[22326] in Perl-Users-Digest
Perl-Users Digest, Issue: 4547 Volume: 10
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Tue Feb 11 00:10:57 2003
Date: Mon, 10 Feb 2003 21:10:11 -0800 (PST)
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Mon, 10 Feb 2003 Volume: 10 Number: 4547
Today's topics:
Posting Guidelines for comp.lang.perl.misc ($Revision: tadmc@augustmail.com
Re: Quick regexp question <dha@panix.com>
Re: RegEx Questions (Tad McClellan)
Re: Request help with this script (Tad McClellan)
Re: Storing `ls -R` into an array <crgutierNO@SPAMdcc.uchile.cl>
Re: Storing `ls -R` into an array <jurgenex@hotmail.com>
Re: Two annoying Perl syntax issues <abigail@abigail.nl>
Re: Two annoying Perl syntax issues <jurgenex@hotmail.com>
Re: Using scalars as hash names <crgutierNO@SPAMdcc.uchile.cl>
Re: Using scalars as hash names <jurgenex@hotmail.com>
Re: What do you think of this? <abigail@abigail.nl>
Why won't my for loop work? (Karen Lofstrom)
Re: Why won't my for loop work? <jurgenex@hotmail.com>
Re: Why won't my for loop work? <jkeen@concentric.net>
Re: Why won't my for loop work? <xyf@nixnotes.org>
Re: Why won't my for loop work? <goldbb2@earthlink.net>
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Mon, 10 Feb 2003 21:13:27 -0600
From: tadmc@augustmail.com
Subject: Posting Guidelines for comp.lang.perl.misc ($Revision: 1.3 $)
Message-Id: <Ec-cnf-mV9fK99WjXTWcrg@august.net>
Outline
Before posting to comp.lang.perl.misc
Must
- Check the Perl Frequently Asked Questions (FAQ)
- Check the other standard Perl docs (*.pod)
Really Really Should
- Lurk for a while before posting
- Search a Usenet archive
If You Like
- Check Other Resources
Posting to comp.lang.perl.misc
Is there a better place to ask your question?
- Question should be about Perl, not about the application area
How to participate (post) in the clpmisc community
- Carefully choose the contents of your Subject header
- Use an effective followup style
- Speak Perl rather than English, when possible
- Ask perl to help you
- Do not re-type Perl code
- Provide enough information
- Do not provide too much information
- Do not post binaries, HTML, or MIME
Social faux pas to avoid
- Asking a Frequently Asked Question
- Asking a question easily answered by a cursory doc search
- Asking for emailed answers
- Beware of saying "doesn't work"
- Sending a "stealth" Cc copy
Be extra cautious when you get upset
- Count to ten before composing a followup when you are upset
- Count to ten after composing and before posting when you are upset
-----------------------------------------------------------------
Posting Guidelines for comp.lang.perl.misc ($Revision: 1.3 $)
This newsgroup, commonly called clpmisc, is a technical newsgroup
intended to be used for discussion of Perl related issues (except job
postings), whether it be comments or questions.
As you would expect, clpmisc discussions are usually very technical in
nature and there are conventions for conduct in technical newsgroups
going somewhat beyond those in non-technical newsgroups.
This article describes things that you should, and should not, do to
increase your chances of getting an answer to your Perl question. It is
available in POD, HTML and plain text formats at:
http://mail.augustmail.com/~tadmc/clpmisc.shtml
For more information about netiquette in general, see the "Netiquette
Guidelines" at:
http://andrew2.andrew.cmu.edu/rfc/rfc1855.html
A note to newsgroup "regulars":
Do not use these guidelines as a "license to flame" or other
meanness. It is possible that a poster is unaware of things
discussed here. Give them the benefit of the doubt, and just
help them learn how to post, rather than assume
A note about technical terms used here:
In this document, we use words like "must" and "should" as
they're used in technical conversation (such as you will
encounter in this newsgroup). When we say that you *must* do
something, we mean that if you don't do that something, then
it's unlikely that you will benefit much from this group.
We're not bossing you around; we're making the point without
lots of words.
Do *NOT* send email to the maintainer of these guidelines. It will be
discarded unread. The guidelines belong to the newsgroup so all
discussion should appear in the newsgroup. I am just the secretary that
writes down the consensus of the group.
Before posting to comp.lang.perl.misc
Must
This section describes things that you *must* do before posting to
clpmisc, in order to maximize your chances of getting meaningful replies
to your inquiry and to avoid getting flamed for being lazy and trying to
have others do your work.
The perl distribution includes documentation that is copied to your hard
drive when you install perl. Also installed is a program for looking
things up in that (and other) documentation named 'perldoc'.
You should either find out where the docs got installed on your system,
or use perldoc to find them for you. Type "perldoc perldoc" to learn how
to use perldoc itself. Type "perldoc perl" to start reading Perl's
standard documentation.
Check the Perl Frequently Asked Questions (FAQ)
Checking the FAQ before posting is required in Big 8 newsgroups in
general, there is nothing clpmisc-specific about this requirement.
You are expected to do this in nearly all newsgroups.
You can use the "-q" switch with perldoc to do a word search of the
questions in the Perl FAQs.
Check the other standard Perl docs (*.pod)
The perl distribution comes with much more documentation than is
available for most other newsgroups, so in clpmisc you should also
see if you can find an answer in the other (non-FAQ) standard docs
before posting.
It is *not* required, or even expected, that you actually *read* all of
Perl's standard docs, only that you spend a few minutes searching them
before posting.
Try doing a word-search in the standard docs for some words/phrases
taken from your problem statement or from your very carefully worded
"Subject:" header.
Really Really Should
This section describes things that you *really should* do before posting
to clpmisc.
Lurk for a while before posting
This is very important and expected in all newsgroups. Lurking means
to monitor a newsgroup for a period to become familiar with local
customs. Each newsgroup has specific customs and rituals. Knowing
these before you participate will help avoid embarrassing social
situations. Consider yourself to be a foreigner at first!
Search a Usenet archive
There are tens of thousands of Perl programmers. It is very likely
that your question has already been asked (and answered). See if you
can find where it has already been answered.
One such searchable archive is:
http://groups.google.com/advanced_group_search
If You Like
This section describes things that you *can* do before posting to
clpmisc.
Check Other Resources
You may want to check in books or on web sites to see if you can
find the answer to your question.
But you need to consider the source of such information: there are a
lot of very poor Perl books and web sites, and several good ones
too, of course.
Posting to comp.lang.perl.misc
There can be 200 messages in clpmisc in a single day. Nobody is going to
read every article. They must decide somehow which articles they are
going to read, and which they will skip.
Your post is in competition with 199 other posts. You need to "win"
before a person who can help you will even read your question.
These sections describe how you can help keep your article from being
one of the "skipped" ones.
Is there a better place to ask your question?
Question should be about Perl, not about the application area
It can be difficult to separate out where your problem really is,
but you should make a conscious effort to post to the most
applicable newsgroup. That is, after all, where you are the most
likely to find the people who know how to answer your question.
Being able to "partition" a problem is an essential skill for
effectively troubleshooting programming problems. If you don't get
that right, you end up looking for answers in the wrong places.
It should be understood that you may not know that the root of your
problem is not Perl-related (the two most frequent ones are CGI and
Operating System related), so off-topic postings will happen from
time to time. Be gracious when someone helps you find a better place
to ask your question by pointing you to a more applicable newsgroup.
How to participate (post) in the clpmisc community
Carefully choose the contents of your Subject header
You have 40 precious characters of Subject to win out and be one of
the posts that gets read. Don't waste them. Take care while
composing them, they are the key that opens the door to getting an
answer.
Spend them indicating what aspect of Perl others will find if they
should decide to read your article.
Do not spend them indicating "experience level" (guru, newbie...).
Do not spend them pleading (please read, urgent, help!...).
Do not spend them on non-Subjects (Perl question, one-word
Subject...)
For more information on choosing a Subject see "Choosing Good
Subject Lines":
http://www.perl.com/CPAN-local/authors/Dean_Roehrich/subjects.post
Part of the beauty of newsgroup dynamics, is that you can contribute
to the community with your very first post! If your choice of
Subject leads a fellow Perler to find the thread you are starting,
then even asking a question helps us all.
Use an effective followup style
When composing a followup, quote only enough text to establish the
context for the comments that you will add. Always indicate who
wrote the quoted material. Never quote an entire article. Never
quote a .signature (unless that is what you are commenting on).
Intersperse your comments *following* each section of quoted text to
which they relate. Unappreciated followup styles are referred to as
"Jeopardy" (because the answer comes before the question), or
"TOFU".
Reversing the chronology of the dialog makes it much harder to
understand (some folks won't even read it if written in that style).
For more information on quoting style, see:
http://web.presby.edu/~nnqadmin/nnq/nquote.html
Speak Perl rather than English, when possible
Perl is much more precise than natural language. Saying it in Perl
instead will avoid misunderstanding your question or problem.
Do not say: I have variable with "foo\tbar" in it.
Instead say: I have $var = "foo\tbar", or I have $var = 'foo\tbar',
or I have $var = <DATA> (and show the data line).
Ask perl to help you
You can ask perl itself to help you find common programming mistakes
by doing two things: enable warnings (perldoc warnings) and enable
"strict"ures (perldoc strict).
You should not bother the hundreds/thousands of readers of the
newsgroup without first seeing if a machine can help you find your
problem. It is demeaning to be asked to do the work of a machine. It
will annoy the readers of your article.
You can look up any of the messages that perl might issue to find
out what the message means and how to resolve the potential mistake
(perldoc perldiag). If you would like perl to look them up for you,
you can put "use diagnostics;" near the top of your program.
Do not re-type Perl code
Use copy/paste or your editor's "import" function rather than
attempting to type in your code. If you make a typo you will get
followups about your typos instead of about the question you are
trying to get answered.
Provide enough information
If you do the things in this item, you will have an Extremely Good
chance of getting people to try and help you with your problem!
These features are a really big bonus toward your question winning
out over all of the other posts that you are competing with.
First make a short (less than 20-30 lines) and *complete* program
that illustrates the problem you are having. People should be able
to run your program by copy/pasting the code from your article. (You
will find that doing this step very often reveals your problem
directly. Leading to an answer much more quickly and reliably than
posting to Usenet.)
Describe *precisely* the input to your program. Also provide example
input data for your program. If you need to show file input, use the
__DATA__ token (perldata.pod) to provide the file contents inside of
your Perl program.
Show the output (including the verbatim text of any messages) of
your program.
Describe how you want the output to be different from what you are
getting.
If you have no idea at all of how to code up your situation, be sure
to at least describe the 2 things that you *do* know: input and
desired output.
Do not provide too much information
Do not just post your entire program for debugging. Most especially
do not post someone *else's* entire program.
Do not post binaries, HTML, or MIME
clpmisc is a text only newsgroup. If you have images or binaries
that explain your question, put them in a publically accessible
place (like a Web server) and provide a pointer to that location. If
you include code, cut and paste it directly in the message body.
Don't attach anything to the message. Don't post vcards or HTML.
Many people (and even some Usenet servers) will automatically filter
out such messages. Many people will not be able to easily read your
post. Plain text is something everyone can read.
Social faux pas to avoid
The first two below are symptoms of lots of FAQ asking here in clpmisc.
It happens so often that folks will assume that it is happening yet
again. If you have looked but not found, or found but didn't understand
the docs, say so in your article.
Asking a Frequently Asked Question
It should be understood that you may have missed the applicable FAQ
when you checked, which is not a big deal. But if the Frequently
Asked Question is worded similar to your question, folks will assume
that you did not look at all. Don't become indignant at pointers to
the FAQ, particularly if it solves your problem.
Asking a question easily answered by a cursory doc search
If folks think you have not even tried the obvious step of reading
the docs applicable to your problem, they are likely to become
annoyed.
If you are flamed for not checking when you *did* check, then just
shrug it off (and take the answer that you got).
Asking for emailed answers
Emailed answers benefit one person. Posted answers benefit the
entire community. If folks can take the time to answer your
question, then you can take the time to go get the answer in the
same place where you asked the question.
It is OK to ask for a *copy* of the answer to be emailed, but many
will ignore such requests anyway. If you munge your address, you
should never expect (or ask) to get email in response to a Usenet
post.
Ask the question here, get the answer here (maybe).
Beware of saying "doesn't work"
This is a "red flag" phrase. If you find yourself writing that,
pause and see if you can't describe what is not working without
saying "doesn't work". That is, describe how it is not what you
want.
Sending a "stealth" Cc copy
A "stealth Cc" is when you both email and post a reply without
indicating *in the body* that you are doing so.
Be extra cautious when you get upset
Count to ten before composing a followup when you are upset
This is recommended in all Usenet newsgroups. Here in clpmisc, most
flaming sub-threads are not about any feature of Perl at all! They
are most often for what was seen as a breach of netiquette. If you
have lurked for a bit, then you will know what is expected and won't
make such posts in the first place.
But if you get upset, wait a while before writing your followup. I
recommend waiting at least 30 minutes.
Count to ten after composing and before posting when you are upset
After you have written your followup, wait *another* 30 minutes
before committing yourself by posting it. You cannot take it back
once it has been said.
AUTHOR
Tad McClellan <tadmc@augustmail.com> and many others on the
comp.lang.perl.misc newsgroup.
------------------------------
Date: Mon, 10 Feb 2003 23:09:24 +0000 (UTC)
From: "David H. Adler" <dha@panix.com>
Subject: Re: Quick regexp question
Message-Id: <slrnb4gc94.5j3.dha@panix2.panix.com>
In article <Xns931E494502E4Delhber1lidotechnet@62.89.127.66>, Bernard
El-Hagin wrote:
> Abigail wrote:
>
>> Bernard El-Hagin (bernard.el-hagin@DODGE_THISlido-tech.net) wrote on
>> MMMCDXLVII September MCMXCIII in
>
> Sarcasm is so last Tuesday.
Oh, it came back in on friday. You must not be on the list. :-)
dha
--
David H. Adler - <dha@panix.com> - http://www.panix.com/~dha/
I am r00t! Yo! Mad leet 5ex0ring with the 0vary-action!
- subbes
------------------------------
Date: Mon, 10 Feb 2003 14:54:20 -0600
From: tadmc@augustmail.com (Tad McClellan)
Subject: Re: RegEx Questions
Message-Id: <slrnb4g4bs.5t4.tadmc@magna.augustmail.com>
Tad McClellan <tadmc@augustmail.com> wrote:
> substr($_, $on, 4) = ','; # replace it with a comma
Oops. Better move that line following the 2 below, it changes
the number of characters...
> substr($_, $on+4) =~ s/ (\d+\.\d+)/,$1/; # comma before first
> substr($_, $on+4) =~ s/(\d+\.\d+) /$1,/g; # comma after all the others
--
Tad McClellan SGML consulting
tadmc@augustmail.com Perl programming
Fort Worth, Texas
------------------------------
Date: Mon, 10 Feb 2003 19:49:14 -0600
From: tadmc@augustmail.com (Tad McClellan)
Subject: Re: Request help with this script
Message-Id: <slrnb4glkq.1pc.tadmc@magna.augustmail.com>
Julia Briggs <julia4_me@yahoo.com> wrote:
> mainly what I desire
> is to make sure that some hacker can't mess with it. Any
> suggestions
Yes.
Check the Perl FAQ.
perldoc -q CGI
"How do I make sure users can't enter values into a form
that cause my CGI script to do bad things?"
> #!/usr/local/bin/perl
You aren't even protecting yourself from yourself, let alone
from external bad folks:
#!/usr/local/bin/perl
use strict;
use warnings;
> &check_url;
> &mailAttachment;
> &confirmation;
Where did you learn to call functions like that?
check_url();
mailAttachment();
confirmation();
> local($check_referer) = 0;
"You should always prefer my() over local() except when you can't."
my($check_referer) = 0;
or
my($check_referer);
> $$_=$query->param($_);
You should use Real References rather than symbolic references.
> open(MAIL, "| $mailprog -t ");
You should always, yes *always*, check the return value from open():
open(MAIL, "| $mailprog -t ") or die "could not run $mailprog $!";
> ## Send out the contents!!
^^
^^ superfluous youthful exuberance
> close (MAIL);
You should check the return value from close() too
when the filehandle is a pipe open.
--
Tad McClellan SGML consulting
tadmc@augustmail.com Perl programming
Fort Worth, Texas
------------------------------
Date: Tue, 11 Feb 2003 01:20:45 +0000 (UTC)
From: Cristian Gutierrez <crgutierNO@SPAMdcc.uchile.cl>
Subject: Re: Storing `ls -R` into an array
Message-Id: <b29j5d$6ut$3@helcaraxe.dcc.uchile.cl>
William English posteo' asi':
> I am doing this:
> @dirContents = `ls -R`;
> When I do a foreach and print each line, I get results like this:
> nt_empty_tree:
> _bin
> _lib
> bin
> data
> lib
> licenses
> userware
> file1
> file2
> When I just do an 'ls -R' from the command line, I get this:
> ./nt_empty_tree:
> _bin/
> _lib/
> bin/
> data/
> lib/
> licenses/
> userware/
> file1
> file2
This is the output of ls -FR. Check what your ls alias is doing with
"which ls".
> If you notice, directories have a "/" at the end and files do not. The
> array however does not have the "/" at the end.
See above.
> How can I tell what is a directory?
if (-d $some_file) {
# we know it's a directory here :)
}
> I know the first element in my example of 'ls -R' from
> the command line has an ":" at the end, so I set the directory path to
> that, but sometimes it is long:
> ./nt_empty_tree/_bin/tools/sae_tools/lib/perl5/bin:
Didn't get it.. what are you trying to do here?
--
Cristian Gutierrez Linux user #298162
crgutier[@]dcc.uchile.cl http://www.dcc.uchile.cl/~crgutier
"Fools ignore complexity. Pragmatists suffer it. Some can avoid it.
Geniuses remove it."
-- Alan Perlis
------------------------------
Date: Tue, 11 Feb 2003 03:29:10 GMT
From: "Jürgen Exner" <jurgenex@hotmail.com>
Subject: Re: Storing `ls -R` into an array
Message-Id: <a6_1a.19148$9y2.10282@nwrddc01.gnilink.net>
William English wrote:
> I am doing this:
>
> @dirContents = `ls -R`;
And why on earth are you writing non-portable code that forks an external
process instead of using Perl's buildin functions 'glob' or 'readdir' or
even 'File::Find'?
Jue
------------------------------
Date: 11 Feb 2003 00:45:27 GMT
From: Abigail <abigail@abigail.nl>
Subject: Re: Two annoying Perl syntax issues
Message-Id: <slrnb4ght7.sp3.abigail@alexandra.abigail.nl>
ixtahdoom (ixtahdoom@yahoo.com) wrote on MMMCDL September MCMXCIII in
<URL:news:2aa9aa08.0302101007.43ca2924@posting.google.com>:
!! Hi,
!!
!! Here are two things that bug me about Perl, and I bet there is an easy
!! solution to both of them. They both are related to the fact that I
!! would like to do an operation in ONE line, rather than TWO. Nitpicky?
!! Yes. I slimmed down the crux of the issues:
!!
!! 1st.
!! I have a file in $temp = "/etc/stuff/foo.gz";
!!
!! I want to make a path called "/etc/stuff", but this always takes me
!! two steps:
!!
!! my $path = $temp;
!! $path =~ s/\/[^\/]*$//;
!!
!! Is there a way to do this in ONE line? I tried join(pop(split..., but
!! Perl didn't like that.
my ($path) = $temp =~ m {(.*)/}; # Assuming $temp contains a /.
!! 2nd.
!!
!! Let's say I use a back-ticked fork, and want to remove the
!! whitespace...
!!
!! my $data = `gzgrep -c 'stuff' $file`;
!! $data = chomp $data;
!!
!! Again, how can I do this chomp during the assignment?
chomp (my $data = `gzgrep -c 'stuff' $file`);
But that's assuming you want to remove a trailing newline, instead
of all the whitespace.
Abigail
--
perl -wle 'eval {die ["Just another Perl Hacker"]}; print ${$@}[$#{@${@}}]'
------------------------------
Date: Tue, 11 Feb 2003 03:34:12 GMT
From: "Jürgen Exner" <jurgenex@hotmail.com>
Subject: Re: Two annoying Perl syntax issues
Message-Id: <Ua_1a.23581$F25.7531@nwrddc02.gnilink.net>
ixtahdoom wrote:
> Here are two things that bug me about Perl, and I bet there is an easy
> solution to both of them. They both are related to the fact that I
> would like to do an operation in ONE line, rather than TWO. Nitpicky?
> Yes. I slimmed down the crux of the issues:
>
> 1st.
> I have a file in $temp = "/etc/stuff/foo.gz";
>
> I want to make a path called "/etc/stuff", but this always takes me
> two steps:
>
> my $path = $temp;
> $path =~ s/\/[^\/]*$//;
>
> Is there a way to do this in ONE line? I tried join(pop(split..., but
> Perl didn't like that.
Yes, there is. Just use
$path = dirname($temp);
from the File::Basename module
> 2nd.
> Let's say I use a back-ticked fork, and want to remove the
> whitespace...
>
> my $data = `gzgrep -c 'stuff' $file`;
> $data = chomp $data;
>
> Again, how can I do this chomp during the assignment?
I think this command doesn't do what you think it does. Do you really want
to assign the number of removed characters to $data?
jue
------------------------------
Date: Tue, 11 Feb 2003 01:23:07 +0000 (UTC)
From: Cristian Gutierrez <crgutierNO@SPAMdcc.uchile.cl>
Subject: Re: Using scalars as hash names
Message-Id: <b29j9r$6ut$4@helcaraxe.dcc.uchile.cl>
Bob Van Der Ploeg posteo' asi':
> Can I use a scalar has a hash key?
> ex:
> foreach $hash_name (@hashes){
> while(($key,$value)=each(%$hash_name)){
> print "$hash_name $key is $value";
> }
> }
This will work as long as @hashes is a hash of hashes (and it seems to
be it). But the print statement will print hash reference (ugly)
first, and then the key/value combination.
--
Cristian Gutierrez Linux user #298162
crgutier[@]dcc.uchile.cl http://www.dcc.uchile.cl/~crgutier
Real Programmers don't write in FORTRAN. FORTRAN is for pipe stress
freaks and crystallography weenies.
------------------------------
Date: Tue, 11 Feb 2003 03:38:13 GMT
From: "Jürgen Exner" <jurgenex@hotmail.com>
Subject: Re: Using scalars as hash names
Message-Id: <Fe_1a.23612$F25.13252@nwrddc02.gnilink.net>
Bob Van Der Ploeg wrote:
> Can I use a scalar has a hash key?
Of course. Hashes are mappings from scalars to scalars, so yes, you can use
scalars as keys.
> foreach $hash_name (@hashes){
> while(($key,$value)=each(%$hash_name)){
> print "$hash_name $key is $value";
This looks more like you want to use a variable for the name(!) of the hash.
That's a totally different animal and while it is possible, you really don't
want to do it unless you know exactly why and how. If you have to ask the
question then you know neither why nor how.
Please see the FAQ about symbolic references ("How can I use a variable as a
variable name?") why your idea is not a good idea and what you should do
instead.
jue
------------------------------
Date: 11 Feb 2003 00:15:11 GMT
From: Abigail <abigail@abigail.nl>
Subject: Re: What do you think of this?
Message-Id: <slrnb4gg4f.sp3.abigail@alexandra.abigail.nl>
Brian McCauley (nobull@mail.com) wrote on MMMCDL September MCMXCIII in
<URL:news:u9hebc70s4.fsf@wcl-l.bham.ac.uk>:
{}
{} The best solution is to ignore alt.perl altogether.
[*]
Score:: -9999
Newsgroups: .*,.*,
Newsgroups: alt\.perl
Newsgroups: soc\.culture\.netherlands
Abigail
--
# Count the number of lines; code doesn't match \w. Linux specific.
()=<>;$!=$=;($:,$,,$;,$")=$!=~/.(.)..(.)(.)..(.)/;
$;++;$*++;$;++;$*++;$;++;`$:$,$;$" $. >&$*`;
------------------------------
Date: Tue, 11 Feb 2003 03:45:01 -0000
From: lofstrom@lava.net (Karen Lofstrom)
Subject: Why won't my for loop work?
Message-Id: <v4gsdtp27haj7b@corp.supernews.com>
I've been learning Perl all of four days :) I had a simple problem to
solve and Perl seemed the best way. Now I'm scratching my head.
I'm an intern at a large academic institute with a wealth of IPs. They
often give out static IPs to researchers who then connect their own
computers (since this is a specialty requiring lots of CPU time). Over the
years more than a few IPs haven't been returned to the pool for assignment
-- people get busy and things slip. So I'm being asked, as the lowest and
least skilled person on the totem pole, to ping all the IPs in certain
subnets to see which are alive. Once we get things sorted out a bit, I'll
be sending polite emails to researchers asking them if they still want
those IPs. Anyway ...
I spent all of one afternoon sitting there manually pinging IPs (up arrow,
backspace, type one new number, wait) until it hit me: Duh, there's a
better way. All I need to do is set up a for loop to increment the IPs and
then redirect the output to a file. I have a tiny bit of experience with
shell scripts (but not doing for loops) and a little bit more experience
with Java. So I settled on Perl as combining the best of both.
After racing through an introduction to Perl and then bouncing off the
camel book, I wrote my first real Perl program. Which didn't compile.
Lots of headbanging later, I got it to compile, but then it didn't run.
Then I got it to run but there's something wrong with the for loop. The
program asks me for a subnet number, creates the file, and then exits,
without doing any pings or writing anything to the file.
#!/usr/bin/perl
#Program named ping.perl pings a subnet
print "Please enter subnet number.\n";
chop($subnet = <STDIN>);
`touch \home\group14\lofstrom\$subnet.txt`;
for($i = 1; $i <255; $i++){
$address = ("xxx.xxx." + $subnet + "." + $i);
`ping $address`;
STDOUT >> $subnet.txt;
}
The xs replace the actual IP, because I don't anyone thinking the worse of
the academic institute because they've got a dumb intern.
The good folks there are swamped with work and don't really have time to
help me learn Perl. So could some kind person here PLEASE help me figure
out what's wrong with my for loop?
--
Karen Lofstrom lofstrom@lava.net
----------------------------------------------------------------------
Jack, you have debauched my sloth.
------------------------------
Date: Tue, 11 Feb 2003 04:09:30 GMT
From: "Jürgen Exner" <jurgenex@hotmail.com>
Subject: Re: Why won't my for loop work?
Message-Id: <_H_1a.3259$SB2.2625@nwrddc03.gnilink.net>
Karen Lofstrom wrote:
> #!/usr/bin/perl
> #Program named ping.perl pings a subnet
> print "Please enter subnet number.\n";
> chop($subnet = <STDIN>);
Are you sure you didn't mean 'chomp' instead?
> `touch \home\group14\lofstrom\$subnet.txt`;
Why are you using backticks if you are ignoring the returned output of the
external command?
Maybe you meant to use 'system' instead?
> for($i = 1; $i <255; $i++){
A more perlish way would be
for ($i = 1..254) {
> $address = ("xxx.xxx." + $subnet + "." + $i);
Ok, here you got a text containing e.g. "123.456.". The numerical value
would be 123.456. Then you are adding $subnet, e.g. 222, which would result
in 345.456.
Then you are adding the numerical value of "." (which is zero) and then $i.
Assuming $i is 222 now you got a total value of 567.456. Doesn't seem right
to me.
Maybe you meant to use string concatenation "." instead of numerical
addition "+"?
> `ping $address`;
Again, why backticks? You are not capturing the output of ping anywhere.
> STDOUT >> $subnet.txt;
Why do you want to shift the file descriptor STDOUT by $subnet.txt bits to
the right? That doesn't make any sense to me.
Maybe you meant to write the last two lines as one single external command?
system "ping $address >> $subnet.txt";
jue
------------------------------
Date: 11 Feb 2003 04:16:39 GMT
From: "James E Keenan" <jkeen@concentric.net>
Subject: Re: Why won't my for loop work?
Message-Id: <b29tf7$p2o@dispatch.concentric.net>
"Karen Lofstrom" <lofstrom@lava.net> wrote in message
news:v4gsdtp27haj7b@corp.supernews.com...
> I've been learning Perl all of four days :) I had a simple problem to
> solve and Perl seemed the best way. Now I'm scratching my head.
>
>
> #!/usr/bin/perl
At this point you should enable strictures and warnings to enforce good Perl
programming practices. use strict will force you to declare variables
properly scoped.
use strict;
use warnings;
> #Program named ping.perl pings a subnet
> print "Please enter subnet number.\n";
> chop($subnet = <STDIN>);
You probably want 'chomp' rather than 'chop'; it's more specific to the task
at hand.
> `touch \home\group14\lofstrom\$subnet.txt`;
> for($i = 1; $i <255; $i++){
> $address = ("xxx.xxx." + $subnet + "." + $i);
What are those + signs doing there? In Perl 5, concatenation of strings is
performed by the dot (.) operator -- not the + sign, which is used for
mathematical addition.
> `ping $address`;
> STDOUT >> $subnet.txt;
I don't understand the previous line. You seem to be using STDOUT (a
filehandle) as a function.
> }
The fact that you're using backticks to set off 2 separate shell processes
gives me a hunch that there's a Perl module out on CPAN somewhere that
already does what you want.
------------------------------
Date: Tue, 11 Feb 2003 04:26:41 GMT
From: ktb <xyf@nixnotes.org>
Subject: Re: Why won't my for loop work?
Message-Id: <slrnb4gurj.dc2.xyf@scab.nixnotes.org>
In article <v4gsdtp27haj7b@corp.supernews.com>, Karen Lofstrom wrote:
> I've been learning Perl all of four days :) I had a simple problem to
> solve and Perl seemed the best way. Now I'm scratching my head.
>
> I'm an intern at a large academic institute with a wealth of IPs. They
> often give out static IPs to researchers who then connect their own
> computers (since this is a specialty requiring lots of CPU time). Over the
> years more than a few IPs haven't been returned to the pool for assignment
> -- people get busy and things slip. So I'm being asked, as the lowest and
> least skilled person on the totem pole, to ping all the IPs in certain
> subnets to see which are alive. Once we get things sorted out a bit, I'll
> be sending polite emails to researchers asking them if they still want
> those IPs. Anyway ...
>
> I spent all of one afternoon sitting there manually pinging IPs (up arrow,
> backspace, type one new number, wait) until it hit me: Duh, there's a
> better way. All I need to do is set up a for loop to increment the IPs and
> then redirect the output to a file. I have a tiny bit of experience with
> shell scripts (but not doing for loops) and a little bit more experience
> with Java. So I settled on Perl as combining the best of both.
>
> After racing through an introduction to Perl and then bouncing off the
> camel book, I wrote my first real Perl program. Which didn't compile.
> Lots of headbanging later, I got it to compile, but then it didn't run.
> Then I got it to run but there's something wrong with the for loop. The
> program asks me for a subnet number, creates the file, and then exits,
> without doing any pings or writing anything to the file.
>
> #!/usr/bin/perl
> #Program named ping.perl pings a subnet
> print "Please enter subnet number.\n";
> chop($subnet = <STDIN>);
> `touch \home\group14\lofstrom\$subnet.txt`;
> for($i = 1; $i <255; $i++){
> $address = ("xxx.xxx." + $subnet + "." + $i);
> `ping $address`;
> STDOUT >> $subnet.txt;
> }
>
If you just want to ping a range of IPs on a subnet and place results in
a file I would do something like this -
#!/usr/bin/perl
use strict;
use warnings;
my $domain = "xxx.xx.xx.";
my $num = 1;
until ($num >= 255) {
system("ping -c 2 $domain$num >> ping.out");
$num++;
}
hth,
kent
--
To know the truth is to distort the Universe.
Alfred N. Whitehead (adaptation)
------------------------------
Date: Mon, 10 Feb 2003 23:46:44 -0500
From: Benjamin Goldberg <goldbb2@earthlink.net>
Subject: Re: Why won't my for loop work?
Message-Id: <3E488034.6AB0226B@earthlink.net>
Karen Lofstrom wrote:
[snip]
> #!/usr/bin/perl
The first lines of your file should be:
use strict;
use warnings;
> #Program named ping.perl pings a subnet
> print "Please enter subnet number.\n";
> chop($subnet = <STDIN>);
This should be chomp(), not chop(). Well, unless you're using perl4,
but don't do that.
> `touch \home\group14\lofstrom\$subnet.txt`;
You could do this faster without using `` and touch.
do { open my($f), ">>", "/home/group14/lofstrom/$subnet.txt" }
or die "Couldn't create $subnet.txt: $!";
> for($i = 1; $i <255; $i++){
This would be better written as:
for my $i ( 1 .. 255 ) {
Assuming, that is, that you really want to start from one, not from
zero, which is, after all, a valid address byte.
> $address = ("xxx.xxx." + $subnet + "." + $i);
The perl concatenation operator is ".", not "+", so this line should be:
my $address = "xxx.xxx." . $subnet . "." . $i;
Or, through the use of interpolation:
my $address = "xxx.xxx.$subnet.$i";
> `ping $address`;
This captures and discards the result of ping.
> STDOUT >> $subnet.txt;
This takes the string "STDOUT", performs the numeric right-shift
operation on it by $subnet concatenated with "net" ... somehow I don't
tihnk that's what you want to do.
> }
>
> The xs replace the actual IP, because I don't anyone thinking the
> worse of the academic institute because they've got a dumb intern.
>
> The good folks there are swamped with work and don't really have time
> to help me learn Perl. So could some kind person here PLEASE help me
> figure out what's wrong with my for loop?
It's not the for loop, but rather it's what you've got inside of the for
loop. I would suggest you do instead, something like the following:
#!/usr/bin/perl -w
use strict;
use warnings;
die "Usage: $0 <subnet>\n" unless @ARGV == 1;
my ($subnet) = @ARGV;
open(STDOUT, ">", "/home/group14/lofstrom/$subnet.txt")
or die "Couldn't open $subnet.txt for append: $!";
$| = 1;
print "Begun pings at ".localtime()."\n";
system("ping xxx.xxx.$subnet.$_") != -1
or die "Error running ping: $!"
for 0..255;
__END__
Well, actually, given the slowness of the ping program, I would try and
do the pings in parallel.
A simple approach for parallel pings would be:
#!/usr/bin/perl -w
use strict;
use warnings;
die "Usage: $0 <subnet>\n" unless @ARGV == 1;
my ($subnet) = @ARGV;
open(my($o), ">", "/home/group14/lofstrom/$subnet.txt")
or die "Couldn't open $subnet.txt for append: $!";
print $o "Begun pings at ".localtime()."\n";
my @p;
open( $p[$_], "-|", "ping xxx.xxx.$subnet.$_")
or die "Couldn't start ping: $!"
for 0..255;
undef $/;
print $o readline shift @o for 0..255;
close $o;
__END__
[untested]
For more complicated things... well... there's a possibility that your
system's ping program itself will do that for you, by allowing multiple
hosts on the commandline. Alternatively, you can use one of the various
perl modules on CPAN designed for this task.
--
"So, who beat the clueless idiot today?"
"Well, we flipped for it, but when Kuno
landed, he wasn't in any shape to fight."
"Next time, try flipping a *coin.*"
------------------------------
Date: 6 Apr 2001 21:33:47 GMT (Last modified)
From: Perl-Users-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Digest Administrivia (Last modified: 6 Apr 01)
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