Perl - Command line input and STDIN

Question

I've created this script below for a assignment I have. It asks for a text file, checks the frequency of words, and lists the 10 words that appear the most times. Everything is working fine, but I need this script to be able to start via the command line as well as via the standard input.

So I need to be able to write 'perl wfreq.pl example.txt' and that should start the script and not ask the question for a text file. I'm not sure how to accomplish this really. I think I might need a while loop at the start somewhere that skips the STDIN if you give it the text file on a terminal command line.

How can I do it?

The script

#! /usr/bin/perl

use utf8;
use warnings;

print "Please enter the name of the file: \n" ;
$file = <STDIN>;
chop $file;


open(my $DATA, "<:utf8", $file) or die "Oops!!: $!";
binmode STDOUT, ":utf8";

while(<$DATA>) {
    tr/A-Za-z//cs;
    s/[;:()".,!?]/ /gio;
    foreach $word (split(' ', lc $_)) {
        $freq{$word}++;
    }
}

foreach $word (sort { $freq{$b} <=> $freq{$a} } keys %freq) {
   @fr = (@fr, $freq{$word});
   @ord = (@ord, $word);
}

for ($v =0; $v < 10; $v++) {
    print " $fr[$v] | $ord[$v]\n";
}

Peter Mortensen · Accepted Answer · 2021-06-15 11:10:49Z

10

Instead of reading from <STDIN>, you can read from <> to get data either from files provided on the command line or from stdin if there are no files.

For example, with the program:

#!/usr/bin/env perl

while (<>) {
  print $_;
}

The command ./script foo.txt will read and print lines from foo.txt, while ./script by itself will read and print lines from standard input.

edited Jun 15, 2021 at 11:10

Peter Mortensen

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answered Oct 24, 2011 at 9:54

Dave Sherohman

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Comments

Peter Mortensen · Accepted Answer · 2021-06-15 11:17:16Z

7

You need to do the following:

my $DATA;
my $filename = $ARGV[0];
unless ($filename) {
    print "Enter filename:\n";
    $filename = <STDIN>;
    chomp $filename;
}
open($DATA, $filename) or die $!;

Though I have to say, user-prompts are very un-Unix like.

edited Jun 15, 2021 at 11:17

Peter Mortensen

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answered Oct 24, 2011 at 9:40

user468687

3 Comments

FictionFighter Over a year ago

This works great and looks like what I was trying for. my $DATA; my $file = $ARGV[0]; if ($file) { open($DATA, $file) or die $!; } else { print "Please enter the name of the file: \n" ; $DATA = STDIN; } I can get it to start via a command line arg now, but when I start it without one it only asks for the file, but never acually goes through the rest of the script and does the sorting and printing. (works with "perl wfreq2.pl example.txt" but not when using "perl wfreq2.pl")

FictionFighter Over a year ago

Not really sure what I'm doing wrong, but I can't get it to start when just stating "perl wfreq.pl" it does ask for the file as a STDIN but it's not moving that along and doing the actual sorting and listing.

TLP Over a year ago

@FictionFighter Dave Sherohman's answer is the one you should use. Using the diamond <> operator is the way to go.

Alex Reynolds · Accepted Answer · 2011-10-24 09:28:18Z

0

perl script.pl < input.txt

The use of the operator < passes input.txt to script.pl as standard input. You can then skip querying for the filename. Otherwise, use $ARGV[0] or similar, if defined.

answered Oct 24, 2011 at 9:28

Alex Reynolds

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5 Comments

FictionFighter Over a year ago

That only gives me error messages. adm@adm-desktop:~/Desktop/perl$ perl wfreq2.pl < example.txt Please enter the name of the file: Oops!!: No such file or directory at wfreq2.pl line 11, <STDIN> line 1.

Alex Reynolds Over a year ago

As I said, skip querying for the filename and just loop over <> in the while loop.

user554546 Over a year ago

Is there something (ie the name of another file) in input.txt?

Alex Reynolds Over a year ago

This is way more complicated than it needs to be, honestly. Read up on "perl standard input" on your favorite search engine.

FictionFighter Over a year ago

No the input textfile consist of list of random words that are to be counted and listed.

Peter Mortensen · Accepted Answer · 2021-06-15 11:09:30Z

You can check for a command-line argument in @ARGV, which is Perl's array that automagically grabs command line arguments, and --if present-- process them (else continue with input from STDIN). Something like:

use utf8;
use strict;  #Don't ever forget this!  Always, always, ALWAYS use strict!
use warnings;

if(@ARGV)
{
  #Assume that the first command line argument is a file to be processed as in your original code.
  #You may or may not want to care if additional command line arguments are passed.  Up to you.
}
else
{
  #Ask for the filename and proceed as normal.
}

Note that you might want to isolate the code for processing the file name (i.e., the opening of DATA, going through the lines, counting the words, etc.) to a subroutine that you can easily call (perhaps with an argument consisting of the file name) in each branch of the code.

Oh, and to make sure I'm clear about this: always use strict and always use warnings. If you don't, you're asking for trouble.

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