在学习ros、slam的同时，不要被ubuntu出现的各种bug而消耗太多时间：
比如ubuntu系统更新、还源，有可能你找了网上的教程换了源，最后还是没有sudo apt-get update起来
1.制定好学习计划，把更多的时间放在理论、代码等消化环节
2.ubuntu由于是开源，内核、显卡、声卡、驱动、各种配置，当你遇到相关问题的时候，你去谷歌或者去百度，可能有人和你遇到了相同的问题，哪怕遇到相同的问题，因为不同ubunut版本、对方PC的硬件情况、软件配置情况与你不同。同一个bug可能会有多个答案，你可能尝试了许多个，都无法解决你的问题，这时候：如果不是大问题，不要花你太多时间，放到以后去解决（初学者非常容易遇到一个ubuntu问题，搞一天，搞几天。然后就放弃学习了），这部分涉及到时间管理的问题。
3.提高问题检索能力：
报错不可怕，可怕的是胡乱找错误，一般运行某个命令，出错，先找第一个erro然后按顺序去检索，看看能不能解决。

今天我sudo apt-get update遇到了一个

Hit:1 http://packages.microsoft.com/repos/code stable InRelease
Ign:2 http://mirrors.tuna.tsinghua.edu.cn/ros/ubuntu bionic InRelease          
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease             
Hit:4 http://mirrors.tuna.tsinghua.edu.cn/ros/ubuntu bionic Release            
Hit:5 http://dl.google.com/linux/chrome/deb stable InRelease                   
Ign:6 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease     
Ign:8 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease   
Ign:9 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease    
Err:10 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release              
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:11 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release  
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Hit:12 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:13 http://ppa.launchpad.net/videolan/master-daily/ubuntu bionic InRelease  
Err:14 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release    
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Err:15 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release     
  Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate.  Could not handshake: Error in the certificate verification. [IP: 101.6.15.130 443]
Reading package lists... Done                                
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

我百度repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release' no longer has a Release file.

有教程：
默认的源是http的，但替换用的镜像源是https，所以休要额外的安装关于HTTPS的包。

apt install apt-transport-https
apt install ca-certificates
apt update

但是我试试了没有用，
然后百度Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification.
有答案说：

各种更新证书的方式也试了，没有一个有效。
最后发现是 机器时间设置 问题，因为证书校验是 时间敏感的。 而我的机器时间还停留在去年。
搜到了 stackoverflow.com/questions/2… 这里，才看到了 答案：
Another cause of this problem might be that your clock might be off. Certificates are time sensitive.
To check the current system time:
date -R
You might consider installing NTP to automatically sync the system time with trusted internet timeservers from the global NTP pool. For example, to install on Debian/Ubuntu:
apt-get install ntp
apt-get install ntp

我试了发现没有用，然后发现有人说是缺少ca-certificates包
重装什么的试试也没有用，

最后还是检索Certificate verification failed: The certificate is NOT trusted：把软件源里的https改成http就行了