How to conditionally define a new command in LaTeX?

Question

I'm trying to compile a LaTeX code with an older version of some package. Thus, I'm having a few issues due to undefined commands.

How can I add conditional code so that I can define some work-around commands of this older package version?

% I want something like this:
if command \foobar is not defined
  \newcommand{\foobar}{FooBar}

% I don't need an "else" clause here, but it would be good
% to know how to add one, in case I need it in future

end if

Answer 1

The LaTeX kernel command \providecommand{\foobar}{FooBar} does exactly what you want.

Answer 2

I think that \providecommand is what you want for this, as lockstep says, but it's worth mentioning the \@ifundefined LaTeX command which tests for whether or not a command is defined and executes some code if it is or isn't. It's more flexible than \providecommand in that it doesn't just deal with defining a particular command. Here's a simple example that I use when anglicising a few commands:

\makeatletter
\@ifundefined{centre}{%
\newenvironment{centre}{\center}{\endcenter}%
}{}
\makeatother

So if the command \centre is not defined then it defines the environment centre.

Answer 3

The ifthen package provides some easy-to-use macros for this. For example:

\ifthenelse{\isundefined{\foobar}}{
  %% Do this if it is undefined
}{
  %% Do this if it is defined
}

You can also do it in plain TeX (which doesn't require any external packages):

\ifx\foobar\undefined
  %% Do this if it is undefined
\else
  %% Do this if it is defined
\fi

Answer 4

The e-TeX primitives that LaTeX wraps (apparently) are \ifdefined and \ifcsname. Use like this:

\ifdefined \foobar \else
  \newcommand{\foobar}{FooBar}
\fi

Use \ifcsname if you need to construct the command name with macros:

\ifcsname foobar\endcsname \else
  \newcommand{\foobar}{FooBar}
\fi

The \ifdefined and \ifcsname control sequences do not exist in the original tex program, but they do in all latex/xetex/pdftex variants on my Ubuntu system.