lsof

To print out a list of all open files, which means programs accessing files at the moment, use the lsof command. You'll get a long list; it's best to use it with grep to filter the content. To quickly do some math calculations on the command line, use the PC calculator. screen is a very useful command to detach from an SSH connection without actually disconnecting or exiting from it, which is very useful to pause your work and later go back to exactly the same point where you left, or to work from another computer. This can save a tremendous amount of time. First, in order to create a new detachable session, type screen. Now do your work, for example, type a text in VI. Now, imagine your day at work is over and you go home. Without the screen, you would now need to save your changes, close VI, and logout from the server. With a screen, just use the key combination Ctrl + A + D to detach from the current SSH session. If you have successfully detached from a session, a line will appear saying detached from and then the screen session ID. Now, in order to prove that we can reattach to this session, just log out from the server and then log back in to the server. Then, back on the server type screen -list to get a list of all the detached screens. To reattach to your screen, use the screen ID: $ screen -r 23433.pts-l_localhost. As you can see, we are exactly back where we left off. If you want to stop your screen session, type exit. Here, we showed you the most fundamental use cases for these programs.