Reading specified line using Perl program and command-line arguments

Question

So, let's say I am writing a Perl program:

./program.perl 10000 < file

I want it to read the 10000th line of "file" only. How could I do it using input redirection in this form? It seems that I keep getting something along the lines of 10000 is not a file.

I thought this would work:

#!/usr/bin/perl -w
$line_num = 0;
while ( defined ($line = <>) && $line_num < $ARGV[0]) {
    ++$line_no;
    if ($line_no == $ARGV[0]) {
        print "$line\n";
        exit 0;
    }
}

But it failed miserably.

Please use strict, as it would have caught your typo ($line_num vs. $line_no). Please also show us the error messages, as perl's complaint that it "Can't open 10000: No such file or directory..." is a good clue as to what else has gone wrong. — pilcrow
– pilcrow, Commented Jul 15, 2012 at 1:35

ruakh · Accepted Answer · 2012-07-14 21:38:44Z

3

If there are command-line arguments, then <> opens the so-named files and reads from them, and if not, then it takes from standard-input. (See "I/O Operators" in the perlop man-page.)

If, as in your case, you want to read from standard-input whether or not there are command-line arguments, then you need to use <STDIN> instead:

while ( defined ($line = <STDIN>) && $line_num < $ARGV[0]) {

answered Jul 14, 2012 at 21:38

ruakh

185k29 gold badges292 silver badges324 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

ruakh Over a year ago

@JordanCarney: You're welcome! By the way, I should also have mentioned -- you don't need to create your own $line_num variable; there's a predefined $. that does exactly this. (See the perlvar man-page.)

Thor · Accepted Answer · 2012-07-14 22:35:05Z

1

Obligatory one-liner:

perl -ne 'print if $. == 10000; exit if $. > 10000'

$. counts lines read from stdin. -n implicitly wraps program in:

while (<>) {
   ...program...
}

answered Jul 14, 2012 at 22:35

Thor

47.7k12 gold badges125 silver badges140 bronze badges

1 Comment

tchrist Over a year ago

print and exit if $. == 10_000

TLP · Accepted Answer · 2012-07-16 15:01:10Z

1

You could use Tie::File

use Tie::File;
my ($name, $num) = @ARGV;
tie my @file, 'Tie::File', $name or die $!;

print $file[$num];
untie @file;

Usage:

perl script.pl file.csv 10000

edited Jul 16, 2012 at 15:01

answered Jul 14, 2012 at 21:40

TLP

68.3k10 gold badges97 silver badges156 bronze badges

Comments

RobEarl · Accepted Answer · 2013-07-30 08:41:42Z

0

You could also do this very simply using awk:

awk 'NR==10000' < file

or sed:

sed -n '10000p' file

edited Jul 30, 2013 at 8:41

answered Sep 23, 2012 at 14:51

RobEarl

7,9226 gold badges38 silver badges52 bronze badges

Collectives™ on Stack Overflow

Reading specified line using Perl program and command-line arguments

4 Answers 4

1 Comment

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related