I can "telnet" to the server and run these commands, when the command sent out the result, I can input another command.

How do you know the command finished sending out the result? By the prompt you are given once it's done. Or maybe because you recognize the format of the output. Or maybe you assumed it was done based on the passing of time. Those are examples of the aforementioned "end of job marker".

A computer cannot detect the appropriate condition without being told what it is. There's too much variety in the possibilities. They usually come in two varieties:

• A prefix that indicates how much data is coming.

  For example,

  • Perl strings are stored as a structure that include length field.
  • The header of IP packets indicate how big the packet is.
  • HTTP response can indicate the size of the response in the Content-Length header.

• A sentinel value indicates the end has been received.

  For example,

  • C strings end at the first occurrence of byte '\0'.
  • bash indicates it's ready for another command by echoing $.
  • HTTP response without a Content-Length header are ended by socket closure.

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