Perl：如何从子目录读取文件？

Question

I'm really new to Perl, so excuse me if I missed an obvious answer. I will appreciate all help.

I'm trying to read a file that is located in a subdirectory, so I can extract some information, and print the information into a new file.

I know how to read a file in the current directory (and print into a new file):

open IN, "$file_name" or die "No such file: !$";
open OUT, ">$new_file_name" or die " No such file: !$";

while (<IN>) {
print OUT $_;
}

but how do I do this to a file that is located in a subdirectory?

Many thanks,

Nika

EDIT:

Thank-you all very very much! I've solved the problem!

Answer 1

open takes a path , so

my $file_name = 'subdir/file.ext';

Here's your code with many improvements:

open my $IN, '<', $in_qfn
    or die "Can't open \"$in_qfn\": $!\n";

open my $OUT, '>', $out_qfn
    or die "Can't create \"$out_qfn\": $!\n";

while (<$IN>) {
    print $OUT $_;
}

I'd actually use ARGV and STDOUT for maximum flexibility if possible.

while (<>) {
    print;
}

Answer 2

There are two obvious ways:

1. Access the file by its full path name, ie the value of $file_name must be something like directoryname/filename.txt or you write "directoryname/$filename" in your first line of code.

2. Change the working directory. That is, use the Perl chdir function to change the working directory before accessing the file: chdir "directoryname"; Keep in mind that this affects all subsequent file systems operations. If you want to go back, use chdir ".."; .

In either case, it is very helpful to familiarize yourself with the concept of working directory, special directory names ( . and .. ), the path separator / , relative and absolute paths and how file systems work in general.

Answer 3

Just give it a full path to the directory. you can use the pwd command to get the full path if you are in the directory where file is located. Note, you should always use the qualified/full path to files because you never know from where the script might be invoked. (it's just good programming practice)

use strict;
use warnings;

my $filename = 'your_full/path/sub_dir/data.txt';
if (open(my $fh, '<:encoding(UTF-8)', $filename)) {
while (my $row = <$fh>) {
chomp $row;
print "$row\n";
}
} else {
warn "Could not open file '$filename' $!";
}

Answer 4

Your variables (or strings) may contain the relative path to the file so simply say

open IN, "foo/$file1"

Oh and use 3-argument open .

Answer 5

You can simply give the name of the file in the subdirectory. Use the forward slash to separate the directory from the sub directory.

Here's your program written in a bit more modern fashion:

use strict;
use warnings;
use autodie;
use feature qw(say);

my $file_name = "path/to/file";
my $new_file_name = "path/to/new/file";

open my $in_fh, "<" $file_name;
open my $out_fh, ">", $new_file_name;

while my $line ( <$in_fh> ) {
    print {$out_fh} $line;
}
close $in_fh;
close $out_fh;

• use strict; requires you to declare your variables which can save you a lot of hassle. For example, you say $foo on place and $Foo (with a capital F ) in another. With use strict; , Perl will catch these errors for you. You declare a variable with my or our . 99% of the time, you will use my .
• use warnings; turns on all sorts of warnings. For example, you are attempting to do something with a variable that is undefined.
• use autodie automatically will kill your Perl script in many circumstances. For example, you can't open or close a file. Or, what if the disk is full, and your print statement can't print to the file?
• use feature qw(say); allows you to use say instead of print . The say command will automatically add a NL to the end of the line for you which is a lot more convenient than it sounds.
• Use three parameter open and close , and use variable names for the file handle. This makes passing the file handle easier to do to subroutines.
• Don't use the default $_ when you don't have to. There are many issues with using it (which I am not going to get into right now. It really doesn't add clarity to the program, and will many times get you into trouble.
• Use close when you finish with a file.

You should get a good book on Modern Perl . For example, in your program, I could have done this:

use strict;
use warnings;
use autodie;
use feature qw(say);

use File::Copy;    # Adds a `copy` command to Perl

use constant {     # Adds the concept of a constant to Perl
    IN_FILE   =>  'path/to/file',
    OUT_FILE  =>  '/paht/to/new/file',
};

copy IN_FILE, OUT_FILE;   # That's it!

No need to open files. No need to use loops. The copy command will do it for you.

These modules like File::Copy are available in the standard installations of Perl. You need to learn about them, so you can use them. This is why you should get a good book and go through it.

Have fun with Perl, and please learn the right way of doing things. It makes Perl a much easier language to use and learn.