Deleting a line from a huge file in Perl

Question

I have huge text file and first five lines of it reads as below :

This is fist line
This is second line
This is third line
This is fourth line
This is fifth line

Now, I want to write something at a random position of the third line of that file which will replace the characters in that line by the new string I am writing. I am able to achieve that with the below code :

use strict;
use warnings;

my @pos = (0);
open my $fh, "+<", "text.txt";

while(<$fh) {
    push @pos, tell($fh);
}

seek $fh , $pos[2]+1, 0;
print $fh "HELLO";

close($fh);

However, I am not able to figure out with the same kind of approach how can I delete the entire third line from that file so that the texts reads below :

This is fist line
This is second line
This is fourth line
This is fifth line

I do not want to read the entire file into an array, neither do I want to use Tie::File. Is it possible to achieve my requirement using seek and tell ? A solution will be very helpful.

Answer 1

A file is a sequence of bytes. We can replace (overwrite) some of them, but how would we remove them? Once a file is written its bytes cannot be 'pulled out' of the sequence or 'blanked' in any way. (The ones at the end of the file can be dismissed, by truncating the file as needed.)

The rest of the content has to move 'up', so that what follows the text to be removed overwrites it. We have to rewrite the rest of the file. In practice it is often far simpler to rewrite the whole file.

As a very basic example

use warnings 'all';
use strict;
use File::Copy qw(move);

my $file_in = '...';
my $file_out = '...';  # best use `File::Temp`

open my $fh_in,  '<', $file_in  or die "Can't open $file_in: $!";
open my $fh_out, '>', $file_out or die "Can't open $file_out: $!";

# Remove a line with $pattern
my $pattern = qr/this line goes/;

while (<$fh_in>) 
{
    print $fh_out $_  unless /$pattern/;
}
close $fh_in;
close $fh_out;

# Rename the new fie into the original one, thus replacing it
move ($file_out, $file_in) or die "Can't move $file_out to $file_in: $!";

This writes every line of input file into the output file, unless a line matches a given pattern. Then that file is renamed, replacing the original (what does not involve data copy). See this topic in perlfaq5.

Since we really use a temporary file I'd recommend the core module File::Temp for that.

---

This may be made more efficient, but far more complicated, by opening in update '+<' mode so to overwrite only a portion of the file. You iterate until the line with the pattern, record (tell) its position and the line length, then copy all remaining lines in memory. Then seek back to the position minus length of that line, and dump the copied rest of the file, overwriting the line and all that follows it.

Note that now the data for the rest of the file is copied twice, albeit one copy is in memory. Going to this trouble may make sense if the line to be removed is far down a very large file. If there are more lines to remove this gets messier.

---

Writing out a new file and copying it over the original changes the file's inode number. That may be a problem for some tools or procedures, and if it is you can instead update the original by either

• Once the new file is written out, open it for reading and open the original for writing. This clobbers the original file. Then read from the new file and write to the original one, thus copying the content back to the same inode. Remove the new file when done.

• Open the original file in read-write mode ('+<') to start with. Once the new file is written, seek to the beginning of the original (or to the place from which to overwrite) and write to it the content of the new file. Remember to also set the end-of-file if the new file is shorter,

  truncate $fh, tell($fh); 
  

after copying is done. This requires some care and the first way is probably generally safer.

If the file weren't huge the new "file" can be "written" in memory, as an array or a string.