perl outrage

[trochee]

non-geeks, avert thine eyes.

I just had to rant somewhere about a very bad paragraph of perl:

opendir (DIR, $dir) || die "cannot open $dir\n";
while ($_ = readdir(DIR))
{
    if(/^foo/){;}       #OK
    else{next;}

   if(/^bar/){next;}   # skip bar data
   print STDERR "$_ ";
   `csh -c "cat $dir/$_/* | $pipethrough >> $dir/temporary"`;
}
`csh -c "cat  $dir/temporary | anonymize > $output"`;
print STDERR "\nDone\n";
closedir(DIR);
`rm $dir/temporary`;
exit;
# more code here...

Augh. I'll rewrite this when I have time.

Bonus question: How many processes are started by the following?

`csh -c "cat  $dir/temporary | anonymize > $output"`;

Here's my rewrite:

opendir (DIR, $dir) or die "cannot open $dir: $!\n";
while ($_ = readdir <DIR>)
{
   next unless /^foo/; # must begin with foo
   next if /^bar/;     # skip beginning with bar

   warn "$_\n";
   `cat $dir/$_/* | $pipethrough >> $dir/temporary`;
}
`anonymize < $dir/temporary > $output`;
warn "\nDone\n";

closedir(DIR) or die "couldn't close directory $dir: $!\n";
unlink($dir/temporary) or warn "couldn't unlink $dir/temporary: $!\n";

Comments

Re: augh!

[lx.livejournal.com]

Oh! You know, it occurred to me that the backtick (qx) might invoke its own process but I tested that it didn't when using bash as the shell rather than csh using the $$ pid sequence. I guess I assumed it would be the same with csh, but I get very different results. With bash:

#!/usr/bin/perl

print "my PID is $$\n";
print `bash -c "./test2.pl"`

test2.pl just outputs its own pid, and I consistently got a pid one off of the initial pid, like so:

$ ./test.pl
my PID is 21729
my PID is 21730

So it seemed that the bash component of the qx() wasn't invoking its own process, since it would have to get a pid before test2.pl did, but the numbers are sequential. However, doing it with csh I got:

$ ./test.pl
my PID is 21746
my PID is 21767

Somehow these two scripts are 21 process increments apart. It's not clear to me how that's happening. Do you know, am I misreading something here, is my testing procedure flawed, or is the csh call actually somehow grabbing 21 processes (or causing the OS to skip 21 pids?). I ran it repeatedly and each time I get a skip of 21 pids between test.pl and test2.pl.

Weird.

Re: augh!

[trochee.livejournal.com]

csh is so broken that it wouldn't surprise me if it needed 21 pids just to patch bugs:

http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/

Re: augh!

[lx.livejournal.com]

Haha, thanks for the link. I've never actually even seen csh in active use (though I have seen the ostensibly fixed tcsh on MacOS X as the default shell), now I'll know to avoid it with good reason. ;)

Of course, if the 21 pid thing is really csh, that amounts to something like 25 additional procs to this script for each invocation of that system call! :) Woooo.