How to write from n-th row to a file using perl - perl

Question

I have a source text in a file and looking for a code that would take the second (or n-th - in general) row from this file and print to a seperate file.
Any idea how to do this?

Answer 1

You can do this natively in Perl with the flip-flop operator and the special variable $. (used internally by ..), which contains the current line number:
# prints lines 3 to 8 inclusive from stdin:
while (<>)
{
print if 3 .. 8;
}
Or from the command line:
perl -wne'print if 3 .. 8' < filename.txt >> output.txt
You can also do this without Perl with: head -n3 filename.txt | tail -n1 >> output.txt

Answer 2

You could always:
Read all of the file in and but it into one variable.
Split the variable at the newline and store in an array
Write the value at the index 1 (for the second row) or the n-1 position to the separate file

Answer 3

use like this script.pl > outfile (or >> outfile for append)
this uses lexical filehandles and 3 arg open which are preferred to global filehandles and 2 arg open.
#!/usr/bin/perl
use strict;
use warnings;
use English qw( -no_match_vars );
use Carp qw( croak );
my ( $fn, $line_num ) = #ARGV;
open ( my $in_fh, '<', "$fn" ) or croak "Can't open '$fn': $OS_ERROR";
while ( my $line = <$in_fh> ) {
if ( $INPUT_LINE_NUMBER == $line_num ) {
print "$line";
}
}
note: $INPUT_LINE_NUMBER == $.
here's a slightly improved version that handles arbitrary amounts of line numbers and prints to a file.
script.pl <infile> <outfile> <num1> <num2> <num3> ...
#!/usr/bin/perl
use strict;
use warnings;
use English qw( -no_match_vars );
use Carp qw( croak );
use List::MoreUtils qw( any );
my ( $ifn, $ofn, #line_nums ) = #ARGV;
open ( my $in_fh , '<', "$ifn" ) or croak "can't open '$ifn': $OS_ERROR";
open ( my $out_fh, '>', "$ofn" ) or croak "can't open '$ofn': $OS_ERROR";
while ( my $line = <$in_fh> ) {
if ( any { $INPUT_LINE_NUMBER eq $_ } #line_nums ) {
print { $out_fh } "$line";
}
}

Answer 4

I think this will do what you want:
line_transfer_script.pl:
open(READFILE, "<file_to_read_from.txt");
open(WRITEFILE, ">File_to_write_to.txt");
my $line_to_print = $ARGV[0]; // you can set this to whatever you want, just pass the line you want transferred in as the first argument to the script
my $current_line_counter = 0;
while( my $current_line = <READFILE> ) {
if( $current_line_counter == $line_to_print ) {
print WRITEFILE $current_line;
}
$current_line_counter++;
}
close(WRITEFILE);
close(READFILE);
Then you'd call it like: perl line_transfer_script.pl 2 and that would write the 2nd line from file_to_read_from.txt into file_to_write_to.txt.

Answer 5

my $content = `tail -n +$line $input`;
open OUTPUT, ">$output" or die $!;
print OUTPUT $content;
close OUTPUT;