In a current project, I have the need to make sure that a script can only run a single copy at a time. I'd like to hear comments on the sub routine below, which gets called at the beginning of a script. I'm hoping to 'solidify' it enough for reuse in my workplace (across our Unix and Win32 environments).

The latest tweak I've been contemplating is whether calling flock() on PID_FILE would gain me anything. Perhaps preventing a race condition if 2 processes were launched simultaneously? 

sub single_instance{
    $SIG{INT} = sub {exit()};
    my ($prog) = $0 =~ m|(?:.*[/\\])?(.*)$|;
    my $tmp_dir = $^O =~ "MSWin32" ? "C:/Windows/Temp" : "/tmp";
    my $lockfile = "$tmp_dir/$prog.pid"; 
    local *PID_FILE; 
    if (-e $lockfile){ 
        open(PID_FILE, "<$lockfile") or die("single_instance(): Can't 
+open $lockfile: $!\n"); 
        my $running_pid = <PID_FILE>; 
        close(PID_FILE); 
        chomp($running_pid); 
        if (kill 0, $running_pid){ 
            die("Already running as pid $running_pid\n"); 
            } 
        } 
    open(PID_FILE, "+>$lockfile") or die("single_instance(): Can't ope
+n $lockfile: $!\n"); 
    print PID_FILE "$$\n"; 
    close(PID_FILE); 
    chmod(0666, $lockfile);
    eval "END {unlink '$lockfile'}"; 
    die("single_instance(): $@\n") if($@); 
    }

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-Nitrox

In reply to Determine if script is already running by Nitrox

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