shell脚本中控制后台任务的数量

有时需要同时扫描几千个服务器，所有需要在shell中并发操作。虽然perl里可以很方便地对fork出来的进程做好
控制，但是有时就是一个小任务，所以直接就shell搞定了。

#!/bin/bash
fun(){
    echo "hello i am $1.  $(date  '+%T')"
    sleep 1
}
i=0
PIDS=''
while read job
do
    fun $job &
    PIDS="$PIDS $!"
    i=$((i+1))
    if [ $i -ge 4 ];then
        wait $PIDS
        echo "finish  jobs:$PIDS || $(date '+%T')"
        PIDS=''
        i=0
    fi


done <list

pm@debian:~$ time ./multi.sh
hello i am 1.  23:41:23
hello i am 4.  23:41:23
hello i am 2.  23:41:23
hello i am 3.  23:41:23
finish  jobs: 13482 13483 13485 13487 || 23:41:24
hello i am 5.  23:41:24
hello i am 8.  23:41:24
hello i am 6.  23:41:24
hello i am 7.  23:41:24
finish  jobs: 13495 13496 13497 13500 || 23:41:25
hello i am 9.  23:41:25
hello i am 11.  23:41:25
hello i am 12.  23:41:25
hello i am 10.  23:41:25
finish  jobs: 13508 13509 13510 13513 || 23:41:26
hello i am 13.  23:41:26
hello i am 14.  23:41:26
hello i am 15.  23:41:26
hello i am 16.  23:41:26
finish  jobs: 13521 13522 13524 13526 || 23:41:27

real    0m4.049s
user    0m0.012s
sys    0m0.000s

如果把后台任务数修改为8的话

pm@debian:~$ time ./multi.sh
hello i am 7.  23:43:54
hello i am 5.  23:43:54
hello i am 3.  23:43:54
hello i am 8.  23:43:54
hello i am 4.  23:43:54
hello i am 1.  23:43:54
hello i am 6.  23:43:54
hello i am 2.  23:43:54
finish  jobs: 13597 13598 13600 13602 13603 13604 13606 13607 || 23:43:55
hello i am 9.  23:43:55
hello i am 10.  23:43:55
hello i am 11.  23:43:55
hello i am 15.  23:43:55
hello i am 14.  23:43:55
hello i am 12.  23:43:55
hello i am 13.  23:43:55
hello i am 16.  23:43:55
finish  jobs: 13622 13623 13625 13627 13628 13630 13631 13632 || 23:43:56

real	0m2.061s
user	0m0.004s
sys	0m0.008s

如果觉得没有必要记录后台的PID
也可以直接使用这样的形式：

fun(){
echo "hello i am $1.  $(date  '+%T')"
sleep 1
}
i=0
while read job
do
fun $job &
i=$((i+1))
if [ $i -ge 8 ];then
wait
echo "finish  jobs|| $(date '+%T')"
i=0
fi


done