Custom Commands

Sometimes in LaTeX there is not a package that does exactly what you want, or you want to make a specific action repeatable. In these cases, you can define your own commands. This is done using the \newcommand command or the \newenvironment command.

When defining new commands or environments you will want to add in \makeatletter and \makeatother to allow for the use of @ and other special characters in the command. This is done as follows:

\makeatletter
% Define new commands here
\makeatother

New Commands

The syntax for \newcommand is:

\newcommand{\commandname}[num]{definition}

where \commandname is the name of the command, num is the number of arguments the command takes (up to 9), and definition is what the command does. For example, if you wanted to define a command that bolds text, you could use:

\newcommand{\bold}[1]{\textbf{#1}}

This would allow you to use \bold{some text} to bold the text "some text". If you wanted to define a command that takes two arguments, you could use:

\newcommand{\add}[2]{#1 + #2}

This would allow you to use \add{2}{3} to get "2 + 3".

These are simple examples, but you can define more complex commands. For example, you could define a command that creates a new section with a label and a reference:

\newcommand{\newsection}[2]{\section{#1}\label{#2}\ref{#2}}

This would allow you to use \newsection{Introduction}{sec:intro} to create a new section titled "Introduction" with the label "sec:intro" and a reference to it.

A particularly useful command may be extending the roman numeral command to be more inline with your needs. For example:

\newcommand{\Rom}[1]{\textrm{\MakeUppercase{\romannumeral #1}}}
\newcommand{\rom}[1]{\textrm{\romannumeral #1}}
\newcommand{\Romit}[1]{\textit{\textrm{\MakeUppercase{\romannumeral #1}}}}
\newcommand{\romit}[1]{\textit{\textrm{\romannumeral #1}}}

This would allow you to use \Rom{1} to get "I", \rom{1} to get "i", \Romit{1} to get "I", and \romit{1} to get "i".

However, you may find it impractical to define a new command for each case here, in this case you can define a command with optional arguments instead. This is done by using the following syntax:

\newcommand{\roms}[2][n]{%
    \if S#1 \textrm{\MakeUppercase{\romannumeral #2}}%
    \else \if I#1 \textit{\textrm{\MakeUppercase{\romannumeral #2}}}%
    \else \if i#1 \textit{\textrm{\romannumeral #2}}%
    \else \textrm{\romannumeral #2}%
    \fi \fi \fi
}

This would allow you to use \roms{1} to get "i", \roms[I]{1} to get "I", \roms[i]{1} to get "i", and \roms[S]{1} to get "I".

New Environments

The syntax for \newenvironment is:

\newenvironment{name}[num]{before}{after}

where name is the name of the environment, num is the number of arguments the environment takes (up to 9), before is what happens before the environment, and after is what happens after the environment. For example, if you wanted to define an environment that bolds text, you could use:

\newenvironment{boldenv}{\textbf\bgroup}{\egroup}

This would allow you to use:

\begin{boldenv}
some text
\end{boldenv}

This gives you large levels of control over your documents and gives you the ability to create tools to make your time writing LaTeX easier.

Additional Notes

When defining new commands or environments, you should be careful not to overwrite existing commands or environments.
You can use \renewcommand to redefine an existing command.
You can use \providecommand to define a command only if it does not already exist.
You can use \newcommand* to define a command that does not allow for paragraph breaks in its arguments.
You can use \newenvironment* to define an environment that does not allow for paragraph breaks in its contents.

You may want to save your custom commands in a separate file like a .sty file and include it in your main document, if this is stored in your texmf directory you can use it in any document you write and call it as a package.