ssh force tty return extra non-printable character

Question

I know ^M will show up when ssh -t, ref1, but I don't catch the situation when ssh -t into Centos 7.2(even Centos 6.2 is OK, but no difference between their environment vars). Anyone know about this? Or maybe related to ref2?

$ ssh -t centos72 'echo $HOME' 2>/dev/null|cat -A
^[[?25h^[[0G^[[K^[[?25h^[[0G^[[K/home/user^M$
$ ssh centos72 'echo $HOME' 2>/dev/null|cat -A
/home/user$
$ ssh -t ubuntu1404 'echo $HOME' 2>/dev/null|cat -A
/home/user^M$
$ ssh ubuntu1404 'echo $HOME' 2>/dev/null|cat -A
/home/user$

Answer 1

These are control characters for your terminal or VT100 codes. They are needed to make your terminal working with the remote psoudoterminal, to show your prompt and other stuff.

Verbose description of the tags from another SO question and from wikipedia:

• CSI ?25h : Shows the cursor.
• CSI n G : Moves the cursor to column n. (not ANSI.SYS)
• CSI n K : Erases part of the line. If n is zero (or missing), clear from cursor to the end of the line. If n is one, clear from cursor to beginning of the line. If n is two, clear entire line. Cursor position does not change.
• and once more the first two.
• ^M : Carriage Return (on the end of the line)

These are some basic sanity checks to make sure the terminal is ready. Ubuntu sends them probably only after the query from the local terminal.