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# Bash Use Alternate Value
The `:+` operator is a parameter expansion modifier in Bash that substitutes an alternate string value if, and only if, the specified parameter is both set and not null. It acts as a conditional expansion mechanism, evaluating the state of a variable without altering its actual stored value.
## Syntax
```bash theme={"dark"}
${parameter:+word}
```
* **`parameter`**: The name of the variable being evaluated.
* **`word`**: The alternate string or expression to be substituted.
## Evaluation Logic
The Bash expansion engine evaluates the state of `parameter` and yields the following results:
1. **Unset**: If `parameter` has not been declared in the environment, the expression expands to a null string (nothing is substituted).
2. **Null**: If `parameter` is declared but empty (e.g., `parameter=""`), the expression expands to a null string.
3. **Set and Not Null**: If `parameter` contains any string value of length greater than zero, the expression expands to the evaluated result of `word`.
## Technical Characteristics
* **Non-Destructive**: The operation is strictly an expansion. The underlying value of `parameter` is never modified or overwritten by this operator.
* **Dynamic Evaluation of `word`**: The `word` token is not treated as a strictly literal string. If the condition is met, `word` is dynamically parsed and subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion before the final substitution occurs.
* **The Colon Modifier (`:`)**: The presence of the colon dictates the strictness of the state test.
* `${parameter:+word}` tests if the parameter is **unset or null**.
* `${parameter+word}` (omitting the colon) tests *only* if the parameter is **unset**. If `parameter` is set to a null string, `${parameter+word}` will successfully expand to `word`, whereas `${parameter:+word}` will not.
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