Preprocessor directives are similar to a WikiLink in form, except they begin with ! and may contain parameters. The general form is:

[[!directive param="value" param="value"]]

This gets expanded before the rest of the page is processed, and can be used to transform the page in various ways.

The quotes around values can be omitted if the value is a simple word. Also, some directives may use parameters without values, for example:

[[!tag foo]]

A preprocessor directive does not need to all be on one line, it can be wrapped to multiple lines if you like:

[[!directive foo="baldersnatch"
bar="supercalifragalisticexpealadocious" baz=11]]

Also, multiple lines of quoted text can be used for a value. To allow quote marks inside the quoted text, delimit the block of text with triple-quotes:

[[!directive text="""
1. "foo"
2. "bar"
3. "baz"
"""]]

ikiwiki also has an older syntax for preprocessor directives, which requires a space in directives to distinguish them from wikilinks. This syntax has several disadvantages: it requires a space after directives with no parameters (such as [[pagecount ]]), and it prohibits spaces in wikilinks. ikiwiki now provides the !-prefixed syntax shown above as the preferred alternative. However, ikiwiki still supports wikis using the older syntax, if the prefix_directives option is not enabled. For backward compatibility with existing wikis, this option currently defaults to off, so ikiwiki supports the old syntax.