> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash Use Default Value
The `:-` operator is a shell parameter expansion modifier used to provide a default value during variable substitution. It evaluates the state of a parameter and returns a specified fallback string if the parameter is either **unset** or **null** (an empty string).
## Syntax
```bash theme={"dark"}
${parameter:-word}
```
* **`parameter`**: The name of the variable being evaluated.
* **`word`**: The fallback expression. This is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion before being returned.
## Evaluation Mechanics
The operator evaluates the `parameter` against three distinct memory states:
1. **Set and Not Null**: If `parameter` contains a value of length > 0, the expansion yields the value of `parameter`. The `word` is ignored.
2. **Unset**: If `parameter` has not been declared in the current shell execution environment, the expansion yields the evaluation of `word`.
3. **Null**: If `parameter` is declared but contains an empty string (`""`), the expansion yields the evaluation of `word`.
*Note: The `:-` operator is strictly a substitution mechanism. It does **not** assign the evaluated `word` back to the `parameter`. To perform substitution and assignment simultaneously, the `:=` operator is used.*
## State Visualization
```bash theme={"dark"}
# State 1: Set and not null
VAR="data"
echo "${VAR:-fallback}"
# Output: data
# State 2: Unset
unset VAR
echo "${VAR:-fallback}"
# Output: fallback
# State 3: Null (empty string)
VAR=""
echo "${VAR:-fallback}"
# Output: fallback
```
## Technical Distinction: `:-` vs `-`
The inclusion of the colon (`:`) dictates how the shell handles null values.
* **`${parameter:-word}`**: Tests for both **unset** and **null**.
* **`${parameter-word}`**: Tests strictly for **unset**. If the parameter is null, the shell considers it a valid state and returns the empty string.
```bash theme={"dark"}
VAR=""
# Omitting the colon treats the null variable as valid
echo "${VAR-fallback}"
# Output: (empty string)
# Including the colon treats the null variable as invalid
echo "${VAR:-fallback}"
# Output: fallback
```
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