if ';' STRING ';' STRING ( ';' STRING )?

Macro

if         ::= 'if' ';' condition ';' expansion ( ';' expansion )?
condition  ::= true | false
false      ::= 'false' | ''
true       ::= ! false
expansion  ::= ...

The ${if} macro allows a conditional expansion. The first argument is the condition. the condition is either false (empty or ‘false’) or otherwise true. If the condition is true, the value of the macro is the second argument, the first expansion. Otherwise, if a third argument is specified this is returned. If no third argument is specified, an empty string is returned.

Notice that if a macro is not set, it will revert to its literal value. This can be confusing. For example, if the macro foo is not set, the ${if;${foo};TRUE;FALSE} will be TRUE. If you want to handle unset as false, the def might be of use.

Examples

# expands to 'B'
aorb = ${if;;A;B}

# Display ${foo} if set to a non-empty string that is not false, otherwise 'Ouch'. See also ${def} 
whatisfoo = ${if;${foo};${foo};Ouch}

# Include a file conditionally. If ${test} is not empty or false, the file is included
-include ${if;${test};test.bnd}