When Snort is run as a daemon , the daemon creates a PID file in the log directory. In Snort 2.6, the -pid-path command line switch causes Snort to write the PID file in the directory specified.
Additionally, the -create-pidfile switch can be used to force creation of a PID file even when not running in daemon mode.
The PID file will be locked so that other snort processes cannot start. Use the -nolock-pidfile switch to not lock the PID file.