Defining commands

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In LaTeX, you can define a new command, override an existing command or, in LaTeX2e only, provide a new command if it doesn’t already exist, using the following commands:

\newcommand{cmd}[args][opt]{def}
\renewcommand{cmd}[args][opt]{def}
\providecommand{cmd}[args][opt]{def} in LaTeX2e

The arguments of these are as follows:

cmd: The name of the new or redefined command. A backslash (\) followed by a string of lower and/or uppercase letters or a \ followed by a single nonletter. For \newcommand the name must not be already defined and must not \begin with \end; for \renewcommand it must already be defined. The \providecommand command is identical to the \newcommand command if a command with this name does not exist; if it does already exist, the \providecommand does nothing and the old definition remains in effect.
args: An integer from 1 to 9 denoting the number of arguments of the command being defined. The default is for the command to have no arguments.
opt (LaTeX2e only): If present, then the first of the number of arguments specified by args is optional with a default value of opt; if absent, then all of the arguments are required.
def: The text to be substituted for every occurrence of cmd; a parameter of the form #n in cmd is replaced by the text of the nth argument when this substitution takes place.

Example

% Define a new command:
\newcommand{\super}{supercalifragilisticexpialidocious}

Renewing the command overwrites the behaviour of a previously defined command. If you attempt to renew a command that hasn’t been defined yet, you will get an error.

% Redefine the command:
\renewcommand{\super}{superman}

If you provide a command, the command is created if it doesn’t already exist. If the command does already exist, there will be no effect. The original command’s behaviour will not be affected.

% Define a command only if it's missing:
\providecommand{\super}{supercalifragilisticexpialidocious}

Here’s the full document and its output:

\documentclass[a4paper,12pt]{article}

\begin{document}

\newcommand{\super}{supercalifragilisticexpialidocious}
\noindent\super

\renewcommand{\super}{superman}
\noindent\super

\providecommand{\super}{supercalifragilisticexpialidocious}
\noindent\super

\end{document}