blddevtiers/scripts/$APP/$Build/result.txt’);

problem is variables are not taking inside the open function for that path where I want to write the file.

Please help me how to read the variables declared outside OPEN

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