Simple Process Locking In Perl

My co-work Rick asked for a simple process lock control in Perl that would work over a server load balanced pool with a common NFS share. The easiest way is a loop that tries to create a directory, if the command fails then it waits and tries again, if the command succeeds then it has the lock and can do it's job. When done it removes the directory and another process gets to continue.

We started to build in an extra step so that if the script dies during the "meat and potatoes" section anther process can check if the PID from the previous process is still running and if not remove it. We did not get that far, I will let you figure the rest out.

The first usleep command is just to prevent process hammering, you should set it to match your expected time for a process to run. The second usleep is to simulate process congestion and is only for testing, you should remove it from your project.

#!/usr/bin/perl -w

use Time::HiRes qw(usleep nanosleep);

my $COUNTER=0;
my $dir = "/mnt/share/templock.dir";

while (! mkdir($dir) )
{
 $COUNTER++; 
 usleep(100);

 printf ("$$ waiting: $COUNTER\n");
 if ($COUNTER > 1000)
 {
  #open(my $fh, "<", $dir/PID");
  #my $row = <$fh>;
  
  die("squak\n");
 }
}

open(my $fh, ">", "$dir/PID");
printf $fh "$$\n";
close($fh);

#meat and potatoes
usleep(10000);


unlink("$dir/PID");
rmdir($dir);

Some tail or some head

Often times I find that head and tail don't put out enough. Some times I just want more. I don't want to create aliases for them because there are times that I just need it to work normally and I am too lazy to absolute path to remove the alias.

This is how I am able to me some tail and some head with more.

In my home I have a bin directory that is just for my bin scripts, but you could put this in /usr/local/bin/ if you like.

I wanted one script that does both, so I use basename and the built in BASH test replacement operation to strip the command down to either head or tail. I created the script as somehead and then made a symlink that points to it called sometail.

$(basename ${0//some/}) -n $(( $(tput lines) - 3 ))

Notice this uses tput to get the number of lines in the terminal and then subtracts three, just so it does not overflow the screen and you get an idea of where the output started.

Remember to

chmod 755 somehead
ln -s somehead sometail