Control Keys

Control keys—those that you type by holding down the CONTROL (or CTRL) key and hitting another key—are another type of special character. These normally don't print anything on your screen, but the operating system interprets a few of them as special commands. You already know one of them: RETURN is actually the same as CTRL-M (try it and see). You have probably also used the BACKSPACE or DEL key to erase typos on your command line.

Actually, many control keys have functions that don't really concern you—yet you should know about them for future reference and in case you type them by accident.

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Perhaps the most difficult thing about control keys is that they can differ from system to system. The usual arrangement is shown in Table 1-7, which lists the control keys that all major modern versions of UNIX support. Note that DEL and CTRL-? are the same character.

You can use the stty command to find out what your settings are and change them if you wish; see Chapter 8 for details. If the version of UNIX on your system is one of those that derive from BSD (such as SunOS and OS X), type stty all to see your control-key settings; you will see something like this:

erase  kill   werase rprnt  flush  lnext  susp   intr   quit   stop   eof
^?     ^U     ^W     ^R     ^O     ^V     ^Z/^Y  ^C     ^\     ^S/^Q  ^D

Table 1-7. Control keys

Control key

stty name

Function description

CTRL-C

intr

Stop current command

CTRL-D

eof

End of input

CTRL-\

quit

Stop current command if CTRL-C doesn't work

CTRL-S

stop

Halt output to screen

CTRL-Q

 

Restart output to screen

DEL or CTRL-?

erase

Erase last character

CTRL-U

kill

Erase entire command line

CTRL-Z

susp

Suspend current command (see Chapter 8)

The ^X notation stands for CTRL-X. If your UNIX version derives from System III or System V (this includes AIX, HP/UX, SCO, Linux, and Xenix), type stty -a.

The resulting output will include this information:

intr = ^c; quit = ^|; erase = DEL; kill = ^u; eof = ^d; eol = ^`;
swtch = ^`; susp = ^z; dsusp <undef>;

The control key you will probably use most often is CTRL-C, sometimes called the interrupt key. This stops—or tries to stop—the command that is currently running. You will want to use this when you enter a command and find that it's taking too long, you gave it the wrong arguments, you change your mind about wanting to run it, or whatever.

Sometimes CTRL-C doesn't work; in that case, if you really want to stop a job, try CTRL-\. But don't just type CTRL-\; always try CTRL-C first! Chapter 8 explains why in detail. For now, suffice it to say that CTRL-C gives the running job more of a chance to clean up before exiting, so that files and other resources are not left in funny states.

We've already seen an example of CTRL-D. When you are running a command that accepts standard input from your keyboard, CTRL-D tells the process that your input is finished—as if the process were reading a file and it reached the end of the file. mail is a utility in which this happens often. When you are typing in a message, you end by typing CTRL-D. This tells mail that your message is complete and ready to be sent. Most utilities that accept standard input understand CTRL-D as the end-of-input character, though many such programs accept commands like q, quit, exit, etc.

CTRL-S and CTRL-Q are called flow-control characters. They represent an antiquated way of stopping and restarting the flow of output from one device to another (e.g., from the computer to your terminal) that was useful when the speed of such output was low. They are rather obsolete in these days of high-speed networks. In fact, under the latter conditions, CTRL-S and CTRL-Q are basically a nuisance. The only thing you really need to know about them is that if your screen output becomes "stuck," then you may have hit CTRL-S by accident. Type CTRL-Q to restart the output; any keys you may have hit in between will then take effect.

The final group of control characters gives you rudimentary ways to edit your command line. DEL acts as a backspace key (in fact, some systems use the actual BACKSPACE or CTRL-H key as "erase" instead of DEL); CTRL-U erases the entire line and lets you start over. Again, these have been superseded.[19] The next chapter will look at bash's editing modes, which are among its most useful features and far more powerful than the limited editing capabilities described here.

 

[18] This should also teach you something about the flexibility of placing I/O redirectors anywhere on the command line—even in places where they don't seem to make sense.

[19] Why are so many outmoded control keys still in use? They have nothing to do with the shell per se; instead, they are recognized by the tty driver, an old and hoary part of the operating system's lower depths that controls input and output to/from your terminal.