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# Bash String Is Empty
The `-z` operator is a unary string evaluation operator in Bash that returns true (exit status `0`) if the length of its string operand is exactly zero, and false (exit status `1`) otherwise. It is executed within the `test` built-in command or conditional expression constructs (`[ ]` and `[[ ]]`) to evaluate the initialization or population state of a variable.
## Syntax
```bash theme={"dark"}
# POSIX-compliant test command
test -z "$STRING"
# POSIX-compliant single bracket syntax
[ -z "$STRING" ]
# Bash-specific double bracket keyword syntax
[[ -z $STRING ]]
```
## Evaluation Mechanics
* **Unset Variables:** By default, if the operand is an uninitialized (unset) variable, Bash expands it to an empty string prior to evaluation. The `-z` operator processes this as a zero-length string and evaluates to true (`0`). However, if the `nounset` shell option is active (`set -u` or `set -o nounset`), referencing an unset variable (e.g., `[ -z "$VAR" ]`) triggers an "unbound variable" error and halts execution. To safely evaluate a potentially unset variable under `set -u`, use default parameter expansion (e.g., `[ -z "${VAR:-}" ]`) to provide an empty fallback without triggering the error.
* **Whitespace:** A string containing only whitespace characters (e.g., `" "`) has a length greater than zero. The `-z` operator evaluates this as false (`1`).
* **Complement Operator:** The logical inverse of `-z` is `-n`, which evaluates to true if the string length is non-zero.
## Quoting and Parsing Rules
The parsing behavior of `-z` depends heavily on the conditional construct used and the application of double quotes around the operand:
* **Single Brackets (`[ ]`):** The operand undergoes word splitting and pathname expansion. If the variable is unquoted and empty, the expression expands to `[ -z ]`. The `test` built-in interprets this as a single-argument test evaluating the literal string `"-z"`, which evaluates to true (exit status `0`) because the string `"-z"` has a non-zero length. If the unquoted variable contains spaces, it splits into multiple arguments (e.g., `VAR="a b"` expanding to `[ -z a b ]`), which results in a `too many arguments` error. **Double quoting (`"$VAR"`) is strictly required.**
* **Double Brackets (`[[ ]]`):** This construct is a shell keyword that suppresses word splitting and pathname expansion. The expression `[[ -z $VAR ]]` is parsed safely even if `$VAR` is empty or contains spaces. While quoting is not strictly required here, it is universally recommended for consistency.
## Exit Status Codes
| Condition | Exit Status | Result |
| :-------------------------------------------------- | :---------- | :-------------------------------------------------------------------- |
| `length(STRING) == 0` | `0` | True |
| `length(STRING) > 0` | `1` | False |
| Missing operand in `[ ]` or `test` (e.g., `[ -z ]`) | `0` | True (POSIX single-argument rule evaluates literal `"-z"`) |
| Missing operand in `[[ ]]` (e.g., `[[ -z ]]`) | `0` | True (Bash treats `-z` as a literal string if no operand is provided) |
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