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Piping Microphone Audio Over Netcat
Send microphone audio to another computer using netcat and arecord. Connect to the stream using "nc [other ip] 3333|aplay" You can set up two-way communication by piping audio the reverse direction on another port: Machine #1: $arecord -D hw:0,0 -f S16_LE -c2|nc -l 3333 &;nc -l 3334|aplay Machine #2: $$ip=[machine1_ip];arecord -D hw:0,0 -f S16_LE -c2|nc $ip 3334 &;nc $ip 3333|aplay

back ssh from firewalled hosts
host B (you) redirects a modem port (62220) to his local ssh. host A is a remote machine (the ones that issues the ssh cmd). once connected port 5497 is in listening mode on host B. host B just do a ssh 127.0.0.1 -p 5497 -l user and reaches the remote host'ssh. This can be used also for vnc and so on.

Read and write to TCP or UDP sockets with common bash tools
Ever needed to test firewalls but didn't have netcat, telnet or even FTP? Enter /dev/tcp, your new best friend. /dev/tcp/(hostname)/(port) is a bash builtin that bash can use to open connections to TCP and UDP ports. This one-liner opens a connection on a port to a server and lets you read and write to it from the terminal. How it works: First, exec sets up a redirect for /dev/tcp/$server/$port to file descriptor 5. Then, as per some excellent feedback from @flatcap, we launch a redirect from file descriptor 5 to STDOUT and send that to the background (which is what causes the PID to be printed when the commands are run), and then redirect STDIN to file descriptor 5 with the second cat. Finally, when the second cat dies (the connection is closed), we clean up the file descriptor with 'exec 5>&-'. It can be used to test FTP, HTTP, NTP, or can connect to netcat listening on a port (makes for a simple chat client!) Replace /tcp/ with /udp/ to use UDP instead.

Triple monitoring in screen
This command starts screen with 'htop', 'nethogs' and 'iotop' in split-screen. You have to have these three commands (of course) and specify the interface for nethogs - mine is wlan0, I could have acquired the interface from the default route extending the command but this way is simpler. htop is a wonderful top replacement with many interactive commands and configuration options. nethogs is a program which tells which processes are using the most bandwidth. iotop tells which processes are using the most I/O. The command creates a temporary "screenrc" file which it uses for doing the triple-monitoring. You can see several examples of screenrc files here: http://www.softpanorama.org/Utilities/Screen/screenrc_examples.shtml

Place the argument of the most recent command on the shell
When typing out long arguments, such as: $ cp file.txt /var/www/wp-content/uploads/2009/03/ You can put that argument on your command line by holding down the ALT key and pressing the period '.' or by pressing <ESC> then the period '.'. For example: $ cd 'ALT+.' would put '/var/www/wp-content/uploads/2009/03/ as my argument. Keeping pressing 'ALT+.' to cycle through arguments of your commands starting from most recent to oldest. This can save a ton of typing.

Every Nth line position # (SED)
sed extract every nth line. Generic is: $ sed -n 'STARTPOSITION,${p;n;*LINE}' foo where n;*LINE = how many lines. thus p;n;n; is "for every 3 lines" and p;n;n;n;n; is "for every 5 lines"

Lists all listening ports together with the PID of the associated process
The PID will only be printed if you're holding a root equivalent ID.

Transfer large files/directories with no overhead over the network
This invokes tar on the remote machine and pipes the resulting tarfile over the network using ssh and is saved on the local machine. This is useful for making a one-off backup of a directory tree with zero storage overhead on the source. Variations on this include using compression on the source by using 'tar cfvp' or compression at the destination via $ ssh user@host "cd dir; tar cfp - *" | gzip - > file.tar.gz

How to secure delete a file
Instead, install apt-get install secure-delete and you can use: -- srm to delete file and directory on hard disk -- smem to delete file in RAM -- sfill to delete "free space" on hard disk -- sswap to delete all data from swap

Print without executing the last command that starts with...
It happens that sometime you remember that you used a special command short time before and you want to check the command again. WIth this command you can just put the beginning of a command and then bash will look for you and it will print back safely withou executing


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