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Trigger a command each time a file is created in a directory (inotify)
Listens for events in the directory. Each created file is displayed on stdout. Then each fileline is read by the loop and a command is run. This can be used to force permissions in a directory, as an alternative for umask. More details: http://en.positon.org/post/A-solution-to-the-umask-problem%3A-inotify-to-force-permissions

List Listen Port by numbers
Show TCP Listen ports sorted by number (bugs: IPV6 addresses not supported)

Get yesterday's date or a previous time
With this command you can get a previous or future date or time. Where can you use this? How about finding all files modified or created in the last 5 mins? touch -t `echo $(date -d "5 minute ago" "+%G%m%d%H%M.%S")` me && find . -type f -newer me List all directories created since last week? touch -t `echo $(date -d "1 week ago" "+%G%m%d%H%M.%S")` me && find . -type d -cnewer me I'm sure you can think of more ways to use it. Requires coreutils package.

Calculate pi with specific scale
a() function in bc is for arctangent, available if using -l option included.

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"

Sharing file through http 80 port
From the other machine open a web navigator and go to ip from the machine who launch netcat, http://ip-address/ If you have some web server listening at 80 port then you would need stop them or select another port before launch net cat ;-) * You need netcat tool installed

Enable verbose boot in Mac OS X Open Firmware

Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.

flush memcached via netcat

Block all IP addresses and domains that have attempted brute force SSH login to computer
I use iptables. To rate limit connections. Very easy and no ban lists to manage.


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