Check if directory exists in ~/ using Perl

Question

I noticed strange behavior of the -d flag to check if a file is a directory and if the directory exists. It behaves differently when using ~/my_dir as my path.

The following code will return false, even though the directory my_dir exists, but if I'll change it to a full path like /home/ricky/my_dir, then the if statement will return true.

#!/usr/bin/perl -w
#
use strict;

if ( -d "~/my_dir") {
    print "Found \n";
}
else {
    print "Not found \n";
}

What's the difference?

Solution 1

~ is a shell shortcut to indicate the home directory, it is unknown to perl. You have to use the environment variable HOME here -- for instance using "$ENV{HOME}/my_dir".

Solution 2

~ is not a real path. Shells such as /bin/bash expand it to the value of the $HOME environment variable. Perl does not. So, you should use something like this instead:

if ( -d "$ENV{HOME}/my_dir" ) {
    ...
}

Comments

[Ricky Levi]

BTW, using system vs back-ticks makes ~/ work in the 'system' function, which is also a difference that I just found ... ( also symlinks have the same issue ) - thanks for the answer :)

[ikegami]

Unknown to the system, even. Perl doesn't do anything with the path but pass it to the system.

[aschepler]

@Ricky That's because the system function runs a /bin/sh, which will do its shell substitutions on your command line.

[Ricky Levi]

@aschepler I just run system('echo $SHELL'); and it says /bin/bash .. so I'm not sure this is correct.

[aschepler]

That's just the environment variable. Try system(q(ps $$)).

[Ricky Levi]

@aschepler I'm not sure what I'm suppose to see ... this is the output 17507 pts/0 R+ 0:00 ps 17507

[Keith Thompson]

-d is the correct way to tell whether a directory exists. chdir has the side effect of changing to the directory; it will also fail if dir_name is a directory but you don't have permission to change to it. And this doesn't address the OP's problem, which is the use of ~.