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Monday, August 10, 2009

Posted by venu k
10 comments | 8:57 AM
#Locking a terminal using a shell script.
#This script locks your terminal until you enter correct password (open2world)
#It also traps signals and interrupts
#You can't terminate the script by Ctrl+c or Ctrl+\ or Ctrl+z and Ctrl+d

trap "" 1 2 3 20
lines=`tput lines`
b=`expr $lines / 2 - 4 `

## center function

columns=`tput cols`
until [ -z "$1" ]
for ((i=1;i<$(((columns-${#1})/2));i++)) do echo -n " " done echo -e "\033[1m \033[5m \033[42m $1 \033[0m " shift done } ## End of center function while true do clear tput cup $b 0
center "Press any key to unlock"
read key
echo -n "enter password : "
read -s password
while true
if [ $password = "open2world" ]
break 2
# breaks second outer loop
echo -e "\n You are an illegal user "
echo -e "\n Enter any key"

Analysis :

The trap command allows you to execute a command when a signal is received by your script. It works like this:

trap arg signals

"signals" is a list of signals to interrupt and "arg" is a command to execute when one of the signals is received. If "arg" is not supplied script doesn't do any thing after receiving signal.

Note: Two signals "SIGKILL" "SIGSTOP" are unable to trap

Ex: trap "echo signal received" 1 2 3 15
(or use SIGHUP SIGINT SIGQUIT SIGTERM instead of 1 2 3 15 )

What is signal:

A signal is a message which can be sent to a running process. Some times called
software interrupts.

Some of signals and its value:

       Signal             Value     Action   Comment
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT (Ctrl+c) 2 Term Interrupt from keyboard
SIGQUIT (Ctrl+\) 3 Core Quit from keyboard
SIGTERM 15 Term Termination signal
SIGTSTP (Ctrl+z) 20 Stop Stop typed at tty

Note: There are different standards(POSIX,SUSv) to determine signals and its values.
I tested Above signals in Fedora 9, Other systems may support them or not depending on system standard(I think almost all standards supports above signals and values).

More stuff:

I used "tput cols" and "tput lines" commands to collect number of columns and lines.
You can also directly use bash variables $LINES ,$COLUMNS to get columns and lines.But
you should to run your script in current shell only. If you run your script in sub shell
current shell will not export $LINES and $COLUMNS variable values.


  1. @anonymous: It seems just copy/paste formatting issues

    # Insert breaks after
    ## End of center function
    while true
    clear tput cup $b 0

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