解决：connect() to unix:/run/php/php8.1-fpm.sock failed (13: Permission denied)

装好了nginx与php8.1-fpm，两都能正常运行，但PHP文件不能正常显示，查看nginx的error.log，有这样的记录：

connect() to unix:/run/php/php8.1-fpm.sock failed (13: Permission denied) while connecting to upstream, client: 192.168.0.114, server: localhost, request: "GET /test.php HTTP/1.1", upstream: "fastcgi://unix:/run/php/php8.1-fpm.sock:", host: "192.168.0.11"

原因是　php8.1-fpm 的apt安装使用了缺省用户www-data，nginx的apt安装使用了缺省用户nginx，/run/php/php8.1-fpm.sock的所有者是www-data, nginx用户无权访问，导致以上报错。

查看nginx的缺省配置文件：

user  nginx;
worker_processes  auto;

error_log  /var/log/nginx/error.log notice;
pid        /var/run/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    keepalive_timeout  65;

    #gzip  on;

    include /etc/nginx/conf.d/*.conf;
}

把第一条user nginx改成user  www-data即可。

记得重启nginx使配置生效。