gettext: sh-format
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.
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