What does r do in the following script

Question

I am using following script to reboot my router using Telnet      usr bin env python  import os import telnetlib from time import sleep  host    192 168 1 1  user    USER  password    PASSWORD  cmd    system restart   tn   telnetlib Telnet host  sleep 1   tn read until  Login     tn write user     n r   sleep 1   tn read until  Password     tn write password     n r   sleep 1   tn write cmd     n r     I don t know why but removing   r  from above code make the script non-working  So what does   r  do in this script and when to use   r  in general   Note  I know about  Carriage Return  but still could not figure out its use in my script  I am running this script in Linux

User · Accepted Answer

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                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    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

User · Answer

Actually  this has nothing to do with the usual Windows   Unix  r n vs  n issue  The TELNET procotol itself defines  r n as the end-of-line sequence  independently of the operating system  See RFC854

User · Answer

r is the ASCII Carriage Return  CR  character    There are different newline conventions used by different operating systems  The most common ones are    CR LF   r n   LF   n   CR   r     The  n r  LF CR  looks unconventional   edit  My reading of the Telnet RFC suggests that    CR LF is the standard newline sequence used by the telnet protocol  LF CR is an acceptable substitute       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

User · Answer

r  means  carriage return  and it is similar to   n  which means  line break  or more commonly  new line   in the old days of typewriters  you would have to move the carriage that writes back to the start of the line  and move the line down in order to write onto the next line   in the modern computer era we still have this functionality for multiple reasons  but mostly we use only   n  and automatically assume that we want to start writing from the start of the line  since it would not make much sense otherwise   however  there are some times when we want to use JUST the   r  and that would be if i want to write something to an output  and the instead of going down to a new line and writing something else  i want to write something over what i already wrote  this is how many programs in linux or in windows command line are able to have  progress  information that changes on the same line   nowadays most systems use only the   n  to denote a newline  but some systems use both together   you can see examples of this given in some of the other answers  but the most common are    windows ends lines with   r n  mac ends lines with   r  unix linux use   n     and some other programs also have specific uses for them   for more information about the history of these characters

[python] What does "\r" do in the following script?

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