CentOS 7相关操作

防火墙操作

开启防火墙
sudo systemctl start firewalld.service
查看防火墙状态
sudo systemctl status firewalld.service
关闭防火墙（临时）
sudo systemctl stop firewalld.service
永久开启防火墙（打开自启动）
sudo systemctl enable firewalld.service
永久关闭防火墙（关闭自启动）
sudo systemctl disable firewalld.service

开放端口7474，并使配置生效

sudo firewall-cmd --zone=public --permanent --add-port=7474/tcp
sudo firewall-cmd --reload

列出所有端口
sudo firewall-cmd --list-ports

关于防火墙更详细的配置操作可见博客Centos7 防火墙配置详解（非常详细！）

换源

CentOS系统为主要使用yum源
（目前测试结果为中科大yum源速度最快，快于163yum源和阿里yum源）

首先备份原有的yum源文件

cd /etc/yum.repos.d/
sudo mv CentOS-Base.repo CentOS-Base.repo.bak

在该目录新建CentOS-Base.repo文件，并将如下内容复制进去（CentOS 8等版本换中科大yum源见CentOS 源使用帮助）

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

清理旧yum源本地缓存
sudo yum clean all
制作中科大yum源缓存
sudo yum makecache

配置多个版本的Java环境(使用alternatives管理)

新建文件夹/usr/java，并转到该目录下
```
sudo mkdir /usr/java
cd /usr/java
```
下载Java安装包，下载至/usr/java
网址：Oracle Java Downloads或者Azul Zulu OpenJDK Downloads

例如下载两个版本jdk8(jdk-8u371-linux-x64.tar.gz)和jdk17(jdk-17_linux-x64_bin.tar.gz)

解压

sudo tar -zxvf jdk-8u371-linux-x64.tar.gz
sudo tar -zxvf jdk-17_linux-x64_bin.tar.gz

将JDK注册到alternatives中，命令：

# 创建JDK目录的软链接，并交由alternatives管理(命令最后一个数字为优先级，此处使用版本号作为优先级，因为java17更常用，所以优先级更高，设为默认)
update-alternatives --install /usr/bin/java java /usr/java/jdk1.8.0_371/ 8
update-alternatives --install /usr/bin/java java /usr/java/jdk-17.0.7/ 17

如果原来alternatives中有系统自带的java版本，需要全部删除掉，否则后续路径可能出问题

删除方法例如：
update-alternatives --remove java /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.372.b07-1.el7_9.x86_64/jre/bin/java
查看jdk版本
java -version
切换jdk版本
sudo update-alternatives --config java

选择对应选项即可切换版本