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Allow to launch nc like a daemon, in background until you still stop it.
You can stop it with kill %1 (jobs method) or kill PID.
The -k option can force nc to listen another connection, but if you use redirection, it will work only one time.
The loop's inside doesn't do anything, but we can imagine to send a message to screen when a connection is established
An example config file is placed in the sample output along with the command line call to use it.
The rsync daemon here is setup on the destination, thus requiring the read only = false flag. Also it uses uid and gid of root, change as required.
This is like ping -a, but it does the opposite. It alerts you if the network is down, not up. Note that the beep will be from the speaker on the server, not from your terminal.
Once a second, this script checks if the Internet is accessible and beeps if it is not. I define the Net as being "UP", if I can ping Google's public DNS server (18.104.22.168), but of course you could pick a different static IP address. I redirect the beep to /dev/console so that I can run this in the background from /etc/rc.local. Of course, doing that requires that the script is run by a UID or GID that has write permissions to /dev/console (usually only root).
Question: I am not sure if the -W1 flag works under BSD. I have only tested this under GNU/Linux using ping from iputils. If anybody knows how portable -W is, please post a comment.