Well... interesting. I wonder why cat acts that way. Interestingly, I'd played around with FIFOs some time ago, and there's a fine way to cheat: "tail -f".
k...@minastirith:~$ mknod /tmp/FIFO p k...@minastirith:~$ tail -f /tmp/FIFO > /tmp/FIFO.log & [1] 13082 k...@minastirith:~$ date > /tmp/FIFO k...@minastirith:~$ date > /tmp/FIFO k...@minastirith:~$ cat /tmp/FIFO.log Wed Jul 14 13:22:04 EDT 2010 Wed Jul 14 13:22:07 EDT 2010 -Ken On Wed, July 14, 2010 12:26 pm, Michael ODonnell wrote: > > Some years back I created a little FIFO-reader utility that can be > used to relay data via a named pipe (FIFO) such that it keeps the output > end of the FIFO open despite multiple opens/closes of the input end by one > or more writers. > > This is necessary because the naive approach only works once, as > illustrated here: > > # mknod /tmp/FIFO p # (could also have used > mkfifo) # cat /tmp/FIFO & # Have an instance of cat waiting > for data. [1] 25868 # (bash confirms > backgrounded process) # date > /tmp/FIFO # 1st date opens > FIFO, writes it, closes it. > Wed Jul 14 11:45:04 EDT 2010 # cat shows data but exits as next read() = > 0. > [1]- Done cat /tmp/FIFO # (bash confirms exit of backgrounded > process) # date > /tmp/FIFO # 2nd date hangs - nobody reading > the FIFO.... > > If I had used my FIFO-reader utility instead of cat in this example > I could have had as many instances of date (or whatever) spewing > sequentially into the FIFO and it would have relayed everything and none of > the writers would have blocked. > > I'm now working with a cantankerous old app that can't easily be > modified and it'd be handy to have multiple sequential invocations of that > app each spew some logging data into a FIFO (without blocking) so it could > be processed by a single persistent instance of a filter connected to the > output of that FIFO. I'm therefore about to dredge up my little utility > and put it back into service in this situation but before I do I wonder if > anybody knows of something that's part of the standard Gnu/Linux stuff > that already solves this problem. > > I thought I'd recently heard that some surprisingly complex options > are/were being added to cat and half expected to find this behavior is now > an option, but maybe that was just some smarta** taking a cheap-shot at > the Gnu folks... > > _______________________________________________ > gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org > http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/ > > > -- > This message has been scanned for viruses and > dangerous content by MailScanner, and is believed to be clean. > > -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. _______________________________________________ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
