FIFO - Parsing and Piping

[elluk]

Hi all.

I have a question around FIFO files.

I would like to read a FIFO and pipe it's contents to netcat. (In this case a printer, but it's the concept I am concerned with).

I will be appending to this FIFO from an application.
I had syslog in mind, but again, this is a POC.

Initially, this is no problem.
But after the first write, the solution fails.
I think netcat is terminating (maybe due to the EOF?).

#Example
mkfifo /tmp/testfifo
nc 192.168.1.1 9100 < /tmp/testfifo &
echo Line1 > /tmp/testfifo
#This line gets piped to nc and delivered.
echo Line2 > /tmp/testfifo
#This line is unable to write.

Like I said, I think this maybe to do with the EOF.
But I don't honestly know.

I guess I need something that reads the FIFO, and executes a new process upon every write? Instead of expecting nc to be open for the whole time?

I hope tail would do this, but no luck.

Can anyone explain what is happening, or perhaps even provide me with a solution?

This strikes me as something that may be a common problem/requirement under various parts of any Unix system.

Any ideas?

[wje_lf]

In some other window, do this:

cat > your_named_pipe

and let it sit there. Then write single lines to it from elsewhere to your heart's content. As long as at least one process has that FIFO open for writing, the process that has it open for reading will not experience end of file.

[elluk]

Thanks wje_lf

I didn't think about that, you were quite right...

Sadly, I was not! It's not the EOF that is causing the problem.

The remote node is terminating the TCP session after a moment of no input. (and therefore netcat terminates).

I think I will need something that executes a new instance of the binary for each write/line/lf or something.

Kinda like a tail, with an execute function!?

Any ideas people?
wje_lf?

[wje_lf]

If I'm understanding you correctly, on the destination host you pipe the netcat data to an application, and that application exits before you want it to, and you wish to fire up that application for each line that netcat wants to give it, without having to restart netcat. Ja?

If so, try something like this.

On the destination host, instead of piping netcat to the application directly, pipe it to a shell script something like the following, which I haven't tested.

#!/bin/sh

while read
do
  # Angels we have heard on high.  (sorry)

  echo "$REPLY" > your_application
done

[elluk]

Hi wje_lf,

If I'm understanding you correctly, on the destination host you pipe the netcat data to an application, and that application exits before you want it to, and you wish to fire up that application for each line that netcat wants to give it, without having to restart netcat. Ja?

Although that not exactly right, it was close enough!

In this case I am just sending data to a network printer (TCP:9100)
It is the printer that is terminating the TCP session.

So I was unable to implement your script at the destination.

However!......

I was able to solve my problem using your techniques from client side.

#Begin "/tmp/ncline"

#!/bin/sh
while read
do echo "$REPLY"|nc printerip 9100
done

#Start the "system"

cat > /tmp/testfifo &
/tmp/ncline < /tmp/testfifo

#Testing!

echo Line1 > /tmp/testfifo
echo Line2 > /tmp/testfifo

With this method I have been able to direct syslog data to my printer.
(And also festival which was pretty funny!)

So thanks goto wje_lf!

Cheers bro!

Keywords: syslog to printer
Keywords: syslog to festival