bash 如果字符串中为空，则插入变量值或默认值
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Insert variable value or default value if empty in a string
提问by Heschoon
I want to insert the value of an environment variable in a string or a default value if the corresponding variable is not initialized.
如果相应的变量未初始化，我想在字符串或默认值中插入环境变量的值。
Example:
例子：
if [ -z $MY_VAR ];
then
MY_VAR="default"
fi
echo "my variable contains $MY_VAR"
I'm however using a lot of variables in my strings and the tests are cluttering my script.
然而，我在我的字符串中使用了很多变量，并且测试使我的脚本变得混乱。
Is there a way to make a ternary expression in my string?
有没有办法在我的字符串中创建三元表达式？
Example of what I want to achieve (it doesn't work):
我想要实现的示例（它不起作用）：
echo "my variable contains ${-z $MY_VAR ? $MY_VAR : 'default'}"
回答by chepner
To actually setthe value of the variable, rather than just expanding to a default if it has no value, you can use this idiom:
要实际设置变量的值，而不是仅在没有值时扩展为默认值，您可以使用以下习惯用法：
: ${MY_VAR:=default}
which is equivalent to your original ifstatement. The expansion has the side effect of actually changing the value of MY_VARif it is unset or empty. The :is just the do-nothing command which provides a context where we can use the parameter expansion, which is treated as an argument that :ignores.
这相当于你原来的if陈述。MY_VAR如果未设置或为空，扩展具有实际更改值的副作用。这:只是一个什么都不做的命令，它提供了一个上下文，我们可以在其中使用参数扩展，它被视为:忽略的参数。
回答by Michael Kohl
请参阅Bash 默认值
→ echo "my variable contains ${MY_VAR:-default}"
my variable contains default
