Why doesn't this path work to open a Windows file in PERL?

Question

I tried to play with Strawberry Perl, and one of the things that stumped me was reading the files.

I tried to do:

open(FH, "D:\test\numbers.txt");

But it can not find the file (despite the file being there, and no permissions issues).

An equivalent code (100% of the script other than the filename was identical) worked fine on Linux.

Answer 1

As per Perl FAQ 5, you should be using forward slashes in your DOS/Windows filenames (or, as an alternative, escaping the backslashes).

Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?

Whoops! You just put a tab and a formfeed into that filename! Remember that within double quoted strings ("like\this"), the backslash is an escape character. The full list of these is in Quote and Quote-like Operators in perlop. Unsurprisingly, you don't have a file called "c:(tab)emp(formfeed)oo" or "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.

Either single-quote your strings, or (preferably) use forward slashes. Since all DOS and Windows versions since something like MS-DOS 2.0 or so have treated / and \ the same in a path, you might as well use the one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, awk, Tcl, Java, or Python, just to mention a few. POSIX paths are more portable, too.

So your code should be open(FH, "D:/test/numbers.txt"); instead, to avoid trying to open a file named "D:<TAB>est\numbers.txt"

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As an aside, you could further improve your code by using lexical (instead of global named) filehandle, a 3-argument form of open, and, most importantly, error-checking ALL your IO operations, especially open() calls:

open(my $fh, "<", "D:/test/numbers.txt") or die "Could not open file: $!";

Or, better yet, don't hard-code filenames in IO calls (the following practice MAY have let you figure out a problem sooner):

my $filename = "D:/test/numbers.txt";
open(my $fh, "<", $filename) or die "Could not open file $filename: $!";

Answer 2

Never use interpolated strings when you don't need interpolation! You are trying to open a file name with a tab character and a newline character in it from the \t and the \n!

Use single quotes when you want don't need (or want) interpolation.

One of the biggest problems novice Perl programmers seem to run into is that they automatically use "" for everything without thinking. You need to understand the difference between "" and '' and you need to ALWAYS think before you type so that you choose the right one. It's a hard habit to get into, but it's vital if you're going to write good Perl.