Gettext/sh 002dformat

From Get docs

15.3.12 Shell Format Strings

Shell format strings, as supported by GNU gettext and the ‘envsubst’ program, are strings with references to shell variables in the form $variable or ${variable}. References of the form ${variable-default}, ${variable:-default}, ${variable=default}, ${variable:=default}, ${variable+replacement}, ${variable:+replacement}, ${variable?ignored}, ${variable:?ignored}, that would be valid inside shell scripts, are not supported. The variable names must consist solely of alphanumeric or underscore ASCII characters, not start with a digit and be nonempty; otherwise such a variable reference is ignored.