UNIX for Dummies Questions & Answers

Hii ,
i would like to know how to finish the program...
the pgm given below works but it doesn't satisfy the condition that it should not terminate process when yu press ctrl-D , or ctrl-Z .
how to use trap command for this??
please help....

Q.Shell scrip to implement terminal locking .It should prompt the user for a password. After accepting the password entered by the user, it must prompt again for password confirmation(to retype password).If a match occurs,it must lock the terminal and prompt for the password.if the proper password is entered,the terminal must be unlocked. Note the script must be written to disregard BREAK,control -D etc. No time limit need to be implemented for the lock duration.

echo "\nEnter the password:\c"
stty -echo
read pass1
echo "\nRetypr password"
read pass2
if [ $pass1 = $pass2 ] ; then
tput clear
echo "\nThe terminal is locked\n"
echo "\nEnter password to unlock:"
read pass3
while [ $pass1 != $pass3 ]
do
echo "\nEnter password to unlock"
read pass3
done
else
echo "\nTerminal could not be locked"
fi
stty echo

You put a line like:

Code:

trap "echo Signal Caught" 0 1 2 3 6 15 20

At the top of your script, the signal number for <ctrl>+<z> is 20, <ctrl>+<d> seems to just cause the read command to see an "enter" and return an empty string which simply cause your script to prompt the user to re-enter the password.
What you do in response to the trap signal is of course up to you, having just "" so that nothing happens my be what you want.
On my Ubuntu install the signal numbers and their meanings are in /usr/include/bits/signum.h.

thanks... it doesnt terminate when i enter <ctrl>+<z> ... but when i enter <ctrl>+<d> it terminates with this msg
"[: 21: !=: argument expected"
so which number should i add for trp so that it doesnt come out when i enter ctrl+d...??

Code:

anusha@anusha-desktop:~$ sh 7a.sh

Enter the password:
Retypr password
















The terminal is locked


Enter password to unlock:
[: 21: !=: argument expected

anusha@anusha-desktop:~$

I don't believe you can trap <ctrl>+<d>, but the solution is to allow your script to handle empty responses thus:
Change:

Code:

if [ $pass1 = $pass2 ] ; then

to

Code:

if [ "${pass1}" = "${pass2}" ] ; then

and change:

Code:

while [ $pass1 != $pass3 ]

to:

Code:

while [ "${pass1}" != "${pass3}" ]

Then if ${pass1} or ${pass2} or ${pass3} are empty they will be an empty string rather than a null value. The braces are not essential but are good scripting practice.

Locking a terminal is an interesting question which has bugged me too.

Ctrl-D is not a signal, it just closes stdin. It's also a function of the terminal, hence controlled by stty. I'm not sure it even can be disabled. Not even 'su -' can ignore ctrl-D and it's the one thing that can ignore everything else... IGNOREEOF=-1 will stop the terminal from quitting from it at least.

If you're changing the terminal settings it'd be wise to save/restore them since they're global to the terminal.

Code:

TTYSAVE=`stty --save`

...

stty $TTYSAVE

-----Post Update-----

Setting the EOF char to something impossible may work. 'stty eof 255'

Thank youTonyFullerMalv.... pgm is working now ....
thank you Corona688 for your information....