multiple calls of 'cat' for one file

Question

The device-file 'debug_port' is an output of some port. I have three scripts:

The first script waits for some info from the port, and at the same time logs data in a log file, then performs its work, with some greps from log and echos to debug_port.

#first.sh
pkill cat
cat debug_port > $logfile1 &
running=$!
# perform its tasks...
kill $running
killall -s SIGPIPE cat
pkill -P $$
rm $logfile1

The second one waits for some other info and performs another work.

#second.sh
pkill cat
cat debug_port > $logfile2 &
running=$!
# <<< there is problem: no output from 'debug_port'
# perform its tasks...
kill $running
rm $logfile2

And caller calls both of them sequentially

#caller.sh
./first.sh
./second.sh

Problem is, I have no output from 'debug_port', in the second script. However I am sure there should be some.

Answer 1

As you are reading a device file. It is possible that it is not seekable: that it is a single stream. If so the a read from one process, will preclude a read of the same byte by another process. Therefore you should not read with two processes at the same time.

If you need to do this, then you need to fork the stream. This may result in more memory ram/disk (depending on implementation) being used, as data has to be stored somewhere. The about of memory use will depend on the lay between the processes.