set -- a b c d e f g h i j
[ "$1" = a ] || echo "error at line $LINENO"
[ "${1}" = a ] || echo "error at line $LINENO"
[ "${1-foo}" = a ] || echo "error at line $LINENO"
[ "${1+foo}" = foo ] || echo "error at line $LINENO"
[ "$1+foo" = a+foo ] || echo "error at line $LINENO"
[ "$10" = a0 ] || echo "error at line $LINENO"
[ "$100" = a00 ] || echo "error at line $LINENO"
[ "${10}" = j ] || echo "error at line $LINENO"
[ "${10-foo}" = j ] || echo "error at line $LINENO"
[ "${100-foo}" = foo ] || echo "error at line $LINENO"