Run a command only if another command returns a zero exit status.
仅当另一个命令返回零退出状态时才运行某个命令。
The exit value is not 0 (zero), as returned in function main.
这个退出值不是0(零)，如main函数中所返回的。
This is because as long as the connection is successful, the last exit status will be zero.
这是因为只要连接成功，最后的退出状态码就是零。
Run a command only if another command returns a non-zero exit status.
仅当另一个命令返回非零退出状态时才运行某个命令。
Run a command only if another command returns a non-zero exit status.
仅当另一个命令返回非零退出状态时才运行某个命令。
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