PETRA : A command line interface, sometimes abbreviated CLI,
is a human machine interface in which communication
between the user and the computer takes place in text mode.
REMI : The user types a command--
that is, they type of the text using the keyboard
to ask the computer to perform an operation.
The computer then displays text corresponding
to the result of the execution of the command or text
corresponding to the questions that an invoked software
application asks of the user.
PETRA : A command line interface can also
be used to launch the execution of various software applications
that use a command interpreter as well as for dialogue with a user.
It is at the core of the interaction between a user
and the computer or any other computing equipment.
REMI : When a command line interface is
ready to receive a command from the user.
It indicates this by displaying a command prompt.
This prompt typically consists of some information
at the beginning of the line, usually the user's account name,
or the computer name, or the current working directory or the date.
And it ends with a well-known character, often the dollar,
or the pound sign or the greater than sign.
This prompt lets the user know that the system is ready to receive a command.
PETRA : One of the features of the Unix operating
system and, by inheritance, the Linux operating system
is that from its earliest beginnings on, it
has had at its disposal more than 100 software applications,
often performing very simple tasks and, most importantly,
all usable from the command line.
REMI : The elementary commands are of the form--
well, command at the beginning, then space, options, than space,
then files_or_data-- knowing that the options and the files or data
are optional.
PETRA : The command appearing at the beginning of the line
is almost always the name of a software application.
This can be an operating system command or another software application written
by a user, often in the C programming language.
REMI : A command line option modifies the execution of a command.
The effect of the option depends on the command.
Generally, the options immediately follow
the name of the command on the command line and they are separated by spaces.
PETRA : Finally, the files or data are the program entries.
For example, if you want to run the application that
displays a calendar in text mode, you type the command cal and press
the Enter key.
And here's the result. If we add the minus j option to it,
it displays the calendar in Julian days.
That's the number of days elapsed since January 1st,
so we type the command cal minus j, press the Enter key
and here's the calendar in Julian days.