# Is this command defined?

Macro sets from the earliest days of TeX programming may be observed to test whether commands exist by using

`\ifx` `\`command `\undefined`stuff› …
(which of course actually tests that the command doesn’t exist). LaTeX programmers can make use of the internal command
`\@ifundefined``{cmd name}``{action1}``{action2}`
which executes action1 if the command is undefined, and action2 if it is defined (cmd name is the command name only, omitting the ‘`\`’ character).

The `\@ifundefined` command is based on the sequence

```\expandafter \ifx \csname cmd name\endcsname \relax
```
which relies on the way `\csname` works: if the command doesn’t exist, it simply creates it as an alias for `\relax`.

So: what is wrong with these techniques?

Using `\undefined` blithely assumes that the command is indeed not defined. This isn’t entirely safe; one could make the name more improbable, but that may simply make it more difficult to spot a problem when things go wrong. LaTeX programmers who use the technique will typically employ `\@undefined`, adding a single level of obscurity.

The `\@ifundefined` mechanism has the unfortunate property of polluting the name space: each test that turns out undefined adds a name to the set TeX is holding, and often all those “`\relax`” names serve no purpose whatever. Even so (sadly) there are places in the code of LaTeX where the existence of the `\relax` is relied upon, after the test, so we can’t get away from `\@ifundefined` altogether.

David Kastrup offers the (rather tricky)

```{\expandafter}\expandafter\ifx \csname cmd name\endcsname\relax ...
```
which “creates” the `\relax`-command inside the group of the first `\expandafter`, therefore forgets it again once the test is done. The test is about as good as you can do with macros.

The e-TeX system system comes to our help here: it defines two new primitives:

• `\ifdefined`, which tests whether a thing is defined (the negative of comparing with `\undefined`, as it were), and
• `\ifcsname` cmd name`\endcsname`, which does the negative of `\@ifundefined` without the `\relax`-command side-effect.
So, in an e-TeX-based system, the following two conditional clauses do the same thing:
```\ifdefined\foo
\message{\string\foo\space is defined}%
\else
\message{no command \string\foo}%
\fi
%
\ifcsname foo\endcsname
\message{\string\foo\space is defined}%
\else
\message{no command \string\foo}%
\fi
```
However, after using the LaTeX `\@ifundefined``{foo}`…, the conditionals will detect the command as “existing” (since it has been `\let` to `\relax`); so it is important not to mix mechanisms for detecting the state of a command.

Since most distributions nowadays use e-TeX as their base executable for most packages, these two primitives may be expected appear widely in new macro packages.