I am not very familar with xterm so I decided to try it out. I installed roots and entered a command I saw on the internet few days ago. the command is .
So I want to will this cause any harm to my n900?

That command looks like it's printing text of battery power left.

It should not harm. May I ask why you don't just use one of the battery widgets that are available?

Be cautious playing in 'root' when you install it.
Can be harmful if you use the wrong commands.

I was afraid that this might've damaged my 900. Thanks for answering my question.

yeah, be very careful executing commands you find on the InterWeb. (especially those that involve "rm -rf *")

All it seems to do is print remaining battery mAh.

You would be better off getting a widget or app.
Not playing with random commands. I wouldn't use it if I could find a GUI app.

Edit:
What does rm -rf do? I understand rm and rmdir etc. Just the '-rf'?

The "-r" option tells it to run recursively (i.e. for all subdirectories as well) and the "-f" option tells it to force the removal (i.e. ignore permissions (if possible), not prompt, etc). If you run it as root, then it will delete everything under the directory you run it from.