How to unfreeze after accidentally pressing Ctrl-S in a terminal?
It’s a situation that has happened quite often to me: after I press (with a different intention) Ctrl-S in a terminal, the interaction (input or output) with it is frozen. It’s probably a kind of “scroll lock” or whatever.
How do I unfreeze the terminal after this?
(This time, I have been working with
apt-shell inside a
urxvt–not sure which of them is responsible for the special handling of Ctrl-S: I was searching the history of commands backwards with C-r, as usual for readline, but then I wanted to go “back” forwards through the history with the usual–at least in Emacs–C-s (1, 2, 3), but that caused the terminal to freeze. Well, scrolling/paging to view past things still works in the terminal, but no interaction with the processes run there.)
Press Ctrl–Q to unfreeze.
This stop/start scheme is software flow control, which is implemented by the OS’s terminal device driver rather than the shell or terminal emulator. It can be configured with the
To disable it altogether, put
stty -ixon in a shell startup script such as
~/.zshrc. To instead just allow any key to get things flowing again, use
Ctrl–Q is indeed the answer. I thought I’d toss in a little history of this that is too long to fit in the margins of ak2’s correct answer.
Back in the dark ages, a terminal was a large piece of equipment that connected to a remote device (originally another terminal because teletypes were so much easier to learn to operate than a telegraph key) over a long wire or via phone lines with modems. By the time Unix was developing, the ASCII code was already well established (although the competing EBCDIC code from IBM was still a force to be reckoned with).
The earliest terminals kept a printed record of every character received. As long as the characters arrived no faster than the print head could type them, at least. But as soon as CRT based terminals were possible, the problem arose that only about 25 lines fit on the CRT, and 25 lines of 80 characters represented enough RAM that no one thought seriously about providing more RAM for characters that had scrolled off the top of the screen.
So some convention was needed to signal that the sending end should pause to let the reader catch up.
The 7-bit ASCII code has 33 code points devoted to control characters (0 to 31 and 127). Some of those had really well established purposes, such as
NUL (blank paper tape leader for threading, gaps, and splices),
DEL (“crossed out” characters on paper tape indicated by punching all seven holes),
TAB. But four were defined explicitly for controlling the terminal device itself (
DC4 aka Ctrl+Q, Ctrl+R, Ctrl+S and Ctrl+T).
My best guess is that some engineer thought that (as mnemonics go), “S” for “Stop” and “Q” for “Continue” weren’t too bad, and assigned
DC3 to mean “please stop sending” and
DC1 to mean “ok, continue sending now”.
Even that convention was already well established by the time Unix was leaving nest at Bell Labs to go out into the world.
The convention is known as software flow control, and is extremely common in real serial devices. It is not easy to implement correctly, as it prevents the use of either of those characters for any other purpose in the communications channel, and the Stop signal has to be handled ahead of any pending received characters to avoid sending more than the receiving end can handle.
If practical, using additional signals out of band from the serial data stream for flow control is vastly preferred. On directly wired connections that can afford the additional signal wires, you will find hardware handshake in use, which frees up those characters for other uses.
Of course, today’s terminal window is not using an actual physical serial port, has scroll bars, and doesn’t really need software handshaking at all. But the convention persists.
I recall the claim that Richard Stallman received complaints about his mapping Ctrl+S to incremental-search in the first releases of emacs, and that he was rather unsympathetic to any user that had to depend on a 7-bit, software flow controlled connection.
Control Keys: perfrom special functions on Shell
- Ctrl–S : Pause Display
- Ctrl–Q : Restart Display
- Ctrl–C : Cancel Operation
- Ctrl–U : Cancel Line
- Ctrl–D : Signal End of File