gettext: sh-format

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1 15.3.3 Shell Format Strings
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1    Shell format strings, as supported by GNU gettext and the ‘envsubst’
1 program, are strings with references to shell variables in the form
1 ‘$VARIABLE’ or ‘${VARIABLE}’.  References of the form
1 ‘${VARIABLE-DEFAULT}’, ‘${VARIABLE:-DEFAULT}’, ‘${VARIABLE=DEFAULT}’,
1 ‘${VARIABLE:=DEFAULT}’, ‘${VARIABLE+REPLACEMENT}’,
1 ‘${VARIABLE:+REPLACEMENT}’, ‘${VARIABLE?IGNORED}’,
1 ‘${VARIABLE:?IGNORED}’, that would be valid inside shell scripts, are
1 not supported.  The VARIABLE names must consist solely of alphanumeric
1 or underscore ASCII characters, not start with a digit and be nonempty;
1 otherwise such a variable reference is ignored.
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