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by Nick_C 4712 days ago
On a side note, does anyone know what ~B actually does? Does it send a SIGINT to the remote terminal? What does ssh mean by the phrase "send a BREAK to a remote system"?

I've tried to use it without success to kill a runaway listing of megabytes of scrolling text, but frantically hitting ctrl-C seems to work much better.
1 comments
thristian 4712 days ago
In the beginning, there was RS232, where each character was a fixed number of bits, optionally with a stop bit and a parity bit. If the sender transmitted too many 0 bits to represent a valid character, that protocol failure was called a 'break', and some receiving equipment would detect such a failure and do something about it, like reset itself to a known-good state. Thus, a 'break' was occasionally a useful thing to send, so sending equipment would often have a special keystroke to cause a 'break' condition.

The Unix 'tty' subsystem was basically designed to support simple serial terminals, and so it had a bunch of behaviour designed to interoperate with the pre-existing 'break' conventions. If a Unix system's serial port received a break, Unix would (optionally) send any processes running via that connection a SIGINT, to represent the 'reset to known-good state' behaviour (this is controlled by the stty command's 'brkint' flag). Also, if the user's terminal didn't provide a specific 'send a break signal' command, Unix could be configured to send a break signal when it received some particular character (^C by default; this is controlled by stty's 'intr' setting).

Of course, nobody uses physical RS232 terminals anymore, but for compatibility reasons the Unix tty API lives on, and the "psuedotty" implementation used for things like terminal emulators maintains compatibility. ssh is basically a tool for exporting the tty API over the network, and so for compatibility it too must have a way to transmit the information "pretend a break condition has occurred on the RS232 connection we're pretending to use."

To summarise: yes, unless you've messed with the stty command, ~B will probably result in a SIGINT. Ctrl-C is probably more reliable since you can hit it much faster than you can type <Enter>~B, but ~B is still useful if your terminal is in 'raw' mode, where ^C is not converted to SIGINT (for example, if you're running an app that wants to bind ^C to some other function).

See also:

    https://en.wikipedia.org/wiki/Universal_asynchronous_receiver/transmitter#Break_condition
    stty(1)
    tcsendbreak(3)
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