问题描述
我经常在使用自动工具(autoconf、automake)的项目的构建脚本中看到这一点.当有人想检查一个 shell 变量的值时,他们经常使用这个习惯用法:
I see this often in the build scripts of projects that use autotools (autoconf, automake). When somebody wants to check the value of a shell variable, they frequently use this idiom:
if test "x$SHELL_VAR" = "xyes"; then
...
这比简单地检查这样的值有什么优势:
What is the advantage to this over simply checking the value like this:
if test $SHELL_VAR = "yes"; then
...
我想我经常看到这个肯定是有原因的,但我不知道它是什么.
I figure there must be some reason that I see this so often, but I can't figure out what it is.
推荐答案
如果您使用的 shell 进行了简单 替换并且 SHELL_VAR
变量不存在(或为空白),那么您需要注意边缘情况.将发生以下翻译:
If you're using a shell that does simple substitution and the SHELL_VAR
variable does not exist (or is blank), then you need to watch out for the edge cases. The following translations will happen:
if test $SHELL_VAR = yes; then --> if test = yes; then
if test x$SHELL_VAR = xyes; then --> if test x = xyes; then
由于 test
的第一个参数丢失,第一个将产生错误.第二个没有这个问题.
The first of these will generate an error since the fist argument to test
has gone missing. The second does not have that problem.
您的案例翻译如下:
if test "x$SHELL_VAR" = "xyes"; then --> if test "x" = "xyes"; then
x
,至少对于 POSIX 兼容的 shell,实际上是多余的,因为引号导致空参数和包含空格的参数都被解释为单个对象.
The x
, at least for POSIX-compliant shells, is actually redundant since the quotes ensue that both an empty argument and one containing spaces are interpreted as a single object.
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