命令出当前命令处理程序扩展版本号。

CMDEXTVERSION: Lists the version number of the current Command Processor Extensions.

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在捕捉到信号后，正在执行的当前命令会尝试在trap接管之前结束执行。

When a signal is caught, the current command being executed attempts to complete before the trap command takes over.

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对命令经常使用别名，因此用户不必知道当前命令的版本号，但必要时可以通过较长的名称访问其他版本。

Commands are often aliased so the user does not have to know a version number for the current command, but can access other versions by longer names if necessary.

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