Error: SSH: connect to host * * port 22: network is unreachable
cause: network is disconnected
solution: restart virtual machine or manually connect or command to restart network connection

sudo service network-manager restart

Error: Unit network.service Not found.
reason: wrong command

Error: ROS error: the window cannot be opened remotely qxcbconnection: could not connect to display
qxcbconnection: could not connect to display
reason: the graphical window cannot be opened remotely

Error: under Ubuntu 16.04, the terminal input sudo The problem of sudo: unable to resolve host Ubuntu: connection timed out occurs
cause: this problem is caused by the inconsistency between the host name (localhost) in hosts and the host name in the “etc” folder of the system disk.
Solution: set hosts to be consistent

Requirements: view the SSH connection of Linux
command:

ps aux | grep sshd

Requirements: use the service command to start the SSH service. The command execution is as follows.
Command:

service sshd start

Requirement: how to view the Pi of the host connected through SSH in Ubuntu
command:

who

Requirements: how to modify user password in Ubuntu
command:

usrname@usrname-laptoppasswd:~$passwd usrname