%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 best way to use this feature is to define two TWikiVariables, one for the THEN case and the other for the ELSE case. For example:

<!--
   * Set LI = Fred is here!
   * Set LO = Fred is there!
-->
%IF{"context authenticated and defined WIKINAME and $ WIKINAME='FredBloggs'" then="%LI%" else="%LO%"}%

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

%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.

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

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

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

• Set OPEN_DAY = yes

You are not allowed to view this TWiki today.

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
authenticated	a user is authenticated
can_login	current environment supports login
inactive	if active links such as 'edit' and 'attach' should be disabled
new_topic	if the topic doesn't already exist
mirror	if this is a mirror
rss	if this is an RSS skin rendering
changes	in changes script (see TWikiScripts)
edit	in edit script (see TWikiScripts)
oops	in oops script (see TWikiScripts)
preview	in preview script (see TWikiScripts)
diff	in rdiff script (see TWikiScripts)
rest	in rest script (see TWikiScripts)
save	in save script (see TWikiScripts)
search	in search script (see TWikiScripts)
view	in view script (see TWikiScripts)
absolute_urls	Set if absolute URLs are required
command_line	the running script was run from the command line, and not from CGI
body_text	when the body text is being processed in a view (useful in plugin handlers)
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)

In addition there is an identifier for each enabled plugin; for example, if GallousBreeksPlugin is installed and enabled, then the context ID GallousBreeksPluginEnabled will be set.

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.