linux使用netstat命令监控unix domain socket（unix socket，.sock文件）状态
UNIX Domain Socket IPC
socket API原本是为网络通讯设计的，但后来在socket的框架上发展出一种IPC机制，就是UNIX Domain Socket。虽然网络socket也可用于同一台主机的进程间通讯（通过loopback地址127.0.0.1），但是UNIX Domain Socket用于IPC更有效率：不需要经过网络协议栈，不需要打包拆包、计算校验和、维护序号和应答等，只是将应用层数据从一个进程拷贝到另一个进程。这是因为，IPC机制本质上是可靠的通讯，而网络协议是为不可靠的通讯设计的。UNIX Domain Socket也提供面向流和面向数据包两种API接口，类似于TCP和UDP，但是面向消息的UNIX Domain Socket也是可靠的，消息既不会丢失也不会顺序错乱。
UNIX Domain Socket是全双工的，API接口语义丰富，相比其它IPC机制有明显的优越性，目前已成为使用最广泛的IPC机制，比如X Window服务器和GUI程序之间就是通过UNIX Domain Socket通讯的。
使用UNIX Domain Socket的过程和网络socket十分相似，也要先调用socket()创建一个socket文件描述符，address family指定为AF_UNIX，type可以选择SOCK_DGRAM或SOCK_STREAM，protocol参数仍然指定为0即可。
UNIX Domain Socket与网络socket编程最明显的不同在于地址格式不同，用结构体sockaddr_un表示，网络编程的socket地址是IP地址加端口号，而UNIX Domain Socket的地址是一个socket类型的文件在文件系统中的路径，这个socket文件由bind()调用创建，如果调用bind()时该文件已存在，则bind()错误返回。
[root@localhost ~]# netstat -ap --unix #显示处于监控状态的unix socket，-a显示所有state状态的链接，因为默认不显示处于listening状态的。-p显示与socket有关的pid/programe name
Active UNIX domain sockets (servers and established)
Proto RefCnt Flags Type State I-Node PID/Program name Path
unix 2 [ ACC ] STREAM LISTENING 29029 3790/master private/defer
unix 2 [ ACC ] STREAM LISTENING 29045 3790/master private/proxywrite
unix 2 [ ACC ] STREAM LISTENING 8972 1/systemd /run/systemd/journal/stdout
unix 5 [ ] DGRAM 8975 1/systemd /run/systemd/journal/socket
unix 16 [ ] DGRAM 8977 1/systemd /dev/log
unix 2 [ ACC ] STREAM LISTENING 27187 3343/NetworkManager /var/run/NetworkManager/private-dhcp
unix 2 [ ACC ] STREAM LISTENING 29018 3790/master private/tlsmgr
unix 2 [ ACC ] STREAM LISTENING 29023 3790/master private/rewrite
unix 2 [ ACC ] STREAM LISTENING 29001 3790/master public/pickup
unix 2 [ ACC ] STREAM LISTENING 29006 3790/master public/cleanup
unix 2 [ ACC ] STREAM LISTENING 29026 3790/master private/bounce
unix 2 [ ACC ] STREAM LISTENING 29009 3790/master public/qmgr
unix 2 [ ACC ] STREAM LISTENING 29038 3790/master public/flush
unix 2 [ ACC ] STREAM LISTENING 29054 3790/master public/showq
unix 2 [ ACC ] STREAM LISTENING 29048 3790/master private/smtp
unix 2 [ ACC ] STREAM LISTENING 29051 3790/master private/relay
unix 2 [ ACC ] STREAM LISTENING 28793 3680/php-fpm: maste /var/run/php-fpm/default.sock
unix 2 [ ACC ] STREAM LISTENING 29057 3790/master private/error
unix 2 [ ACC ] STREAM LISTENING 21637 1/systemd /run/lvm/lvmpolld.socket
[root@localhost ~]# netstat -ap --unix |grep /var/run/php-fpm/default.sock #监控某个指定的sock文件建立链接通信的情况，后面可以用wc等进行统计数量
unix 2 [ ACC ] STREAM LISTENING 28793 3680/php-fpm: maste /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183596 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183259 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183413 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183547 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183241 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183568 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183561 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183505 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 182751 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183452 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183554 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183575 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183582 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183498 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183431 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183526 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183589 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183240 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 182359 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183445 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183438 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183424 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183385 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183478 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 182523 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183540 - /var/run/php-fpm/default.sock
unix 3 [ ] STREAM CONNECTED 183533 - /var/run/php-fpm/default.sock
