Question

I want to process each stdout-line for a shell, the moment it is created. I want to grab the output of test.sh (a long process). My current approach is this:

 ./test.sh >tmp.txt &
 PID=$!
 tail -f tmp.txt | while read line;  do
 echo $line
 ps ${PID} > /dev/null
 if [ $? -ne 0 ]; then
     echo "exiting.."
 fi
 done;

But unfortunately, this will print “exiting” and then wait, as the tail -f is still running. I tried both break and exit

I run this on FreeBSD, so I cannot use the --pid= option of some linux tails.

I can use ps and grep to get the pid of the tail and kill it, but thats seems very ugly to me.

Any hints?

Answer 1

why do you need the tail process?

Could you instead do something along the lines of

./test.sh | while read line; do
  # process $line
done

or, if you want to keep the output in tmp.txt :

./test.sh | tee tmp.txt | while read line; do
  # process $line
done

If you still want to use an intermediate tail -f process, maybe you could use a named pipe (fifo) instead of a regular pipe, to allow detaching the tail process and getting its pid:

./test.sh >tmp.txt &
PID=$!

mkfifo tmp.fifo
tail -f tmp.txt >tmp.fifo &
PID_OF_TAIL=$!
while read line; do
  # process $line
  kill -0 ${PID} >/dev/null || kill ${PID_OF_TAIL}
done <tmp.fifo
rm tmp.fifo

I should however mention that such a solution presents several heavy problems of race conditions :

• the PID of test.sh could be reused by another process;
• if the test.sh process is still alive when you read the last line, you won’t have any other occasion to detect its death afterwards and your loop will hang.