How can a shell output affect the typed text after the command prompt?

Question

I have experienced similar behaviour before, but only now I can reproduce it: Output from a command affecting the text after the next command prompt, ready to be Entered by the user.

Take the following example:

cat /usr/share/terminfo/a/ansi

which has the following effect:

Or, copied and pasted:

1%dP1%dM1%dB1%d@1%dS1%dL1%dD1%dC1%dT1%dA%p1%c2%{1}%-%db%p1%dd
1%t;7%;%?%p2%t;4%;%?%p3%t;7%;%?%p4%t;5%;%?%p6%t;1%;%?%p7%t;8%;%?%p9%t;11%;m +%d;%dR;0123456789]c1%dm1%dmAX[user@untrusted ~]$ R65;1;9c

As you can see, R65;1;9c appears after the command prompt and pressing Enter will have it evaluated as command.

I do remember cases where text not only appeared after the command line, but was also executed again, possibly due to it containing newlines.

Can this be considered bug or is this anticipated behaviour, that some kind of combination of non-printable characters allows writing after the next command prompt?

Answer 1

What you are seeing is the attribute string for the terminal. Terminals might respond differently to one or more of these commands.

A quick way to reproduce them:

$ echo -e '\033[c'

In my case, inside Gnome terminal this types 65;1;9c, inside xterm it types 64;1;2;6;9;15;18;21;22c and in the console, after pressing Ctrl-Alt-F1 I get 6c.

The strings are created by the terminal, so you cannot use them to "insert" arbitrary text.

This page tells you more about their meaning:

   CSI Ps c  Send Device Attributes (Primary DA).
          Ps = 0  or omitted -> request attributes from terminal.  The
          response depends on the decTerminalID resource setting.
            -> CSI ? 1 ; 2 c  ("VT100 with Advanced Video Option")
            -> CSI ? 1 ; 0 c  ("VT101 with No Options")
            -> CSI ? 6 c  ("VT102")
            -> CSI ? 6 2 ; Psc  ("VT220")
            -> CSI ? 6 3 ; Psc  ("VT320")
            -> CSI ? 6 4 ; Psc  ("VT420")
          The VT100-style response parameters do not mean anything by
          themselves.  VT220 (and higher) parameters do, telling the
          host what features the terminal supports:
            Ps = 1  -> 132-columns.
            Ps = 2  -> Printer.
            Ps = 3  -> ReGIS graphics.
            Ps = 4  -> Sixel graphics.
            Ps = 6  -> Selective erase.
            Ps = 8  -> User-defined keys.
            Ps = 9  -> National Replacement Character sets.
            Ps = 1 5  -> Technical characters.
            Ps = 1 8  -> User windows.
            Ps = 2 1  -> Horizontal scrolling.
            Ps = 2 2  -> ANSI color, e.g., VT525.
            Ps = 2 9  -> ANSI text locator (i.e., DEC Locator mode).

Some devices will also output identifying information for the device (Request terminal parameters) with the following escape sequence.

$ echo -e '\033[x'