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How to kill a runaway cat?

Solutons:


If the file(s) in question contain really lots of data sending the signal can actually get to cat before it finishes. What you really observe is the finite speed of your terminal – cat sends the data to the terminal and it takes some time for the terminal to display all of it.

Remember, that usually it has to somehow redraw the whole output window for each line of output (i.e. move the contents of the window one line up and print the next line at the bottom). While there are techniques and algorithms to make this faster than if it was done the straightforward way, it still takes some time.

Thus, if you want to get rid of the output as quickly as possible, hide the terminal window, because then usually no actual redrawing takes place. In graphical environment this can mean either minimising the window or switching to a different virtual desktop, on the Linux virtual console just switch to another one ((Ctrl +)Alt + Fx).

Also notice that if you ran this over a slow network link (SSH over a GSM connection for example), you would definitely see much less output before cat got killed by the signal, because the speed of the terminal wouldn’t be the bottleneck any more.

Terminal Setup

I reckon this is more to do with the way the terminal is set up, than with any buffering issue. Check the output of stty -a | grep intr, you should have intr = ^C; on the output line if CtrlC is enabled at the tty/pty. If it isn’t, you can use stty intr ^C to enable it. Add the line to your .tcshrc or .login to make it permanent (or delete the line that changes it in the first place!).

Failing CtrlC, you can also try sending SIGQUIT with Ctrl. If this doesn’t work, again check stty -a | grep quit to see if it is properly set up.

Terminal Emulator Setup

Also check the settings for your terminal emulator (if you are using one), it may be that there is a shortcut set up at this level (maybe for copy or something) and the CtrlC does not reach the pty level. A shortcut could also be set up somewhere else in your desktop environment or Window system.

A good test on Linux if you are using a terminal emulator is to switch to a Linux console (CtrlAltF1), login there and see if the same behaviour occurs. If it doesn’t then this suggests the problem lies with your Window system or terminal emulator.

It could be an issue with the delay between data being read from the pty device as suggested by peterph. But if this is the case and you do actually have to wait minutes for the data to be displayed, then surely the terminal emulator is buffering way too much data (or your PC is very slow). The answer would be to find a way to reduce that buffer size in your terminal emulator settings or use a different one.

Extra Tip

Something else worth adding; I usually end up in the runaway cat situation when if I accidentally cat a binary file. The other effect of this can be to screw up your terminal settings (if the binary data happens to match various terminal escape codes which it often does). If tput is installed (usually is by default), you can avoid having to restart with the following command:

tput reset

This is what tmux option c0-change-interval and c0-change-trigger designed for. You should use a screen manager for resumable session anyway.

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