shell变量旁边的连字符是什么(What is a hyphen beside a shell variable)

shell变量旁边的连字符是什么(What is a hyphen beside a shell variable)

我在一些脚本中看到有一个连接到shell变量的连字符。 例如：

if [ -z ${X-} ]

变量旁边的这个连字符符号在这里做了什么。 我在网上找不到任何相关文档。

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

if [ -z ${X-} ]

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

最满意答案

这一切都在本手册的Shell参数扩展部分中进行了解释：

${parameter:-word}

如果parameter未设置或为null，则替换word的扩展。 否则， parameter的值将被替换。

就在此之前，有：

省略冒号只会导致对未设置的参数进行测试。

所以：

${X-stuff}

扩展为：

如果设置$X则扩展$X 如果X没有设置的stuff 。

尝试一下：

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

现在你的扩张是

${X-}

因此，如果设置$X ，它会扩展到$X的扩展，如果未设置$X ，则扩展为空字符串。

---

你为什么想做这个？ 对我来说，似乎这是一个解决方法set -u ：

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

---

最后，你的测试

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

（注意引号，它们是必需的）测试X是否为零（无论是否设置X ，即使使用了set -u ）。

It's all explained in the Shell Parameter Expansion section of the manual:

${parameter:-word}

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

Just before this there is:

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

So:

${X-stuff}

expands to:

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

Try it:

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

Now your expansion is

${X-}

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

---

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-}" $

---

Finally, your test

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

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