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I execute command that outputs LOTS of information through SSH. For example, I foolishly add debug information inside a loop that executes million times, or just run cat /dev/urandom for kicks.

The terminal is flooded with information.

Example what I'm talking about

I want to terminate the command ASAP and fix my program. I don't care what it prints. Now, the thing is that I press Ctrl+C ASAP (in the example above I pressed it immediately after running the command), but it still takes its time to print all the information I don't even need.

What I've tried

I tried pressing Ctrl+C so hard that it had funny results when terminal finally caught up:

rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C

rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C
rr-@burza:~/xor$ ^C

I also read about Ctrl+S which apparently is used to tell terminal "stop output, I need to catch up" but apparently it does nothing.

Miscellaneous details

I'd like not to alter the command I run so I can rescue myself in any situation, even if I don't remember that the program I run could end up like that.

My SSH client runs on Cygwin (CYGWIN_NT-6.1-WOW64 luna 1.7.30(0.272/5/3) 2014-05-23 10:36 i686 Cygwin) in MinTTY with terminal type set to xterm-256color.

SSH server runs on Debian (Linux burza 3.2.0-4-686-pae #1 SMP Debian 3.2.51-1 i686 i686 i686 GNU/Linux).

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On which system are you running the actual programs that produce the output? Linux, Unix, or Windows? Linux and UNIX ought to accept Ctrl-O, which means "discard any output that is written to this terminal". –  Mark Plotnick Jun 18 '14 at 18:24
The server runs on Debian. Edited the question. Ctrl-O seems to do nothing as well. Perhaps it's the client thing? –  rr- Jun 18 '14 at 18:25
You might try starting your xterm with the -j option, to enable jump scrolling. The basic problem is that the remote can send data faster than the terminal window can display it - by default, it has to bitblt the contents of the window every time a new line is printed. A whole lot of data can get buffered up by the time your Ctrl-C gets received by the remote system, and your terminal program will try to display all of it. –  Mark Plotnick Jun 18 '14 at 18:32
Just an idea: if you have some exact commands that you usually accidentally execute and they generate a lot of output, why not just append some aliases to .bashrc? –  psimon Jun 18 '14 at 18:36

4 Answers 4

up vote 4 down vote accepted

Some of that output will be buffered. You send your Ctrl+C to the remote end which interrupts the running program. The program exists and the shell send the characters to show you the prompt again. Before the prompt is shown your screen will first show all the data that was buffered and already on it's way to you.

What you're asking is for the program to stop and the data in transit to somehow disappear. That can't happen as it's already en-route.

The only way you can make sure that you don't see this data is to exit the terminal at your end and then re-connect to your remote - but that's probably far more effort than waiting for the buffered data to display.

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I usually run the output into less so that I can kill it via less instead using the q key.

$ cmd | less


$ cat /dev/urandom | less

   ss #2

After hitting q+Enter it'll quit and return back to your normal terminal, leaving it nice and clean.

Why does that happen?

The problem you're encountering is that there are buffers (for STDOUT) that are being queued up with the output of your display. These buffers get filled so quickly that you're unable to interrupt it fast enough to stop it.

                                    ss #1

To disable/limit this effect you can disable the STDOUT buffering which should make things a bit more responsive using stdbuf, but you'll likely have to play with these settings to get things the way you want. To unbuffer STDOUT you can use this command:

$ stdbuf -o0 <cmd>

The man page for stdbuf details the options at your disposal:

    If MODE is 'L' the corresponding stream will be line buffered.  This 
    option is invalid with standard input.

    If MODE is '0' the corresponding stream will be unbuffered.

    Otherwise MODE is a number which may be followed by one of the 
    following: KB 1000, K 1024, MB 1000*1000, M 1024*1024, and so
    on for G, T, P, E, Z, Y.  In this case the corresponding stream will be 
    fully buffered with the  buffer  size  set  to  MODE

For a nice background on how buffering works I highly suggest taking a look at this Pixel Beat articled titled: buffering in standard streams. It even includes nice pictures.


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That's reasonable as long as you remember to append |less to cmd, which unfortunately I often don't. If you run cmd, you still need to wait until it's done printing results computed before receiving ^C. –  rr- Jun 18 '14 at 18:30
That doesn't really explain what's going on. After all, there's no pipe involved, so what buffer are you referring to? –  Gilles Jun 19 '14 at 1:08
@Gilles - sorry, the buffer would be the buffer to the screen. –  slm Jun 19 '14 at 1:10
What buffer? In the kernel? In xterm? –  Gilles Jun 19 '14 at 1:13
@Gilles - give me a minute, I'm looking for the details 8-) –  slm Jun 19 '14 at 1:17

There are several levels of buffering. When you press Ctrl+C, this stops the program from emitting data to the terminal. This doesn't affect data that the terminal emulator hasn't displayed yet.

When you're displaying data at very high speed, the terminal can't keep up and will lag. That's what's going on here: displaying text is at lot more expensive than producing these random numbers. Yes, even with a bitmap font — producing cryptographic-quality random numbers is dirt cheap in comparison. (I just tried on my machine and the X process saturated the CPU, with xterm taking a few % and cat (which the random number generation is accounted against) barely reaching 1%. And that's with a bitmap font.)

If you want this to just stop now, kill the terminal emulator. If you don't wish to do that, at least minimize the window; intelligent terminal emulators (such as xterm) will not map the window, which saves the X CPU time, so the garbage will finish displaying quicker. The X server has high priority, so this will make a big difference to your machine's responsiveness while xterm is processing the data in the background.

When all this is going on in a remote shell, the lag is even worse, because the data produced by cat has to first go through the SSH connection. Your press of Ctrl+C also has to go through the SSH connection; it gets somewhat higher priority (it's sent out of band), but that still takes some time during which more output accumulates. There's no way to suppress data in transit short of closing the SSH connection (which you can do by pressing Enter then ~.).

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It should be enough to find a way to kill the cat command.
For the following proposals you may need a second ssh connection open.

  • Seldom CTRL+z can be more effective than CTRL+c: it can answer faster. After that you suspend the command you can kill it with kill %1 or whatever is its job number.
    This in the hope that you are still able to read anything from the screen (a flooding random binary text can easily mess up your characters set).
    As remembered by Gilles if you minimize the window probably the system will be faster to read the interrupt request than you to kill the process. So suspend/break, minimize, wait a little, maximize again, can be a solution too.
    Of course through an ssh connection I expect that you need however to wait some time.

  • In another terminal/session you can ask pgrep cat (if cat was the command invoked) and identify the cat process is using more your cpu. You can identify it with more precision with pstree:

    pgrep cat | awk '{print "pstree -sp "$1}' | sh | grep sshd

    answer with an output like


    In this case, after you have only to kill the cat PID: kill 23131


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