什么是外壳变量旁边的连字符 [英] What is a hyphen beside a shell variable

问题描述

在某些脚本中，我看到在shell变量上附加了一个连字符.例如:

I saw in some of our scripts that there is a hyphen attached to a shell variable. For example:

if [ -z ${X-} ]

此变量旁边的连字符在这里起什么作用.我找不到在线的任何文档.

What does this hyphen symbol beside the variable do here. I cannot find any documentation online for this.

推荐答案

在

$ {parameter:-word}

如果未设置 parameter 或将其设置为null，则将替换 word 的扩展名.否则，将替换 parameter 的值.

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

就在这之前:

省略冒号只会对未设置的参数进行测试.

Omitting the colon results in a test only for a parameter that is unset.

所以:

${X-stuff}

扩展到:

• 如果设置了 X ，则扩展 $ X
• 东西(如果未设置 X ).

• The expansion of $X if X is set
• stuff if X is unset.

尝试一下:

$ unset X
$ echo "${X-stuff}"
stuff
$ X=
$ echo "${X-stuff}"

$ X=hello
$ echo "${X-stuff}"
hello
$

现在您的扩展名是

${X-}

因此，如果设置了 X ，则可能会扩展为 $ X 的扩展；如果未设置 X ，则扩展为空字符串

so you guess that it expands to the expansion of $X if X is set, and to the null string if X is unset.

您为什么要这样做?对我来说，这似乎是 set -u :

Why would you want to do this? to me it seems that this is a workaround the set -u:

$ set -u
$ unset X
$ echo "$X"
bash: X: unbound variable
$ echo "${X-}"

$

---

最后，您的考试

---

Finally, your test

if [ -z "${X-}" ]

(请注意引号，它们是强制性的)测试 X 是否为nil(不管是否设置了 X ，即使 set -u ).

(note the quotes, they are mandatory) tests whether X is nil (regardless of X being set or not, even if set -u is used).

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