使用 perl 打印文本文件

Question

I am completely new to this and this should be the easiest thing to do but for some reason I cannot get my local text file to print. After trying multiple times with different code I came to use the following code but it doesn't print.

I have searched for days on various threads to solve this and have had no luck. Please help. Here is my code:

#!/usr/bin/perl

$newfile = "file.txt";
open (FH, $newfile);

while ($file = <FH>) {
         print $file;
}

I updated my code to the following:

#!/user/bin/perl

use strict;            # Always use strict
use warnings;          # Always use warnings.

open(my $fh, "<", "file.txt") or die "unable to open file.txt: $!";
                   # Above we open file using 3 handle method
                   # or die die with error if unable to open it.
while (<$fh>) {        # While in the file.
     print $_;     # Print each line
 }
close $fh;             # Close the file

system('C:\Users\RSS\file.txt');

It returns the following: my first report generated by perl. I do not know where this is coming from. Nowhere do I have a print "my first report generated by perl."; statement and it definitely is not in my text file.

My text file is full of various emails, addresses, phone numbers and snippets of emails.

Thank you all for your help. I figured out my problem. I somehow managed to kick myself out of my directory and did not realize it.

Answer 1

This is most likely a combination of a failure to open the file, and a failure to check the return value of open .

If you are completely new to perl, I warmly recommend reading the excellent "perlintro" man page, using either man perlintro or perldoc perlintro on the command line, or taking a look here: https://perldoc.perl.org/perlintro.html .

The "Files and I/O" section there gives a good and concise way of doing this:

open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";

while (<$in>) {     # assigns each line in turn to $_
     print "Just read in this line: $_";
}

This version will give you an explanation and abort if anything goes wrong while trying to open the file. For example, if there is no file named file.txt in the current working directory, your version will quietly fail to open the file, and afterwards it will quietly fail to read from the closed file handle.

Also, always adding at least one of these to your perl scripts will save you a lot of trouble in the long run:

use warnings; # or use the -w command line switch to turn warnings on globally
use diagnostics;

These won't catch the failure to open the file, but will alert on the failed read.

In the first example here you can see that without the diagnostics module, the code fails without any error messages. The second example shows how the diagnostics module changes this.

$ perl -le 'open FH, "nonexistent.txt"; while(<FH>){print "foo"}'
$ perl -le 'use diagnostics; open FH, "nonexistent.txt"; while(<FH>){print "foo"}'
readline() on closed filehandle FH at -e line 1 (#1)
    (W closed) The filehandle you're reading from got itself closed sometime
    before now.  Check your control flow.

By the way, the legendary "Camel Book" is basically the perl man pages formatted for paper printing, so reading the perldocs in the order listed in perldoc perl will give you a high level of understanding of the language in a reasonably accessible and inexpensive manner.

Happy hacking!

Answer 2

This is simple and including explanations.

use strict;            # Always use strict
use warnings;          # Always use warnings.

open(my $fh, "<", "file.txt") or die "unable to open file.txt: $!";
                       # Above we open file using 3 handle method
                       # or die die with error if unable to open it.
while (<$fh>) {        # While in the file.
         print $_;     # Print each line
 }
close $fh;             # Close the file

There is then also the case where you are trying to open a file which is not in a location where you think it is. So consider doing full path, if not in the same dir.

open(my $fh, "<", 'F:\Workdir\file.txt') or die "unable to open < input.txt: $!"; 

EDIT: After your comments, it seems that you are opening an empty file. Please add this at the bottom of that same script and rerun. It will open the file in C:\\Users\\RSS and make sure it does actually contain data?

system('C:\Users\RSS\file.txt');

Answer 3

First, of all as you are starting out, it is better to enable all warnings by 'use warnings' and disable all such expression which can lead to uncertain behavior or are difficult to debug by pragma 'use strict'.

As you are dealing with file stream, it is always recommended to the check if you were able to open the stream. so, try to use croak or die both would terminate the program with a given message.

Instead of reading inside the while condition, I would recommend checking for end of file. So, loop breaks as end is found. Usually, when reading a line you would use it for further processing, so it is good idea to remove end of lines using chomp.

A sample for reading a file in perl can be as follows:

#!/user/bin/perl

use strict;
use warnings;

my $newfile = "file.txt";
open (my $fh, $newfile) or die "Could not open file '$newfile' $!";
while (!eof($fh))
{
    my $line=<$fh>;
    chomp($line);
    print $line , "\n";
}