Hi,
I have a bash script which wants to do some work in parallel, I did this by putting each job in an subshell which is run in the background. While the number of job running simultaneously should under some limit, I achieve this by first put some lines in a FIFO, then just before forking the subshell, the parent script is required to read a line from this FIFO. Only after it gets a line can it fork the subshell. Up to now, everything works fine. But when I tried to read a line from the FIFO in the subshell, it seems that only one subshell can get a line, even if there are apparently more lines in the FIFO. So I wonder why cannot other subshell(s) read a line even when there are more lines in the FIFO.
My testing code looks something like this:

    #!/bin/sh

fifo_path="/tmp/fy_u_test2.fifo"
mkfifo $fifo_path
#open fifo for r/w at fd 6
exec 6 $fifo_path

process_num=5
#put $process_num lines in the FIFO

for ((i=0; i<${process_num}; i++)); do
    echo "$i"
done >&6

delay_some(){
    local index="$1"
    echo "This is what u can see. $index \n"
    sleep 20;
}

#In each iteration, try to read 2 lines from FIFO, one from this shell,
#the other from the subshell
for i in 1 2
do
    date >>/tmp/fy_date
#If a line can be read from FIFO, run a subshell in bk, otherwise, block.
    read -u6
    echo " $$ Read --- $REPLY  --- from 6 \n" >> /tmp/fy_date
    {
        delay_some $i
#Try to read a line from FIFO
        read -u6
        echo " $$ This is in child # $i, read --- $REPLY --- from 6 \n" >> /tmp/fy_date
    } &
done

And the output file /tmp/fy_date has content of:

Mon Apr 26 16:02:18 CST 2010
 32561 Read --- 0  --- from 6 \n
Mon Apr 26 16:02:18 CST 2010
 32561 Read --- 1  --- from 6 \n
 32561 This is in child # 1, read --- 2 --- from 6 \n