Category: Computing

CPAN carries a module called File::Pid, that implements a PID file object that writes to a file. The documentation suggests it be used as:

  use File::Pid;
  my $pidfile = File::Pid->new({
    file => '/some/file.pid',
  });
  $pidfile->write;
  if ( my $num = $pidfile->running ) {
      die "Already running: $num\n";
  }
  $pidfile->remove;

However, if you write() before calling running(), then the PID in the PID file gets overwritten with the current script’s PID, and thus running() always returns the current scripts PID, and thus, if you run the example, it always bombs out.
Instead, you want to put the call to write() after the call to running():

  use File::Pid;
  my $pidfile = File::Pid->new({file => '/some/file.pid',  });
  die "Already running in PID $num" if ( my $num = $pidfile->running );
  $pidfile->write;
  do_seomthing_useful();
  $pidfile->remove;

Easy.

Perl is a reasonable scripting language (as are others, so shh!). It has always had strong regular expression support; those regular expressions can also be used to do substitutions, such as:


my $pet = "I have a dog";
$pet =~ s/dog/cat/;


Neat enough. But lets say I want to look up the parts of the “s///” that define my search text, and my replacement text. Easy enough:


my $pet = "I have a dog";
my $search = "dog";
my $replace = "cat";
$pet =~ s/$search/$replace/;


But lets make our substitution a little more complex – I want to match a URL, and have the host and port lower case, but leave the path as the case it comes in, and I don’t want to be entering expressions as the replacement text! Lets try:


my $url = 'http://www.FOo.COm/wibbLE';
my $search = '^([^:]+://[^/]+)/?(.*)?$';
my $replace = '\L$1\E/$2';
print $url if $url =~ s/$search/$replace/;


Sadly, while $search matches, the replace is the string “\L$1\E/$2“. It appears that we need to use a combination of “/e” mode (evaluate as expression) to evaluage this replacement string. Of course, when we’re doing eval, we want to ensure we don’t have malicious content in $replace, such as “unlink()” and friends who could do Bad Things. So my solution was to escape double quotes from my $replace string, wrap that all in double quotes, and pass in “/ee“:


my $search = '^([^:]+://[^/]+(:\d+)?)/?(.*)?$'; # From database
my $replace = '\L$1\E/$3'; # From database
$replace =~ s/"/\\"/g; # Protection from embedded code
$replace = '"' . $replace . '"'; # Put in a string for /ee
print $url if $url =~ s/$search/$replace/ee;


This will give us:

• Port and host name lower case
• Hostname (and port) will always have a slash after it
• Bad code like unlink() won’t be run
• The expression that we initally set/fetch/got in $replace is just a vanilla replacement term, not arbitary Perl code.