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Generate a list of installed packages on Debian-based systems
This command is useful when you want to install the same packages on another fresh OS install for example. To do that, use: $ sudo dpkg --set-selections < LIST_FILE

View acceptable client certificate CA names asked for during SSL renegotiations
The key is to use the -prexit option at the command line, and then type "quit" instead of CTRL-C to exit OpenSSL. OpenSSL will then dump its last negotiated state, printing out the contents of the renegotiated handshake. Crucial for debugging client certificate configurations on web servers such as IIS, which renegotiate the SSL/TLS connection with the HTTP request in-flight to ask the client for a cert.

quick and dirty formatting for HTML code
Finds all the closing tags in a HTML document via non-greedy regex and adds a linefeed for easier reading/editing . sed -i.bak -r 's_(/[^>]*?>)_\1\n_g' filename.html . This will save a copy of filename.html as filename.html.bak and then add the linefeeds to the original file

add a ip address to a network device
Allows to add more than one ip address to one network device.

Create QR codes from a URL.
like 7300, but doesn't clutter your working directory with old qr.*.png files. This will get the QR barcode, and send it right into ImageMagick's 'display' tool. Usage is the same as 7300; just call this function followed by the URL: $ qrurl http://xkcd.com

Send a backup job to a remote tape drive on another machine over SSH
I use this all the time for taking manual backups of stuff i want to keep but not important enough to backup regularly.

Getting information about model no. of computer
This command gives a model information of a computer. Also useful in determining the host is a VM machine or actual physical machine.

Show what PID is listening on port 80 on Linux

Drop or block attackers IP with null routes
Someone might attack on your system. You can drop attacker IP using IPtables. However, you can use route command to null route unwanted traffic. A null route (also called as blackhole route) is a network route or kernel routing table entry that goes nowhere. Matching packets are dropped (ignored) rather than forwarded, acting as a kind of very limited firewall. The act of using null routes is often called blackhole filtering.

loop over a set of items that contain spaces
If you want to operate on a set of items in Bash, and at least one of them contains spaces, the `for` loop isn't going to work the way you might expect. For example, if the current dir has two files, named "file" and "file 2", this would loop 3 times (once each for "file", "file", and "2"): $ for ITEM in `ls`; do echo "$ITEM"; done Instead, use a while loop with `read`: $ ls | while read ITEM; do echo "$ITEM"; done


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