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<h1 class="subsection">Conditional Variable Assignment</h1> <p>There is another assignment operator for variables, ‘<samp>?=</samp>’. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement: </p> <div class="example"> <pre class="example">FOO ?= bar
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</div> <p>is exactly equivalent to this (see <a href="origin-function">The <code>origin</code> Function</a>): </p> <div class="example"> <pre class="example">ifeq ($(origin FOO), undefined)
FOO = bar
endif
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</div> <p>Note that a variable set to an empty value is still defined, so ‘<samp>?=</samp>’ will not set that variable. </p><div class="_attribution">
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Copyright © 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 Free Software Foundation, Inc. <br>Licensed under the GNU Free Documentation License.<br>
<a href="https://www.gnu.org/software/make/manual/html_node/Conditional-Assignment.html" class="_attribution-link">https://www.gnu.org/software/make/manual/html_node/Conditional-Assignment.html</a>
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