IF Statements

The %IF% construct gives TWiki the power to include content in topics based on the value of simple expressions.

%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.

Note that because of the way TWiki evaluates, then whatever is in the THEN and ELSE parameters will already have been expanded by the time the condition is actually evaluated. The standard FormatTokens can be used in the THEN and ELSE parameters when you need to delay evaluation of (for example) a TWiki variable.

The basic syntax of a condition is the same as the syntax used for queries, with the addition of the following special operators:

context	True if the current context is set (see below)
allows	'X' allows 'Y' is true if web/topic 'X' exists and allows access mode 'Y' for the current user. Web access rights are only checked if there is no topic called 'X'.
istopic	istopic 'X' is true if topic 'X' exists
isweb	isweb 'X' is true if web 'X' exists
ingroup	'X' ingroup 'Y' is true if user 'X' is in group 'Y'
defined	True if a preference variable or url parameter of this name is defined.
$	expands a URL parameter or TWikiVariable name. Plugin handlers are not called. Built-in variables and user-defined preferences are supported. You can pass a limited subset of parameters to TWiki variables by enclosing the variable name in single quotes; for example, $ 'VARIABLE{value}'. The 'VARIABLE{value}' string may not contain quotes (' or ").
{X}	expands to the value of the configuration variable {X} - for example, {ScriptUrlPath}

Examples:

1. TWiki variable defined or not

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

2. Compare TWiki variable

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

3. URL parameter

%IF{ "defined search" then="Search: $percntURLPARAM{search}$percnt" else="No search passed in"}%

4. Range test on URL parameter

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

5. Text comparison of URL parameter

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

6. Configuration item set or not

%IF{ "{AntiSpam}{HideUserDetails}" then="User details are hidden" }%

7. Plugin enabled test

TablePlugin is %IF{ "context TablePluginEnabled" then="enabled" else="disabled" }%.

expands to:
TablePlugin is enabled.

8. Check access permissions

You %IF{"'%TOPIC%' allows 'change'" then="can" else="cannot"}% change this topic.
You %IF{"'Sandbox.TestTopic' allows 'change'" then="can" else="cannot"}% change Sandbox.TestTopic.
You %IF{"'Sandbox' allows 'change'" then="can" else="cannot"}% change Sandbox web

expands to:
You cannot change this topic. You cannot change TestTopic? . You can change Sandbox web

9. Check topic existance

Topic Sandbox.TestTopic %IF{"istopic 'Sandbox.TestTopic'" then="exists" else="does not exist"}%
Web Sandbox.TestTopic %IF{"isweb 'Sandbox'" then="exists" else="does not exist"}%

expands to:
Topic TestTopic? does not exist Web TestTopic? exists

10. Group membership

You %IF{"'%USERNAME%' ingroup 'TWikiAdminGroup'" then="are an admin" else="are a normal user"}% 

expands to:
You are a normal user

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)
rss	if this is an RSS skin rendering
save	in save script (see TWikiScripts)
search	in search script (see TWikiScripts)
textareas_hijacked	provided for use by editors that highjack textareas, and want to signal this fact. This is used by skins, for example, so they can suppress extra controls when textareas have been hijacked.
view	in view script (see TWikiScripts)
rest	in rest script (see TWikiScripts)
registration_supported	registration is supported by the current UserMapper?
registration_enabled	set if {Register}{EnableNewUserRegistration} is on, and registrationis supported
passwords_modifyable	set if the password manager support changing the password / email

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.

Note also that while the query syntax can be used to access form fields, there are some contexts in which an IF statement may be used where there is no topic context, or the topic context is not what you expected.