首先

因为在尝试启动通过tasksel安装的Ubuntu上的Apache并显示初始设定的默认页面时遇到了问题，所以我打算记录一下。

环境

Ubuntu 20.04.3 LTS 是一个操作系统版本。Apache 2.4 是一个网页服务器软件。

操作步骤

我会先观察一下情况。

$ sudo systemctl status apache2.service 
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Fri 2021-12-17 08:27:02 JST; 5 days ago
       Docs: https://httpd.apache.org/docs/2.4/

apachectl[135474]: AH00558: apache2: Could not reliably determine the server's fully qualified domain name, u>
apachectl[135474]: (98)Address already in use: AH00072: make_sock: could not bind to address [::]:80
apachectl[135474]: (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:80
apachectl[135474]: no listening sockets available, shutting down
apachectl[135474]: AH00015: Unable to open logs
apachectl[135460]: Action 'start' failed.
apachectl[135460]: The Apache error log may have more information.
systemd[1]: apache2.service: Control process exited, code=exited, status=1/FAILURE
systemd[1]: apache2.service: Failed with result 'exit-code'.
systemd[1]: Failed to start The Apache HTTP Server.

AH00558: apache2:由于无法可靠地确定服务器的完全合格域名，因此将以下内容添加到Apache的配置文件中。

ServerName localhost:80

我会再次检查一下状态。

$ sudo systemctl status apache2.service
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Wed 2021-12-22 22:33:01 JST; 4s ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 183591 ExecStart=/usr/sbin/apachectl start (code=exited, status=1/FAILURE)

systemd[1]: Starting The Apache HTTP Server...
apachectl[183604]: (98)Address already in use: AH00072: make_sock: could not bind to address [::]:80
apachectl[183604]: (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:80
apachectl[183604]: no listening sockets available, shutting down
apachectl[183604]: AH00015: Unable to open logs
apachectl[183591]: Action 'start' failed.
apachectl[183591]: The Apache error log may have more information.
systemd[1]: apache2.service: Control process exited, code=exited, status=1/FAILUREsystemd[1]: apache2.service: Failed with result 'exit-code'.
systemd[1]: Failed to start The Apache HTTP Server.

先前的错误已经解决，但仍无法启动。由于地址已在使用中：AH00072，因此将在以下进行端口检查。

$ sudo lsof -i | grep http
docker-pr  54172             root    4u  IPv4  680930      0t0  TCP *:http (LISTEN)
docker-pr  54179             root    4u  IPv6  684154      0t0  TCP *:http (LISTEN)

因为之前实行的Docker容器占用了80号端口，导致出现了问题，所以我删除了容器。(此处省略)

启动Apache并检查状态。

$ sudo systemctl start apache2.service
$ sudo systemctl status apache2.service
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2021-12-22 22:50:10 JST; 15s ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 185411 ExecStart=/usr/sbin/apachectl start (code=exited, status=0/SUCCESS)
   Main PID: 185428 (apache2)
      Tasks: 6 (limit: 9370)
     Memory: 18.9M
     CGroup: /system.slice/apache2.service
             ├─185428 /usr/sbin/apache2 -k start
             ├─185429 /usr/sbin/apache2 -k start
             ├─185430 /usr/sbin/apache2 -k start
             ├─185431 /usr/sbin/apache2 -k start
             ├─185432 /usr/sbin/apache2 -k start
             └─185433 /usr/sbin/apache2 -k start

终于启动成功了。然后通过浏览器访问http://localhost，会显示默认页面。