ROOT can be started from the command line, issuing the command root, whose result is the output of the ROOT banner for few seconds and the prompt of CINT. The most trivial command is the question mark, that prints some helpful guidelines (NB: we dropped several lines, indicated by the ellipsis; user input is typed just after the ROOT prompt "root[n]" where n is counting the accepted commands):

root [0] ?
(...)
CINT/ROOT C/C++ interpreter interface.
All commands must be preceded by a . (dot), except
for the evaluation statement { } and the ?.
===========================================================================
             > [file]  : output redirection to [file]
             2> [file] : error redirection to [file]
             >& [file] : output&error redirection to [file]
Help:        ?         : help
             help      : help
             /[keyword] : search keyword in help information
Shell:       ![shell]  : execute shell command
(...)
Evaluation:  S [expr]  : step over expression (no declaration/loop/condition)
             {[statements]} : evaluate statement (any kind)
             x [file]  : load [file] and execute function [file](wo extension)
             X [file]  : load [file] and execute function [file](wo extension)
             E <[file]>: open editor and evaluate {statements} in the file
Load/Unload: L [file]  : load [file]
(...)
Quit:        q         : quit cint
             qqq       : quit cint - mandatory
             qqqqq     : exit process immediately
             qqqqqqq   : abort process
             save      : call emergency routine to save important data
 
ROOT special commands.
===========================================================================
             pwd          : show current directory, pad and style
             ls           : list contents of current directory
             which [file] : shows path of macro file
root [1] .q

ROOT keeps the history of all commands, even across different interactive sessions. On Unix platforms, we can check those commands by opening the text file ~/.root_hist (where the tilde is a short-hand notation for our home directory).