%IF{"CONDITION" then="THEN" else="ELSE"}%

In the example above, if CONDITION evaluates to TRUE, then THEN will be included in the topic; otherwise ELSE will be included.

What can be included in the THEN and ELSE parameters is obviously limited by standard TWiki syntax for parameters.

The basic syntax of a condition is as follows:

expr ::= '(' expr ')' ;
expr ::= andexpr | andexpr 'or' expr ;
andexpr ::= notexpr | notexpr 'and' andexpr ;
notexpr ::= basexpr | 'not' baseexpr ;
basexpr ::= atom | uop atom | atom bop basexpr ;
uop ::= 'context' | 'defined' | '$' ;
bop ::= '=' | '!=' | '>' | '<' | '>=' | '<=' ;
atom ::= context identifier, TWiki variable name, single-quoted string, or configuration item

TWiki variable defined or not
%IF{"defined WIKINAME" then="WIKINAME is defined" else="WIKINAME is not defined"}%

You are %IF{ "$ WIKINAME='TWikiGuest' and not defined OPEN_DAY" then="not" }% allowed to
%IF{ "context view" then="view" else="edit"}% this TWiki today.

URL parameter
%IF{ "defined search" then="Search: %URLPARAM{search}%" else="No search passed in"}%

Configuration item set or not
%IF{ "{AntiSpam}{HideUserDetails}" then="User details are hidden" }%

url param t is %IF{ "0 < $ t and $ t < 1000" then="in" else="out of"}% range.

Text comparison
%IF{ "$'URLPARAM{scope}'='text'" then="Plain text search" }% 

Configuration items are defined in configure. You cannot see the value of a configuration item, you can only see if the item is set or not.

Context identifiers are used in TWiki to label various stages of the rendering process. They are especially useful for skin authors to find out where they are in the rendering process. The following context identifiers are available:

id	context
absolute_urls	Set if absolute URLs are required
authenticated	a user is authenticated
body_text	when the body text is being processed in a view (useful in plugin handlers)
can_login	current environment supports login
changes	in changes script (see TWikiScripts)
command_line	the running script was run from the command line, and not from CGI
diff	in rdiff script (see TWikiScripts)
edit	in edit script (see TWikiScripts)
footer_text	when the footer text is being processed in a view (useful in plugin handlers)
header_text	when the header text is being processed in a view (useful in plugin handlers)
i18n_enabled	when user interface I18N? support is enabled (i.e., user can choose the language for UI)
inactive	if active links such as 'edit' and 'attach' should be disabled
mirror	if this is a mirror
new_topic	if the topic doesn't already exist
oops	in oops script (see TWikiScripts)
preview	in preview script (see TWikiScripts)
rest	in rest script (see TWikiScripts)
rss	if this is an RSS skin rendering
save	in save script (see TWikiScripts)
search	in search script (see TWikiScripts)
view	in view script (see TWikiScripts)

In addition there is a context identifier for each enabled plugin; for example, if GallousBreeksPlugin is installed and enabled, then the context ID GallousBreeksPluginEnabled will be set. Other extensions may set additional context identifiers.

The %IF% statement is deliberately kept simple. In particular, note that there is no way to conditionally execute a Set statement. If you need more sophisticated control over formatting, then consider using the SpreadSheetPlugin.