在bash中，我怎么测试一个变量定义在＆QUOT; -u＆QUOT;模式 [英] In Bash, how do I test if a variable is defined in "-u" mode

问题描述

我刚刚发现设置-u 在bash和它帮助我pviously找到几个$ P $看不见的错误。但我也有一种情况，我需要测试变量是否计算了一些默认值之前定义。我想出了这个最好的是：

I just discovered set -u in bash and it helped me find several previously unseen bugs. But I also have a scenario where I need to test if a variable is defined before computing some default value. The best I have come up with for this is:

if [ "${variable-undefined}" == undefined ]; then
    variable="$(...)"
fi

该作品（只要变量没有字符串值未定义）。我在想，如果有一个更好的办法？

which works (as long as the variable doesn't have the string value undefined). I was wondering if there was a better way?

推荐答案

您可以测试在几个方面不确定的字符串。使用标准的测试条件是这样的：

What Doesn't Work: Test for Zero-Length Strings

You can test for undefined strings in a few ways. Using the standard test conditional looks like this:

# Test for zero-length string.
[ -z "$variable" ] || variable='foo'

这不会与工作设置-u ，但是。

另外，你可以使用条件的分配，这是一个更bash类似的方式来做到这一点。例如：

Alternatively, you can use conditional assignment, which is a more Bash-like way to do this. For example:

# Assign value if variable is unset or null.
: "${variable:=foo}"

由于猛砸方法来处理这​​个前pression扩展，你可以放心地使用设置-u使用没有得到一个击：变量：未绑定变​​量错误。

Because of the way Bash handles expansion of this expression, you can safely use this with set -u without getting a "bash: variable: unbound variable" error.

这篇关于在bash中，我怎么测试一个变量定义在＆QUOT; -u＆QUOT;模式的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！