How can I prevent more than one instance of a script from running?

I use flock -n "$0" true || exit 1 in some of my shell scripts that need locking (i.e. check whether the script itself has an advisory lock on it). The same could be implemented in Perl:

use FindBin qw($Bin $Script);
use Fcntl ':flock';

open my $fh, '>>', "$Bin/$Script" or die "open(>>$Bin/$Script): $!";
flock $fh, LOCK_EX|LOCK_NB or die "Another copy seems to be running";
[download]

The >> mode may be required depending on how flock is implemented (you may get by with just <). Alternatively, remove |LOCK_NB to make your process wait until it has the lock instead of terminating. Lock is automatically removed when the owning process terminates, so no need to manually unlock anything.

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.