To see how long a process has been running on a Linux system, you can use the ps command with the -o and etime options.
For example, to see the elapsed time for the process with PID 12345, run the following command:
ps -o etime= -p 12345
The etime option displays the elapsed time since the process was started, in the format dd-hh:mm:ss.
If you want to see the elapsed time for all processes on the system, you can run the following command:
ps -eo etime,pid,comm | sort
This command will display the elapsed time, PID, and command name for all processes, sorted by elapsed time.
Note: The ps command is a versatile tool that provides a lot of information about processes running on the system. The options used in these examples are just a few of the many available. To learn more about the ps command, refer to the man page or the online documentation.