Why running `cat /etc/localtime` on PuTTY puts a stream of “PuTTY” in the prompt?

Question

I was randomly experimenting and I found that running cat /etc/localtime on PuTTY and on xterm have different effects.

Example:

root@webtest:~# cat /etc/localtime
TZif2

     ▒
      ▒▒Kmp▒▒ǀ▒▒▒p▒Ƀprp▒▒▒▒_Tp▒▒▒p▒A▒p▒nop▒#
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▒▒!▒▒
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                  %▒EC▒FɐG#▒G▒▒I▒I▒▒J▒K▒▒L̿▒M▒▒N▒▒▒OnnP▒▒▒QW▒▒Rle▒S7l▒TLG▒UN▒V,)▒V▒0▒XFX▒▒Y▒(Z▒▒[▒
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 ▒▒c    LMTWESTWETWEMTCETCEST
WET0WEST,M3.5.0/1,M10.5.0
PuTTYPuTTYPuTTYPuTTYPuTTYPuTTYroot@webtest:~# PuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTY

But when I run on xterm, it gives me this:

root@webtest:~# cat /etc/localtime
TZif2

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Why is PuTTY putting PuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTYPuTTY on the prompt?

---

Edit: This question isn't a duplicate of Why using cat on binary files messed up the terminal and how?

The linked question asks why the ouput is different, even when redirecting the output to a file.

My question is asking why it is showing PuTTY in the prompt (note: on the prompt, where we type the commands), on PuTTY.

I'm really sorry, but I'm not being able to express my doubt any better.

Answer 1

It's an FAQ (for PuTTY):

A.7.12 When I cat a binary file, I get ‘PuTTYPuTTYPuTTY’ on my command line.

Don't do that, then.

This is designed behaviour; when PuTTY receives the character Control-E from the remote server, it interprets it as a request to identify itself, and so it sends back the string ‘PuTTY’ as if that string had been entered at the keyboard. Control-E should only be sent by programs that are prepared to deal with the response. Writing a binary file to your terminal is likely to output many Control-E characters, and cause this behaviour. Don't do it. It's a bad plan.

To mitigate the effects, you could configure the answerback string to be empty (see section 4.3.7); but writing binary files to your terminal is likely to cause various other unpleasant behaviour, so this is only a small remedy.

PuTTY's developers started off by copying things, and sometimes they forget the reason. The answerback sequence (for a while) in xterm would echo "xterm". But that went away in 1999:

change default answerback response to an empty string.

Notwithstanding that, you'll still find people saying that xterm does this (by default).

When a terminal sends an answerback response, the result is just as if you had typed that information. Your shell (or other program) will echo the characters.

Answer 2

/etc/localtime is a symbolic link to a binary file, not a text file. Dumping the contents of a binary file to a terminal can have interesting and unexpected results, particularly if some of the binary data happen to match terminal control codes. PuTTY may have a control code to output its name, which xterm does not or if it does, it parses differently.