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Re: Re: Prepending to a file

by dkubb (Deacon)
on Feb 06, 2001 at 13:36 UTC ( #56646=note: print w/replies, xml ) Need Help??

in reply to Re: Prepending to a file
in thread Prepending to a file

One way that I can see how to do this is with the following subroutine called prepend_file():

#!/usr/bin/perl -w use strict; use IO::File; use constant FILE => 'test.txt'; use constant DATA => "this should be the first line\n"; use constant BUFFER_SIZE => '8096'; my $fh = prepend_file(FILE, DATA, BUFFER_SIZE); while(my $line = <$fh>) { print $line; } sub prepend_file { my $file = shift; my $data = shift; my $buffer_size = shift; #Open a temporary and source file handle my $temp_fh = IO::File->new_tmpfile or die "Could not open a temporary file: $!"; my $fh= IO::File->new($file, O_RDWR) or die "Could not open file ", FILE, ": $!"; #Write the first bit of data $temp_fh->syswrite($data); #Copy all the $data from the $fh to the temp file handle $temp_fh->syswrite($data) while $fh->sysread($data, $buffer_size); $temp_fh->sysseek(0, 0); $fh->sysseek(0, 0); #Write out the new file from the temporary file handle $fh->syswrite($data) while $temp_fh->sysread($data, $buffer_size); #could return anything here, I just chose the file handle just #in case we needed to use it for something. return $fh->sysseek(0, 0) && $fh; } __END__

It uses IO::File's new_tmpfile() method to create a temporary file. You then only have to deal with the single filehandle, and IO::File takes care of throwing away the temp file when you're done. I wanted to make sure it could handle most sizes of files, even those that exceed available memory, this is why I used a temporary file and not just memory/slurping.

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Re^3: Prepending to a file
by ibm1620 (Friar) on Oct 01, 2016 at 03:32 UTC

    Instead of

    or die "Could not open file ", FILE, ": $!";

    don't you want

    or die "Could not open file ", $file, ": $!";
      or die "Could not open file ", $file, ": $!";

      Looks good. It seems you found a bug.

      Because filenames may contain spaces and other "invisible" characters, especially when the code generating the filename has a bug (e.g missing chomp), you may want to quote it in the error message. And you could use string interpolation for the filename. So:

       or die "Could not open file '$file': $!";


      Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

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