.. _basic-commands-lesson-label:

==============
Basic Commands
==============

This Lesson is estimated to be completed in 10 minutes.

Description
###########

This page describes the first basic commands that you will need to use when programming in M.

Here we assume that:

* You already have the `GT.M`_ compiler installed in your system.
* You are running in a Linux system with a shell interpreter such a bash or zsh.

GT.M provides a mixture of compiler and interpreter. That is, you can either
write M code directly in an interpreter, or write M code in a source file that
is then compiled and executed.

Our first examples in this Lesson focus on working with the interpreter. This is called `direct`_ mode in GT.M.

.. _direct: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/ch04.html#Operate_in_Direct_Mode
.. _GT.M: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/ch01.html

Start the M Interpreter
#######################

From your shell, start the GT.M interpreter by typing

::

   $ gtm

Here we assume that $ is the prompt symbol of your Linux shell.

In response, you should see the GT.M prompt

::

   GTM>

At this point we can try the following simple command that writes to the
console the version of the GT.M compiler that we are using.

::

   GTM> WRITE $ZVERSION

In response, the system will print a message similar to


::

   GTM> WRITE $ZVERSION
   GT.M V5.5-000 Linux x86_64
   GTM>

and the GT.M prompt will appear again.



Managing Variables
##################

Variables `names`_ in M must start with a letter or the character %, and must continue with letters or numbers.

.. _names: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/ch05s02.html

For example

::

   %TIME
   NAME
   NAME123
   Name12first


The SET Command
###############

The :ref:`set-command-label` is used to set the value of a variable.

Examples

::

   GTM> SET SUM=0
   GTM> SET NAME="Linus Torvalds"
   GTM> SET YEAR=1984

* Variables do not have to be predefined.
* If the variable has not been defined before, the SET command will create the variable.
* There is only one variable type in M: the `string`_ type.
* Variables can be interpreted as numbers depending on the context.

.. _string: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/ch05.html#Data_Types

Space Matters
#############

`Commands`_ must be separated from their arguments by one and only one space.

.. _Commands: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/ch05s07.html

Note that between the SET command and the variable NAME there is only one space.

::

   GTM> SET NAME="Linus Torvalds"

Then,

* There is no space between the variable name and the "=" sign.
* There is no space between the "=" sign and the value.



The WRITE Command
#################

The :ref:`write-command-label` prints the value of a variable to the standard output.

The same rules of spaces applies to the WRITE command. There should be one and
only one space between the WRITE command and its argument.

::

  GTM> WRITE NAME
  Linus Torvalds

or

::

  GTM> WRITE YEAR
  1984

or

::

  GTM> WRITE SUM
  0


Multiple elements can be combined in the same WRITE command by separating them with commas

::

  GTM> WRITE "My Name is: ",NAME
  My Name is: Linus Torvalds


The exclamation point symbol can be used to insert new lines in the output to be printed.

::

  GTM> WRITE "My Name is: ",!,NAME
  My Name is:
  Linus Torvalds


The HALT Command
################

To exit the interpreter, use the :ref:`halt-command-label`.

::

  GTM> HALT

this will return you to the command prompt of your shell interpreter.


Next Lesson: :ref:`basic-coding-style-lesson-label`.