Installing Tools

In GNU/Linux, you can download and install a software by one command (which may be difficult to do in Windows). This is achieved by the package manager. Different GNU/Linux distribution has different package manager. In Ubuntu, the package manager is called apt.

You will download and install some tools needed for the PAs from the network mirrors. Before using the network mirrors, you should check whether the system can access the Internet.

Checking network state

Inside NJU, you should easily obtain an IPv6 address. To test this, try to ping a mirror host with IPv6 support:

ping mirrors.tuna.tsinghua.edu.cn -c 4

You should receive reply packets successfully:

PING mirrors.tuna.tsinghua.edu.cn(2402:f000:1:408:8100::1 (2402:f000:1:408:8100::1)) 56 bytes of data.
64 bytes from 2402:f000:1:408:8100::1 (2402:f000:1:408:8100::1): icmp_seq=1 ttl=54 time=1.56 ms
64 bytes from 2402:f000:1:408:8100::1 (2402:f000:1:408:8100::1): icmp_seq=2 ttl=54 time=1.07 ms
64 bytes from 2402:f000:1:408:8100::1 (2402:f000:1:408:8100::1): icmp_seq=3 ttl=54 time=1.07 ms
64 bytes from 2402:f000:1:408:8100::1 (2402:f000:1:408:8100::1): icmp_seq=4 ttl=54 time=1.24 ms

--- mirrors.tuna.tsinghua.edu.cn ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 7ms
rtt min/avg/max/mdev = 1.071/1.237/1.563/0.203 ms

If you do not have an IPv6 address, you may access the Internet by IPv4 address. You can try to ping a host outside the university LAN:

ping www.baidu.com -c 4

If you can ping Baidu successfully, you should successfully ping the mirror host above, too.

Setting APT source file

Run the following commands to update the APT source file:

bash -c 'echo "deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ hirsute main restricted universe multiverse" > /etc/apt/sources.list'

如果你的系统不是Ubuntu 21.04, 请更换合适的源

使用与系统不相符的源会导致工具包版本冲突, 强行安装将会损坏系统. 如果你使用的系统不是Ubuntu 21.04, 请勿使用上述的源, 你需要自行搜索合适的源并更换.

However, you will receive an error message:

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

This is because the APT source file is owned by root, and you do not have the permission to modify it.

One solution is first switching to the root account. But to avoid switching, an alternative way is to use sudo. If you find an operation requires superuser permission, append sudo before that operation. First, you should add your account to the sudo group:

su -   # we still need to switch to the root account first
adduser username sudo  # change `username` to your user name
exit

To let the changes to take effect, you should log out or reboot. After that, try

whoami
sudo whoami

If it is the first time you run sudo, enter the password of your account. You should find that sudo whoami will output root.

Now you can use sudo to modify the APT source file. This time you should modify it successfully. To check this, you can output the file:

cat /etc/apt/sources.list

You should see it really outputs the line you just echoed.

Updating APT package information

Now you can tell apt to retrieve software information from the sources:

apt-get update

However, you will receive an error message:

E: Could not open lock file /var/lib/apt/lists/lock - open (13: Permission denied)
E: Unable to lock directory /var/lib/apt/lists/

This is because apt-get requires superuser privilege to run. Run the command with superuser privilege. Since it requires Internet accessing, it may cost some time to finish.