Example of using named pipes in Linux Bash

One of the best examples of a practical use of a named pipe...

From http://en.wikipedia.org/wiki/Netcat:

Another useful behavior is using netcat as a proxy. Both ports and hosts can be redirected. Look at this example:

nc -l 12345 | nc www.google.com 80

Port 12345 represents the request.

This starts a nc server on port 12345 and all the connections get redirected to google.com:80. If a web browser makes a request to nc, the request will be sent to google but the response will not be sent to the web browser. That is because pipes are unidirectional. This can be worked around with a named pipe to redirect the input and output.

mkfifo backpipe
nc -l 12345  0<backpipe | nc www.google.com 80 1>backpipe

Here are the commands:

$ mkfifo named_pipe

$ echo "Hi" > named_pipe &

$ cat named_pipe

The first command creates the pipe.

The second command writes to the pipe (blocking). The & puts this into the background so you can continue to type commands in the same shell. It will exit when the FIFO is emptied by the next command.

The last command reads from the pipe.

Open two different shells, and leave them side by side. In both, go to the /tmp/ directory:

cd /tmp/

In the first one type:

mkfifo myPipe
echo "IPC_example_between_two_shells">myPipe

In the second one, type:

while read line; do echo "What has been passed through the pipe is ${line}"; done<myPipe

First shell won't give you any prompt back until you execute the second part of the code in the second shell. It's because the fifo read and write is blocking.

You can also have a look at the FIFO type by doing a ls -al myPipe and see the details of this specific type of file.

Next step would be to embark the code in a script!