What does “\r” do in the following script?

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The '\r' character is the carriage return, and the carriage return-newline pair is both needed for newline in a network virtual terminal session.

From the old telnet specification (RFC 854) (page 11):

The sequence “CR LF”, as defined, will cause the NVT to be
positioned at the left margin of the next print line (as would,
for example, the sequence “LF CR”).

However, from the latest specification (RFC5198) (page 13):

  1. …

  2. In Net-ASCII, CR MUST NOT appear except when immediately followed
    by either NUL or LF, with the latter (CR LF) designating the “new
    line” function. Today and as specified above, CR should
    generally appear only when followed by LF. Because page layout
    is better done in other ways, because NUL has a special
    interpretation in some programming languages, and to avoid other
    types of confusion, CR NUL should preferably be avoided as
    specified above.

  3. LF CR SHOULD NOT appear except as a side-effect of multiple CR LF
    sequences (e.g., CR LF CR LF).

So newline in Telnet should always be '\r\n' but most implementations have either not been updated, or keeps the old '\n\r' for backwards compatibility.

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