UNIX for Beginners Questions & Answers

Greetings!

Been a while since I futzed around with Perl, and came upon a minor headscratcher for the community

Here's the basic code which I'm trying to make tick over:

Code:

#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;

print " starting ";
while (-e "~/.somedir/testFile")
{
    print " file detected ";
    sleep 1;
}
print " file missing ";

exit;

Queerly, this will not work on hidden (.somedir) dirs!

So, not being one to give up too easily, I even tried passing a couple bashlines to the system as well. Here are the better substitute "while" statements which I tried:

Code:

while (`wc -l ~/.somedir/testFile | sed -e 's/ \/.*$//'` eq "0")
while (`ls -a ~/.somedir | grep -e \"testFile\"` eq "testFile")

Of course, none of these worked in our Perl context

Any ideas for a "simple" solution amended from this rag-tag assortment would be quite appreciated!

Thanks again --

I commend you for making use of the pragmas strict, warnings and diagnosis, especially, when you are troubleshooting/learning.
Still, I wish you would had declared your purpose. It is not clear by looking at the posted snippet.
In other words, I cannot infer what you are trying to do by what you have posted. Please, clarify what you are trying to achieve.

exit; is not necessary.

Thank you, Aia, for posting! Sorry for the confusion...

Here's the program flow which I am seeking to achieve with my crude little snippet:
Not too exciting; but I think it about encapsulates what I need to do

To this end, I have formed three different while() statements to interchangeably fit into the main script code:

Code:

while (`wc -l ~/.somedir/testFile | sed -e 's/ \/.*$//'` eq "0")
while (`ls -a ~/.somedir | grep -e \"testFile\"` eq "testFile")
while (-e "~/.somedir/testFile")

The commandlines from 1) and 2) both work well as stand-alone code at the terminal; but do not execute properly from Perl. #2 is particularly well-suited to the work; as it doesn't rely upon the file being of any particular size to allow for successful detection. String comparisons should logically work as expected in this context.

Choice #3 is preferred as a straightforward solution; but, unfortunately, Perl will not find our "testFile" on a hidden directory such as ~/.somedir using the normal

Code:

while (-e "~/.somedir/testFile")

statement layout.

So, that's the problem in a nutshell...

Again, I'd prefer going with the straight-Perl approach which #3 offers. But, if making this work is more complicated than getting the commandline in

Code:

while (`ls -a ~/.somedir | grep -e \"testFile\"` eq "testFile")

to function correctly in this code context, I'll settle for a just "quick fix" of #2 for now.

Thanks again!

That sounds fishy to me as nothing about UNIX except ls cares about hidden files and folders.

You know what? It doesn't work on non-hidden dirs either. It always fails, because ~/ which is a shell feature not a perl one.

Use an absolute path, or substitute HOME a la $ENV{"HOME"}."/folder/file" and -e works fine.

There is no problems for Perl using the hidden directories and files, the issue is with ~. It does not translate to home like in the shell.
Please, try:

Code:

while ( -e "$ENV{'HOME'}/.somedir/testFile" )
{
    $| = 1; # to flush the print without the newline
    print " file detected ";
    sleep 1;
}

Thank you both so much for the input! The Perl code seems to work well now; and I picked up yet another kernel of savvy

One last question, if folks have a moment.

Not furnished with network access yesterday afternoon, I fiddled for a while and found out why

Code:

while (`ls -a ~/.somedir | grep -e \"testFile\"` eq "testFile")

from the OP didn't work either.

Seems as though an unidentified character is being "automagically" appended to the "testFile" string which is returned to Perl. The following updated line works, but it seems a bit "sloppy" to allow anything aside from a neat eq operation with a string comparison operation like this:

Code:

while (`ls -a ~/.somedir | grep -s -e \"testFile\"` ge "testFile")

In the spirit of learning new stuff, is there a way of cleaning up this output with a bit of additional inline instruction to allow something like

Code:

while (`ls -a ~/.somedir | grep -s -e \"testFile\"` eq "testFile")

to actually work as anticipated?

So much to learn...

Thanks again; and have a great day

I get that you are learning or trying to learn. Nevertheless, I question the desire to learn something you will not have any use for it, if you wanted to do it correctly.

Calling two different system programs to pipe them together in order to evaluate the resulted output would be the hallmark of someone that probably needs to do things in a shell and not with Perl. Not considering, that what you are accepting as a valid comparison is not.

Addressing the highlighted red part of the quote: Is it not an `unidentified' character. It's a \n from running the command ls -a