Control-Key Signals

When you type CTRL-C, you tell the shell to send the INT (for "interrupt") signal to the current job; CTRL-Z sends TSTP (on most systems, for "terminal stop"). You can also send the current job a QUIT signal by typing CTRL-\ (control-backslash); this is sort of like a "stronger" version of CTRL-C.[9] You would normally use CTRL-\ when (and only when) CTRL-C doesn't work.

As we'll see soon, there is also a "panic" signal called KILL that you can send to a process when even CTRL-\ doesn't work. But it isn't attached to any control key, which means that you can't use it to stop the currently running process. INT, TSTP, and QUIT are the only signals you can use with control keys.[10]

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You can customize the control keys used to send signals with options of the stty command. These vary from system to system—consult your manpage for the command—but the usual syntax is stty signame char. signame is a name for the signal that, unfortunately, is often not the same as the names we use here. Table 1-7 in Chapter 1 lists stty names for signals found on all versions of UNIX. char is the control character, which you can give using the convention that ^(circumflex) represents "control." For example, to set your INT key to CTRL-X on most systems, use:

stty intr ^X

Now that we've told you how to do this, we should add that we don't recommend it. Changing your signal keys could lead to trouble if someone else has to stop a runaway process on your machine.

Most of the other signals are used by the operating system to advise processes of error conditions, like a bad machine code instruction, bad memory address, or division by zero, or "interesting" events such as a timer ("alarm") going off. The remaining signals are used for esoteric error conditions of interest only to low-level systems programmers; newer versions of UNIX have even more signal types.