1. 系统自动清理
在Centos 7下，系统使用systemd管理易变与临时文件，/tmp目录的清理规则主要取决于/usr/lib/tmpfiles.d/tmp.conf文件的设定，默认的配置内容为：

# This file is part of systemd.

#

# systemd is free software; you can redistribute it and/or modify it

# under the terms of the GNU Lesser General Public License as published by

# the Free Software Foundation; either version 2.1 of the License, or

# (at your option) any later version.

See tmpfiles.d(5) for details
Clear tmp directories separately, to make them easier to override
v /tmp 1777 root root 10d # 清理/tmp下10天前的目录和文件
v /var/tmp 1777 root root 30d # 清理/var/tmp下30天前的目录和文件
Exclude namespace mountpoints created with PrivateTmp=yes
x /tmp/systemd-private-%b-*
X /tmp/systemd-private-%b-/tmp
x /var/tmp/systemd-private-%b-
X /var/tmp/systemd-private-%b-*/tmp

2. 手动清理
注意！不能简单的rm -rf /tmp/*
这样会导致正在使用的临时文件被删除，出现一系列问题
正确的做法：
yum install -y tmpwatch
#删除24小时内未使用的临时文件
tmpwatch 24 /tmp/