> ## 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 Display Error if Null or Unset
The `:?` operator is a parameter expansion modifier in Bash used to assert that a variable is set and not null. If the specified parameter is unset or null, Bash evaluates a provided string, writes it to standard error, and immediately aborts the execution of a non-interactive shell with a non-zero exit status. If the parameter is valid, the expression expands to the parameter's value.
```bash theme={"dark"}
${parameter:?word}
```
## Evaluation Mechanics
The behavior of the expansion depends strictly on the state of `parameter`:
* **Set and Not Null:** The expression evaluates to the value of `parameter`. The `word` is ignored.
* **Unset or Null:** Bash expands `word` and writes the result to standard error.
* **Omitted Word:** If `word` is not provided (e.g., `${parameter:?}`), Bash supplies the default diagnostic message strictly as: `parameter null or not set`.
## Error Output Format
When the error is triggered, Bash prefixes the output message depending on the execution environment:
* **Non-interactive shells (Scripts):** The prefix includes the script's name (`$0`) and the line number where the error occurred. Format: `$0: line N: parameter: word` (e.g., `./script.sh: line 3: parameter: word`).
* **Interactive shells (Terminal):** The prefix strictly uses the shell name. Format: `bash: parameter: word`.
## Execution Context Behavior
The operator interacts directly with the shell's execution state:
* **Non-interactive shells (Scripts):** Triggering the error causes the shell to immediately terminate execution with an exit status of `1`. Subsequent commands are not executed.
* **Interactive shells (Terminal):** Triggering the error prints the message to standard error but does *not* exit the shell session.
## Safe Evaluation via the Null Command
Because the expression expands to the variable's value when valid, using it as a standalone command will cause Bash to attempt to execute the expanded value, typically resulting in a `command not found` error. To evaluate the expansion strictly for its assertion side-effect without executing the resulting string, it must be passed as an argument to the shell's null command (`:`). The null command evaluates its arguments and performs expansions, but takes no action and always returns a zero exit status.
```bash theme={"dark"}
: "${parameter:?word}"
```
## Strict Distinction: `:?` vs `?`
Bash differentiates between the presence and absence of the colon (`:`) in the operator, which alters the evaluation of null (empty) strings:
```bash theme={"dark"}
# Fails if 'var' is UNSET or NULL (empty string)
: "${var:?Error Message}"
# Fails ONLY if 'var' is UNSET. An empty string ("") is considered valid and will not trigger the error.
: "${var?Error Message}"
```
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