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# Bash Ternary Operator
The `?:` operator in Bash is a ternary conditional operator evaluated exclusively within arithmetic expansion contexts (`$((...))`, `((...))`, or `let`). It evaluates a mathematical or logical expression and returns one of two specified integer values based on whether the condition resolves to non-zero (true) or zero (false).
## Arithmetic Ternary Operator
**Syntax:**
```bash theme={"dark"}
condition ? expression_if_true : expression_if_false
```
**Evaluation Mechanics:**
1. **Context Requirement:** The operator is syntactically invalid in standard shell command execution; it must be enclosed in an arithmetic evaluation block.
2. **Condition Resolution:** The `condition` is evaluated as an integer. In Bash arithmetic, any non-zero value represents logical `true`, while `0` represents logical `false`.
3. **Short-Circuit Evaluation:** The operator exhibits short-circuiting behavior. If `condition` is non-zero, only `expression_if_true` is evaluated. If `condition` is zero, only `expression_if_false` is evaluated. Side effects (such as variable assignments) in the unselected branch will not occur.
4. **Return Value:** The integer result of the evaluated expression becomes the return value of the entire ternary operation.
**Syntax Visualization:**
```bash theme={"dark"}
# Assignment via arithmetic expansion
result=$(( condition ? expr1 : expr2 ))
# Standalone arithmetic evaluation
(( var = condition ? expr1 : expr2 ))
```
## Disambiguation: Parameter Expansion (`:?`)
While `?:` is the arithmetic ternary operator, Bash also features a structurally similar parameter expansion operator, `:?` (colon-question mark), used for variable state validation rather than arithmetic logic.
**Syntax:**
```bash theme={"dark"}
${parameter:?word}
```
**Evaluation Mechanics:**
1. **State Check:** The shell evaluates whether `parameter` is unset or null (an empty string).
2. **Execution Flow:**
* If `parameter` is set and not null, the expansion yields the value of `parameter`.
* If `parameter` is unset or null, the expansion evaluates `word` and writes the resulting string to standard error (`stderr`). If `word` is omitted, a default system error message is printed.
3. **Termination:** If the `:?` expansion triggers the error condition within a non-interactive shell (such as a script), the shell immediately aborts execution and exits with a non-zero status.
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