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Calculate days on which Friday the 13th occurs
Simply change the years listed in the first seq, and it will print out all the months in that span of years that have Friday the 13ths in them.

Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Find and copy files from subdirectories to the current directory

Scan your LAN for unauthorized IPs
populate the auth.hosts file with a list of IP addresses that are authorized to be in use and when you run this command it will return the addresses that are pingable and not in the authorized list. Can be combined with the "Command line Twitter" command to tweet unauthorized access.

Remotely sniff traffic and pass to snort
I have a small embedded linux device that I wanted to use for sniffing my external network, but I didn't want to recompile/cross-compile snort for the embedded platform. So I used tcpdump over ssh to pass all the traffic as pcap data to a "normal" Linux system that then takes the pcap data and passes it to snort for processing.

Get your external IP address if your machine has a DNS entry
Using DynDNS or a similar service not only allows access to your home machine from outside without needing to know what IP the ISP has assigned to it but it also comes in handy if you want to know your external IP address. The only purpose of the sed command is to remove the leading "host.na.me has address " part from the output. If you don't need to discard it you can simply use $ host $HOSTNAME

Count to 65535 in binary (for no apparent reason)
Yes, it's useless.

Add .gitignore files to all empty directories recursively from your current directory

Duplicate installed packages from one machine to the other (RPM-based systems)

Listen to BBC Radio from the command line.
This command lets you select from 10 different BBC stations. When one is chosen, it streams it with mplayer. Requires: mplayer with wma support.


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