until

until test-commands; do consequent-commands; done

只要test-commands的退出状态不为0, 就会一直执行consequent-commands.
until结构的退出状态是最后一次consequent-commands执行的退出状态。如果consequent-commands一次都没有被执行，until结构的退出状态就为0.

$ let i=0
$ until (( i >= 10 )); do (( i++ ));echo $i; done
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while

while test-commands; do consequent-commands; done

只要test-commands的退出状态为0, 就会一直执行consequent-commands.
while结构的退出状态是最后一次consequent-commands执行的退出状态。如果consequent-commands一次都没有被执行，while结构的退出状态就为0.

$ while (( i < 10 )); do (( i++ ));echo $i; done
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