Walkthrough: Starting ROOT (5 minutes)

ROOT is a robust, complex environment for performing physics analysis, and you can spend a lifetime learning it.1

To actually run ROOT, just type:

> root


The window in which you type this command will become your ROOT command window. You’ll see some “Welcome to ROOT” text in the window.

You can type .help to see a list of ROOT commands. You’ll probably get more information than you can use right now. Try it and see.

For the moment, the most important ROOT line command is the one to quit ROOT. To exit ROOT, type .q. Do this now and then start ROOT again, just to make sure you can do it.


Sometimes ROOT will crash. If it does, it can get into a state for which .q won’t work. Try typing .qqq (three q) if .q doesn’t work; if that still doesn’t work, try five q, then seven q. Unfortunately, if you type ten q, ROOT won’t respond, “You’re welcome.”

OK, that’s a dumb joke; I should leave the humor to xkcd. But the tip about .qqq, .qqqqq, and .qqqqqqq is legitimate. Sometimes I find just typing q or using Ctrl-C also works.


ROOT can function as a calculator. If you want, in ROOT type 2+3 or sqrt(2) or whatever. I’m not going to dwell on this aspect of ROOT, but it’s good to know it’s there.2

xkcd e_to_the_pi_minus_pi

Figure 6: https://xkcd.com/217/ by Randall Munroe. If you want to try this in ROOT, it’s exp(TMath::Pi()) - TMath::Pi()


That’s three lifetimes so far.


One of those ROOT quirks that makes you go “uhh…”: If you want to take the sine of 30 degrees you have to use sin(30.*TMath::Pi()/180.)