Accessing Default directory in perl

Question

I am having a perl script which is taking a directory path as input and then script iterates through all the files present in that directory. For each file it calls some other function.

Here is the script:

#!/usr/bin/perl

use JSON;   
use XML::Simple;
use File::Spec;

$num_args = $#ARGV + 1;
if ($num_args != 1) {
    print "\nUsage: $0 <input directory>\n";
    exit;
}

my $dir = $ARGV[0];

opendir(DIR, $dir) or die "cannot open directory";

@docs = grep(/\.xml$/,readdir(DIR));

foreach $file (@docs) 
{
    my $abs_path = join("",$dir,$file);
    #print $abs_path;
    my $return_object = some_function_call($abs_path);
    print_output($return_object);
 }

 sub print_output
 {
    my $output_object = $_[0];
    print $output_object;
 }

My question here is when i am reading input directory, in case if does not exists how can make above script read from "Some Default Directory location". I tried adding some logic but did not work for me. This is my first perl script so please excuse me if this is too basic to ask.

I am working in LINUX environment.

Answer 1

Before opendir(DIR, $dir) or die "cannot open directory"; check if directory exists and if not assign $dir variable default value:

$default_dir = "....";

if (!-e $dir)
{
    $dir = $default_dir;
}

Answer 2

The first and most critical mistake you have made is to not use

use strict;
use warnings;

These two pragmas provide more help than any other two lines of code you could possibly add. Never write Perl code without them.

use JSON;   
use XML::Simple;
use File::Spec;

You do not use either of these modules, so for this code, they are redundant.

$num_args = $#ARGV + 1;
if ($num_args != 1) {
    print "\nUsage: $0 <input directory>\n";
    exit;
}

This can be written

if (@ARGV != 1) {
    print ...
    exit;
}

Because an array in scalar context returns its size.

opendir(DIR, $dir) or die "cannot open directory";
@docs = grep(/\.xml$/,readdir(DIR));
foreach $file (@docs) {
    my $abs_path = join("",$dir,$file);

Unless you really need the precision of readdir, what you most often want is glob(). This code will replace the above code:

my @docs = glob "$dir/*.xml";

It will return the relative path (not the absolute path, as your variable name implies -- unless the path argument is absolute). For example: examples/foo.xml.

sub print_output {
    my $output_object = $_[0];
    print $output_object;
}

I assume this sub is just an example, because it does nothing, and can be reduced to a simple print.

As to your question about default directory to read from, that to me is at odds with the requirement of an argument (where you check @ARGV). But you can do a simple check for existence:

if (not -e $dir) {
    $dir = "default";
}

Answer 3

The constant pragma and a conditional statement modifier would make your default logic rather readable:

....
use constant DEFAULT_DIR => '/path/to/default/dir';
....

my $dir = $ARGV[0];
$dir = DEFAULT_DIR unless -e $dir;
....

(And, yes, do take notice of others' suggestions: strict, warnings, lexical dirhandles, pruning unneeded modules, avoiding $#array, etc.)