string integer "double-quoted string" 'single-quoted string` `back-quoted string`
The data Operator
Double-quoted strings and back-quoted strings are dynamically expanded before the condition is evaluated. Single-quoted strings are static literals and are not dynamically expanded.
When used as an existence check, the condition evaluates to
the data is non-zero. Otherwise, the condition evaluates to
For integer existence checks,
false; all other values are
For string existence checks, an empty string is
false. All other
All other data types are disallowed in existence checks.