Checking if the other end of a fifo is open

Dave Smith dave at
Thu Apr 9 11:40:33 MDT 2009

I'm writing a bash script that communicates with a daemon process using 
a fifo. I am trying to bulletproof the script to safeguard myself if the 
daemon process crashes before it starts writing to the fifo. UNIX 
hackers will know that opening a fifo for reading will block until the 
writing side has opened the fifo for writing (and vice versa). I open 
the fifo for reading like this in bash:

  exec 4< $my_fifo

However, doing this is dangerous because the "exec" call will block 
until the daemon process opens the fifo for writing. This is usually 
fine, but if the daemon process *crashes* before it opens the fifo for 
writing, my script will hang indefinitely.

So my question is this: Is there a way to check if a fifo is already 
open for writing so I can safely open it for reading. Even better: Can I 
do some "exec" magic with a timeout, such that if the open blocks for 
more than a few seconds, I can bail out somehow?

Here is one caveat: I need to open the fifo exactly *once* and read many 
times, so I can't just do multiple "read" commands on it (which opens 
and closes the fifo each time). I'd be happy to be proven wrong on this 
point. :)



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