First steps with ROOT

Note

Before you can use ROOT, you must have a working ROOT installation. → See Installation Guide.

Working with an interactive ROOT session

Starting and quitting ROOT

ROOT can be started at the system prompt. To that end, you can type:

$ root

and the ROOT prompt is displayed:

   ------------------------------------------------------------------
  | Welcome to ROOT 6.32.08                        https://root.cern |
  | (c) 1995-2022, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for linuxx8664gcc on Nov 14 2024, 13:00:54                 |
  | From tags/6-32-08@6-32-08                                        |
  | With c++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0                     |
  | Try '.help'/'.?', '.demo', '.license', '.credits', '.quit'/'.q'  |
   ------------------------------------------------------------------

root [0]

To display a list of ROOT commands, type .help (or .?):

root [0] .help

To quit the ROOT prompt, type .q (or .quit or .exit):

root [0] .q

Command line options

These are some command line options you can use when starting ROOT:

-b: ROOT session runs in batch mode, without graphics display. This mode is useful in case you do not want to set the DISPLAY.

-n: Does not execute the logon script and logoff script as specified in .rootrc.

-q: Exits after processing the command line macro files.

-l: Does not show the ROOT banner.

-a: Displays the ROOT splash screen.

-x: Exits on exception.

dir: If dir is a valid directory, change to it (cd) before executing ROOT.

-?, -h, --help: Prints usage.

-config: Prints the cmake configure options.

Running C++ code

ROOT uses the interactive C++ interpreter Cling that is built on top of the Low Level Virtual Machine (LLVM) and the Clang libraries. Cling provides a command line prompt and a just-in-time (JIT) compiler for compilation.
For more information on Cling, → see Cling.

Note

Cling provides a user experience that is closer to a typical interpreter, e.g. IPython. Therefore, unlike pure C++ language, no semicolon (;) is required at the end of the line.

Simple commands

You can use ROOT to execute simple commands at the ROOT prompt. Every command typed at the ROOT prompt is stored in the .root_hist file in your home directory.

Examples

Simple operations:

root [0] 21+21
(int) 42
root [1] sqrt(42)
(double) 6.4807407

Relational operators:

root [2] 42 > 98
(bool) false

Calling a function from a ROOT class like TMath::Pi:

root [3] TMath::Pi()
(double) 3.1415927

Multi-line commands

You can use ROOT to execute multi-line commands at the ROOT prompt.

  1. To start a multi-line command, type at the ROOT prompt:{

  2. Type one command per line.

  3. To end the multi-line command, type:}

Example

root [0] {
root (cont'ed, cancel with .@) [1] int j = 0;
root (cont'ed, cancel with .@) [2] for (int i = 0; i < 3; i++)
root (cont'ed, cancel with .@) [3] {
root (cont'ed, cancel with .@) [4] j = j + i;
root (cont'ed, cancel with .@) [5] std::cout << "i = " << i << ", j = " << j << std::endl;
root (cont'ed, cancel with .@) [6] }
root (cont'ed, cancel with .@) [7] }
i = 0, j = 0
i = 1, j = 1
i = 2, j = 3
root [8]

You can also write the commands to a file, called a ROOT macro, and then execute and compile it. For more information on ROOT macros, → see ROOT macros and shared libraries.

Special interpreter commands

ROOT provides a set of commands to perform special actions from the ROOT prompt. They are all prefixed by a dot:

root [0] .<command>

Here a selection of commands:

.?: Provides the full list of all commands. (Also .help)

.? edit: Prints a list of keyboard shortcuts for editing the input text at the ROOT prompt.

.!<OS_command>: Accesses the shell of the operating system. For example .!ls or .!pwd.

.x <file_name>: Executes a ROOT macro. file_name can be also a precompiled macro or a regular shared library userlib.so (or .dll) with a function userlib(args).

.U <file_name>: Unloads a file.

.L <file_name>: Loads a ROOT macro or library.

.L <file_name>+: Compiles a ROOT macro.

.class: Lists the available ROOT classes.

.Class X: Shows what cling knows about class X.

.files: Shows all loaded files.

.include: Shows the include paths.

.I path: Adds an include path.

ROOT command line tools

ROOT also provides many command line tools at the system prompt for simple file operations or automating common operations performed on ROOT classes. → See ROOT command line tools

Using ROOT from Python

ROOT provides Python bindings, called PyROOT, which allow to access all the ROOT C++ functionality from Python.

Therefore, ROOT can be used interactively from the Python prompt. The first step consists in importing the ROOT module:

$ python
>>> import ROOT

After that, we can use ROOT as we did from C++. The global C++ namespace is accessible via the ROOT module, e.g.:

>>> ROOT.TMath.Pi()
3.141592653589793

ROOT tutorials

ROOT tutorials are available online on the Reference Guide tutorial page.

Moreover, when you install ROOT, a tutorials directory is created, containing all ROOT tutorials.

Starting with a simple tutorial

A good tutorial to start with is df000_simple.C , which shows a simple usage of RDataFrame. To execute it, type at the ROOT prompt:

root [0] .x df000_simple.C

A window with the plot of a histogram should appear as a result.

The same tutorial is also available in Python → df000_simple.py .

A more thorough introduction to RDataFrame can be found → here.

Browsing through the rest of tutorials

The Reference Guide tutorial page groups tutorials by topic. For example, dataframe tutorials can be found → here.

Tutorials are written in C++, Python or both. For instance, the following dataframe tutorial is available in both languages:

Moreover, some tutorials are also published in the form of Jupyter notebooks. If that is the case, below the tutorial link there are two buttons to open the notebook statically (View notebook) or in SWAN (Open in SWAN). Please note that SWAN is an online notebook service that requires a CERN account.