Unix Tip: Using stty to Your Advantage

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The stty (set the options for a terminal) command can be very useful once you get the knack of using it. Stty can give you quick manual control over the characteristics of your terminal. If ever you have found yourself working on a terminal in which the terminal settings did not match your actual terminal, you will know how annoying this situation can be. It is often experienced when you attempt to backspace to fix a typo on the command line and, instead of seeing the cursor move backwards on the line erasing characters, you see an increasingly long string of ^H or ^? characters appearing at the end of your command line.

boson> echo "Please send me the lsit^H^H^H

What this tells you is that ^H (control-H) is not functioning as an erase. Using stty, however, you can reset your erase character like this:

boson> stty erase \^h

To see a list of your current stty settings, type the following command:

stty -a

This command will list your current settings in a format like this:

boson> stty -a

speed 38400 baud;

rows = 80; columns = 132; ypixels = 0; xpixels = 0;

csdata ?

eucw 1:0:0:0, scrw 1:0:0:0

intr = ^c; quit = ^\; erase = ^h; kill = ^u;

eof = ^d; eol = <<undef> eol2 = <undef> swtch = <undef>

start = ^q; stop = ^s; susp = ^z; dsusp = ^y;

rprnt = ^r; flush = ^o; werase = ^w; lnext = ^v;

-parenb -parodd cs8 -cstopb -hupcl cread -clocal -loblk -crtscts -crtsxoff -parext

-ignbrk brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -iuclc

ixon -ixany -ixoff imaxbel

isig icanon -xcase echo echoe echok -echonl -noflsh

-tostop echoctl -echoprt echoke -defecho -flusho -pendin iexten

opost -olcuc onlcr -ocrnl -onocr -onlret -ofill -ofdel tab3

The standard control characters are:

control-d end of file (eof)

control-h backspace (erase)

control-z suspend (susp)

control-u delete text from start of line (kill)

control-c interrupt

The most commonly used stty command is undoubtedly the "stty erase ^h" command which is often used in scripts

if [ `tty | grep -ci console` -eq 0 ]
then
	stty ERASE ^H
	TERM=SUN
fi

You can also display your settings in a format which allows you to restore them at a later time. This is done with the "stty -g" command, the output of which (on one of my Solaris systems) looks like this:

boson> stty -g

2502:1805:f00bf:8a3b:17:1c:7f:15:4:0:0:0:11:13:1a:19:12:f:17:16:0:0:1:1:0:00:0

:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0

If you want to save and restore your terminal settings, you can use the output of the stty -g command to create a command that will do this for you. First, let's examine what the command would look like:

boson> echo stty `stty -g`

stty 2502:1805:f00bf:8a3b:3:1c:f:15:4:0:0:0:11:13:1a:19:12:f:17:16:0:0:1:1:0:00

:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0

The "stty 2502:1805 ..." command can be used to restore the current settings at a later time. So, let's store that command in a file and make it work as a script:

boson> echo stty `stty -g` > restore-tty; chmod 755 restore-tty

Now, let's test the script by first resetting our erase character from ^H to ^A and then restoring it back to its previous setting using our script.

boson> stty erase \^a

boson> echo This is a mess^H^H^H

This is a mess

Obviously, control-H isn't working any longer as a backspace character, though control-A should work just fine. To restore the previous settings, we invoke our script:

boson> ./restore-tty

Control-H should now work as before.

If you want to change your stty settings in a script and then restore them, you don't have to build a script to preserve the initial settings. You can save your settings in a variable and then use the stty command to restore from that:

oldSettings=`stty -g`			# save stty settings
stty -echo raw
<other commands>
stty $oldSettings 			# restore stty settings
Other stty settings allow you to change the usable size of your terminal. For example, if you need a narrower display, you could set your terminal width to be narrower by setting your columns from the normal 80 to something more narrow:

stty columns 60

If you list your files after running this command, you will note the changes in your display.

boson> ls /etc
SnmpAgent.d            labelit                rc1.d
TIMEZONE               lib                    rc2
TIMEZONE.temp          link                   rc2.d
acct                   llc2                   rc3
aliases                log                    rc3.d
becomes this:

boson> ls /etc
SnmpAgent.d            nodename
TIMEZONE               nscd.conf
TIMEZONE.temp          nsswitch.conf
acct                   nsswitch.dns
aliases                nsswitch.files
If you have ever worked in a terminal in which you noticed data scrolling off the top of your screen when piped to the more or the less command, you have probably suspected that your terminal settings were wrong. Whenever you see this behavior, you should look at your stty rows variable; it is likely to be set too high.

Similarly, if you notice that the lines in your display wrap around the edge of your screen, your columns setting may be too large.

The stty command can also come in handy for scripts in which you wish to hide text that is entered by a user. By using the stty echo and -echo commands, you can turn input echoing on and off as you see fit. If you want to prompt a user to enter a password, for example, you can do something like this:

echo -n "Enter you very secret password: "

stty -echo

read password

stty echo

The "stty -echo" command turns off input echoing and the "stty echo" command turns it back on again.

You can then proceed with the remainder of the script knowing that the password was not displayed on the screen where someone who should not know it might have noticed.  

This story, "Unix Tip: Using stty to Your Advantage" was originally published by ITworld.

Copyright © 2006 IDG Communications, Inc.

  
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