为什么autoconf脚本中的条件变量在变量前加"x"? [英] Why do conditionals in autoconf scripts prefix variables with "x"?

问题描述

为什么autoconf脚本中的条件语句在变量前加"x"?例如，GNU提供的用于测试Boost的宏具有以下条件，例如

Why do conditional statements in autoconf scripts prefix their variables with "x"? For example, the macro provided by GNU to test for Boost has conditionals such as

if test "x$want_boost" = "xyes"; then

为什么未将其定义为:

if test "$want_boost" = "yes"; then

推荐答案

在某些早期的shell中，测试空字符串变量并不像现在这样容易，因此最好的选择是查看"x $ variable "等于"x".另外，由于显然使用了test，因此比尝试正确引用/转义'$ x！="y"'这样的序列更简单，而又不会失去理智和/或可移植性.

In some early shells, testing for an empty string variable wasn't as easy as it is now, so the best alternative was to see if "x$variable" was equal to just "x". Also, since that's apparently using test, that's simpler than trying to properly quote/escape sequences like '$x != "y"' without losing sanity and/or portability.

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