bash - 如何判断字符串是否未在 Bash shell 脚本中定义

Question

如果我想检查空字符串，我会这样做

[ -z $mystr ]

但是如果我想检查变量是否已经定义了怎么办？还是 Bash 脚本没有区别？

Answer 1

我认为 Vinko 暗示(如果未说明)您所寻求的答案的 answer ，尽管它没有简单地拼写出来。区分VAR是否已设置但为空或未设置，可以使用:

if [ -z "${VAR+xxx}" ]; then echo "VAR is not set at all"; fi
if [ -z "$VAR" ] && [ "${VAR+xxx}" = "xxx" ]; then echo "VAR is set but empty"; fi

您可能可以将第二行的两个测试合并为一个:

if [ -z "$VAR" -a "${VAR+xxx}" = "xxx" ]; then echo "VAR is set but empty"; fi

但是，如果您阅读 Autoconf 的文档，您会发现他们不建议将术语与 '-a' 结合使用，而是建议使用单独的简单测试与 &&。我还没有遇到有问题的系统；这并不意味着它们过去不存在(但它们现在可能非常罕见，即使它们在遥远的过去并不那么罕见)。

您可以找到这些和其他相关的详细信息shell parameter expansions , test or [命令和 conditional expressions在 Bash 手册中。

---

我最近通过电子邮件询问了这个问题的答案:

You use two tests, and I understand the second one well, but not the first one. More precisely I don't understand the need for variable expansion

if [ -z "${VAR+xxx}" ]; then echo "VAR is not set at all"; fi

Wouldn't this accomplish the same?

if [ -z "${VAR}" ]; then echo "VAR is not set at all"; fi

公平的问题 - 答案是“不，你的更简单的替代方案不会做同样的事情”。

假设我在你的测试之前写了这个:

VAR=

您的测试会说“VAR 根本没有设置”，但我的测试会说(暗示因为它什么也没回显)“VAR 已设置但其值可能为空”。试试这个脚本:

(
unset VAR
if [ -z "${VAR+xxx}" ]; then echo "JL:1 VAR is not set at all"; fi
if [ -z "${VAR}" ];     then echo "MP:1 VAR is not set at all"; fi
VAR=
if [ -z "${VAR+xxx}" ]; then echo "JL:2 VAR is not set at all"; fi
if [ -z "${VAR}" ];     then echo "MP:2 VAR is not set at all"; fi
)

输出是:

JL:1 VAR is not set at all
MP:1 VAR is not set at all
MP:2 VAR is not set at all

In the second pair of tests, the variable is set, but it is set to the empty value. This is the distinction that the ${VAR=value} and ${VAR:=value} notations make. Ditto for ${VAR-value} and ${VAR:-value}, and ${VAR+value} and ${VAR:+value}, and so on.

---

As Gili points out in his answer, if you run bash with the set -o nounset option, then the basic answer above fails with unbound variable. It is easily remedied:

if [ -z "${VAR+xxx}" ]; then echo "VAR is not set at all"; fi
if [ -z "${VAR-}" ] && [ "${VAR+xxx}" = "xxx" ]; then echo "VAR is set but empty"; fi

或者您可以使用 set +u 取消 set -o nounset 选项(set -u 等同于 set - o 名词集).