vnstat:tldr:1882a

The command "vnstat -tr ${30}" uses the program "vnstat" to display the traffic usage statistics of a network interface in real-time.

Here is a breakdown of the command:

• "vnstat" is a command-line tool used for monitoring network traffic and generating statistics.
• "-t" or "--top" is an option that specifies to display the top (most used) network interfaces.
• "r" or "--rate" is an option that specifies to display the traffic rate in real-time.
• "${30}" represents a variable that holds the value "30". It could be a placeholder for a number of seconds or minutes during which the real-time statistics are displayed before the command stops running.

Overall, the command would display the real-time traffic usage statistics of the top network interfaces for a duration of 30 seconds (or any other value specified by "${30}").