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<h1><center>shell并发控制</center></h1>
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作者：行癫（盗版必究）
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------
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## 一：FD文件描述符
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#### 1.FD简述
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FD：文件描述符/文件句柄
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进程使用文件描述符用来管理进程打开的文件
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```shell
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[root@xingdiancloud ~]# ll /proc/$$/fd
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总用量 0
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lrwx------. 1 root root 64 4月 16 23:51 0 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 1 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 2 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 255 -> /dev/pts/0
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[root@xingdiancloud ~]# touch file1
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启用自定义文件描述符打开文件
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[root@xingdiancloud ~]# exec 6<> file1
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[root@xingdiancloud ~]# ll /proc/$$/fd
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总用量 0
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lrwx------. 1 root root 64 4月 16 23:51 0 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 1 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 2 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 255 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 6 -> /root/file1
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[root@xingdiancloud ~]# echo "xingdian" > /proc/$$/fd/6
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[root@xingdiancloud ~]# cat file1
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xingdian
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[root@xingdiancloud ~]# rm -rf file1
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[root@xingdiancloud ~]# ll /proc/$$/fd
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总用量 0
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lrwx------. 1 root root 64 4月 16 23:51 0 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 1 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 2 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 255 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 6 -> '/root/file1 (deleted)'
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[root@xingdiancloud ~]# cat /proc/$$/fd/6
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xingdian
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释放文件描述符
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[root@xingdiancloud ~]# exec 6<&-
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[root@xingdiancloud ~]# ll /proc/$$/fd
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总用量 0
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lrwx------. 1 root root 64 4月 16 23:51 0 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 1 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 2 -> /dev/pts/0
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lrwx------. 1 root root 64 4月 16 23:51 255 -> /dev/pts/0
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```
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注意：
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exec打开一个文件
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exec关闭一个文件(释放文件句柄)
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当一个文件FD未被释放，删除源文件也不会影响FD
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#### 2.管道简述
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匿名管道
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```shell
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[root@xingdiancloud ~]# rpm -qa | grep rpm
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```
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命令管道
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```shell
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[root@xingdiancloud ~]# mkfifo /tmp/xingdian
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[root@xingdiancloud ~]# file /tmp/xingdian
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/tmp/xingdian: fifo (named pipe)
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[root@xingdiancloud ~]# tty
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/dev/pts/0
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[root@xingdiancloud ~]# rpm -qa > /tmp/xingdian
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另一个终端
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[root@xingdiancloud ~]# grep bash /tmp/xingdian
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bash-5.1.8-6.el9.x86_64
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bash-completion-2.11-4.el9.noarch
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[root@xingdiancloud ~]# tty
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/dev/pts/1
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```
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#### 3.FD案例
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```shell
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[root@xingdiancloud ~]# cat a.sh
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#!/bin/bash
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exec 7<> /etc/hosts
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exec 8<> /etc/hostname
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while read -u 7 line
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do
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echo $line
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done
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while read -u 8 line2
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do
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echo $line2
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done
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exec 7<&-
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exec 8<&-
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[root@xingdiancloud ~]# bash a.sh
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127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
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::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
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xingdiancloud
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```
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4.并发案例
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```shell
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#!/usr/bin/bash
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#固定线程
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read -p "请输入线程数量：" diange
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diangefile=/root/diangefile
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mkfifo $diangefile //虚拟管道
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exec 8<>$diangefile //文件描述
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rm -rf $diangefile
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for i in `seq $diange`
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do
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echo >&8 //将内容写入到描述文件中,写如了空行
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done
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for i in {1..100}
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do
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read -u 8 //read 读取描述文件中的内容
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{
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ip=192.168.101.$i
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ping -c1 $ip &>/dev/null
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if [ $? -eq 0 ];then
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echo "$ip is up"
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else
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echo "$ip is down"
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fi
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echo >&8 //等到进程执行完之后再往管道里丢内容以支持后面的循环
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}&
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done
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wait
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echo "完成"
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exec 8<&- //释放描述文件
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```
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