CentOS清除SSH登录记录、历史命令及日志缓存

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清除 SSH 登录记录

SSH 登录日志以二进制方式存储在以下文件内，需要使用对应命令查看，不然会显示乱码。

日志文件 查看命令 日志内容
/var/log/wtmp last 登录成功日志，包含用户名、IP 地址和时间记录
/var/log/btmp lastb 登录失败日志，包含信息同上
/var/log/lastlog lastlog 各用户的最近登录日志
/var/log/secure 用cat查看 各类需要输入口令的登录日志

清除 SSH 登录日志使用下面命令。

cat /dev/null > /var/log/wtmp
cat /dev/null > /var/log/btmp
cat /dev/null > /var/log/lastlog
cat /dev/null > /var/log/secure

清除 Bash 历史命令

Bash 执行过的命令存在用户目录下的.bash_history文件里，用history命令查看。

清除所有历史命令记录，第二条命令表示立即更新日志文件。

history -c
history -w

如果只需清除当前会话用过的命令记录，使用history -r命令清除。

删除重复软件包

用于解决因同时安装32位和64位软件包导致的问题。先安装yum-utils软件。

yum -y install yum-utils

列出所有重复的软件包。

package-cleanup --dupes

删除重复软件包，过程中需确认操作。

package-cleanup --cleandupes

清除 YUM 缓存

用法一：清除已启用软件仓库里的所有类型缓存。

yum clean all

延伸：Ubuntu / Debian 用apt-get clean命令清除缓存，还可用apt-get autoremove清除重复多余软件包。

用法二：添加--enablerepo='*'选项临时清除禁用状态的软件仓库缓存。

yum --enablerepo='*' clean all

用法三：清除指定类型 YUM 缓存。

命令 作用
yum clean expire-cache 清除元数据和镜像列表的获取时间，使下次使用 YUM 时重新验证缓存。如果缓存数据仍有效，则不会删除任何重要数据。
yum clean packages 清除缓存目录下的软件包。
yum clean headers 清除旧版 YUM 用来确认软件包依赖关系的文件，在下次运行 YUM 时重新下载。
yum clean metadata 清除 YUM 用于确认软件包远程可用性的文件，在下次运行 YUM 时重新下载。
yum clean dbcache 清除加速元数据访问的 sqlite 缓存，下次运行时重新创建（注：不适用仅包含 .xml 数据的软件仓库，其清除后不会重新创建）。
yum clean rpmdb 清除任何本地 rpmdb 中的缓存数据。
yum clean plugins 清除已启用 YUM 插件的缓存数据。
上述列表中的expire-cache是较温和选项，可以用来替代clean all。

清除系统日志文件

已排除上面提到的日志文件。其中/var/log/messages是重要日志文件，记录了几乎所有系统错误及重要的相关信息。如果系统出了问题，该文件是经常要检查的项目之一。

cat /dev/null > /var/log/boot.log
cat /dev/null > /var/log/cron
cat /dev/null > /var/log/dmesg
cat /dev/null > /var/log/firewalld
cat /dev/null > /var/log/maillog
cat /dev/null > /var/log/messages
cat /dev/null > /var/log/spooler
cat /dev/null > /var/log/syslog
cat /dev/null > /var/log/tallylog
cat /dev/null > /var/log/yum.log

关于系统日志文件的路径设置可以查看/etc/rsyslog.conf文件，CentOS 旧版本则是/etc/syslog.conf。

释放内存缓存

内存缓存分 Page Cache 和 Buffer Cache。前者从硬盘读取的文件缓存，譬如 find 查找文件首次速度慢，而后就变快了。后者是 slab 分配器中的对象（Dentry 缓存、Inode 缓存）。为避免清除缓存引发可能的内存 Buffer 数据丢失，清除前先运行sync命令使数据写到硬盘。

下面命令将同时清除两种类型缓存。如要选择性清除替换其中数字，1 表示 Page Cache，2表示 Buffer Cache。

sync && echo 3 > /proc/sys/vm/drop_caches

用free -h命令检查运行前后的内存 buff/cache 用量，available 表示剩余可用容量。

可以crontab -e添加定时释放内存缓存任务。例如每天凌晨 4 点运行一次。

0 4 * * * sync && echo 3 > /proc/sys/vm/drop_caches
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