如何在Makefile中执行算术运算? [英] How do I perform arithmetic in a makefile?

问题描述

是否可以对Makefile中的变量执行某些操作? 例如，定义

Is it possible to perform some operations on variables in a makefile? For instance, defining

JPI=4
JPJ=2

是否可以在同一个makefile文件中定义等于$(JPI)* $(JPJ)扩展值的变量JPIJ?

Is it possible to define in the same makefile a variable JPIJ equal to the expanded value of $(JPI)*$(JPJ)?

推荐答案

使用第一行是选择Bash shell来代替默认值(sh).通常，sh不支持算术扩展.但是在Ubuntu中，/bin/sh由 Dash提供，它支持此功能.这样该行可以跳过.

The first line is to choose the Bash shell instead of the default (sh). Typically, sh doesn't support arithmetic expansion. However in Ubuntu, /bin/sh is provided by Dash, which supports this feature. So that line could be skipped.

双美元符号是因为我们想要由外壳完成的扩展.注意:JPI和JPJ变量先由make扩展，然后将表达式传递给bash，如下所示:

The double dollar sign is because we want the expansion to be done by the shell. Note: the JPI and JPJ variables are expanded by make first, then the expression is passed to bash like this:

$(( 4 * 2 ))

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