Giving both input & output file at command line - perl

Question

Si I have this line in the perl script which prints the output to the STDOUT/console
printf "Line no. $i"
What code shall I include in the program to direct this output to an output file given by user at the command line itself (as undermentioned)
Right now ,the following portion asks the user for input file:
print "enter file name";
chomp(my $file=<STDIN>);
open(DATA,$file) or die "error reading";
But I dont want to ask the user for either of input/output file.
What I want is a way in which user could give in the input as well as output file from command line while running the program.
perl input_file output_file program.pl
What code shall i just include for this.

Answer 1

You can use shift to read the command line arguments to your script. shift reads and removes the first element of an array. If no array is specified (and not inside a subroutine), it will implicitly read from #ARGV, which contains the list of arguments passed to your script. For example:
use strict;
use warnings;
use autodie;
# check that two arguments have been passed
die "usage: $0 input output\n" unless #ARGV == 2;
my $infile = shift;
my $outfile = shift;
# good idea to sanitise the arguments here
open my $in, "<", $infile;
open my $out, ">", $outfile;
while (<$in>) {
print $out $_;
}
close $in;
close $out;
You could call this script like perl script.pl input_file output_file and it would copy the contents of input_file to output_file.

Answer 2

The easiest approach here is to ignore input and output files within your program. Just read from STDIN and write to STDOUT. Let the user redirect those filehandles when calling your program.
Your program looks something like this:
#!/usr/bin/perl
use strict;
use warnings;
while (<STDIN>) {
# do something useful to the data in $_
print;
}
And you call it like this:
$ ./your_program.pl inputfile.txt > outputfile.txt
This is known as the "Unix Filter Model" and it's the most flexible way to write programs that read input and produce output.

Answer 3

You can use #ARGV variable ,
use strict ;
use warnings ;
if ( #ARGV != 2 )
{
print "Usage : <program.pl> <input> <output>\n" ;
exit ;
}
open my $Input,$ARGV[0] or die "error:$!\n" ;
open my $Output,">>" .$ARGV[1] or die "error:$!\n";
print $Output $_ while (<$Input> ) ;
close ($Input) ;
close ($Output) ;
Note:
You should run the program perl program.pl input_file output_file this format.