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Using NMAP to check if a port is open or close
Using NMAP to check to see if port 22(SSH) is open on servers and network devices.

Verify/edit bash history command before executing it
Bash history commands are those that begin with the character ! (eg. the most popular 'sudo !!' Explained here => http://www.commandlinefu.com/commands/view/13). By default bash immediately executes the history command. Setting this shell option will make bash first allow you to verify/edit an history command before executing it. To set this option permanently, put this command in ~/.profile or ~/.bashrc file. To unset this option issue following command. $shopt -u histverify

Watch for when your web server returns
If your web server is down, this command will periodically attempt to connect to it. If the output is blank, your server is not yet up. If you see HTML, your server is up. Obviously, you need to replace the Google URL with your web server URL... * 'watch' -- a command for re-executing a command and displaying the output * '-n 15' -- tells watch to redo the command every 15 seconds * 'curl' -- a handy utility for getting the source of a web page * '-s' -- tells curl to be silent about failing * '--connect-timeout 10' -- Try to connect for 10 seconds

Execute a command on logout
Execute a command on shell logout,and run it until is finished,then shell is closed.

Create a mirror of a local folder, on a remote server
Create a exact mirror of the local folder "/root/files", on remote server 'remote_server' using SSH command (listening on port 22) (all files & folders on destination server/folder will be deleted)

Change every instance of OLD to NEW in file FILE
Very quick way to change a word in a file. I use it all the time to change variable names in my PHP scripts (sed -i 's/$oldvar/$newvar/g' index.php)

Split and join with split and cat.
`split -b 1k file` splits files into 1k chunks. Rejoin them with `cat x* > file`.

Remove security limitations from PDF documents using ghostscript (for Windows)
#4345 also works under windows

Belgian banking "structured communication"
Derived from current time down to minutes.

analyze traffic remotely over ssh w/ wireshark
When using tcpdump, specify -U option to prevent buffering.


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