问题：

python --version的结果：

Command 'python' not found, but can be installed with:



 sudo apt install python3 


 sudo apt install python 


 sudo apt install python-minimal



You also have python3 installed, you can run 'python3' instead.

cat /etc/apt/sources.list的结果：

# deb cdrom:[Ubuntu 18.04.1 LTS _Bionic Beaver_ - Release amd64 (20180725)]/ bionic main restricted



# See http://help.ubuntu.com/community http://packages.ros.org/ros/ubuntu/y/UpgradeNotes for how to upgrade to


# newer versions of the distribution.


deb [trusted=yes] http://br.archive.ubuntu.com/ubuntu/ bionic restricted main


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic main restricted



## Major bug fix updates produced after the final release of the


## distribution.


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic-updates main restricted



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu


## team. Also, please note that software in universe WILL NOT receive any


## review or updates from the Ubuntu security team.


deb http://br.archive.ubuntu.com/ubuntu/ bionic universe


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic universe


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic-updates universe



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 


## team, and may not be under a free licence. Please satisfy yourself as to 


## your rights to use the software. Also, please note that software in 


## multiverse WILL NOT receive any review or updates from the Ubuntu


## security team.


deb http://br.archive.ubuntu.com/ubuntu/ bionic multiverse


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic multiverse


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic-updates multiverse



## N.B. software from this repository may not have been tested as


## extensively as that contained in the main release, although it includes


## newer versions of some applications which may provide useful features.


## Also, please note that software in backports WILL NOT receive any review


## or updates from the Ubuntu security team.


deb http://br.archive.ubuntu.com/ubuntu/ bionic-backports restricted universe multiverse main


# deb-src http://br.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse



## Uncomment the following two lines to add software from Canonical's


## 'partner' repository.


## This software is not part of Ubuntu, but is offered by Canonical and the


## respective vendors as a service to Ubuntu users.


# deb http://archive.canonical.com/ubuntu bionic partner


# deb-src http://archive.canonical.com/ubuntu bionic partner



deb http://security.ubuntu.com/ubuntu bionic-security restricted main


# deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted


deb http://security.ubuntu.com/ubuntu bionic-security universe


# deb-src http://security.ubuntu.com/ubuntu bionic-security universe


deb http://security.ubuntu.com/ubuntu bionic-security multiverse


deb http://archive.canonical.com/ bionic partner


# deb-src http://archive.canonical.com/ bionic partner


# deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse


deb http://br.archive.ubuntu.com/ubuntu/ bionic-updates main multiverse universe restricted


deb http://my-private-server.test bionic main


# deb-src http://my-private-server.test bionic main 

sudo apt update的结果：

E: Conflicting values set for option Trusted regarding source http://br.archive.ubuntu.com/ubuntu/ bionic


E: The list of sources cannot be read.

/etc/apt/sources.list的结果：

bash: /etc/apt/sources.list: Permission denied

答案1：

此行中有[trusted=yes]作为选项：

deb [trusted=yes] http://br.archive.ubuntu.com/ubuntu/ bionic restricted main

要修复，请将该选项添加到有相同URL的行，或者简单地运行下面命令删除：

sudo sed -i '/bionic restricted main$/s/^deb [trusted=yes]/deb/' /etc/apt/sources.list

答案2：

你已经安装了python3，并且没有必要将python2.x与python3.x一起安装，因为python2.7在2020年1月1日结束了它的生命，使用命令python3启动Python3.x解释器，命令python启动Python2.x解释器。

移除 E: Conflicting values set for option Trusted regarding source http://br.archive.ubuntu.com/ubuntu/ bionic 第5行第/etc/apt/sources.list行中的错误项应为：

deb [arch=amd64 trusted=yes] http://br.archive.ubuntu.com/ubuntu/ bionic main restricted

其中斜体文本是可选的。

deb http://br.archive.ubuntu.com/ubuntu/ bionic main restricted