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<h1 class="section">Basics of Variable References</h1> <p>To substitute a variable’s value, write a dollar sign followed by the name of the variable in parentheses or braces: either ‘<samp>$(foo)</samp>’ or ‘<samp>${foo}</samp>’ is a valid reference to the variable <code>foo</code>. This special significance of ‘<samp>$</samp>’ is why you must write ‘<samp>$$</samp>’ to have the effect of a single dollar sign in a file name or recipe. </p> <p>Variable references can be used in any context: targets, prerequisites, recipes, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program: </p> <div class="example"> <pre class="example">objects = program.o foo.o utils.o
program : $(objects)
cc -o program $(objects)
$(objects) : defs.h
</pre>
</div> <p>Variable references work by strict textual substitution. Thus, the rule </p> <div class="example"> <pre class="example">foo = c
prog.o : prog.$(foo)
$(foo)$(foo) -$(foo) prog.$(foo)
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</div> <p>could be used to compile a C program <samp>prog.c</samp>. Since spaces before the variable value are ignored in variable assignments, the value of <code>foo</code> is precisely ‘<samp>c</samp>’. (Don’t actually write your makefiles this way!) </p> <p>A dollar sign followed by a character other than a dollar sign, open-parenthesis or open-brace treats that single character as the variable name. Thus, you could reference the variable <code>x</code> with ‘<samp>$x</samp>’. However, this practice can lead to confusion (e.g., ‘<samp>$foo</samp>’ refers to the variable <code>f</code> followed by the string <code>oo</code>) so we recommend using parentheses or braces around all variables, even single-letter variables, unless omitting them gives significant readability improvements. One place where readability is often improved is automatic variables (see <a href="automatic-variables">Automatic Variables</a>). </p><div class="_attribution">
<p class="_attribution-p">
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/Reference.html" class="_attribution-link">https://www.gnu.org/software/make/manual/html_node/Reference.html</a>
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