Trapping Signals of Terminal in Linux

It frequently happens that while programming either by designing or by accidently the control falls in an indefinite loop. There has to be a way of getting out of such indefinite loops. This facility is provided by the programming environment. Generally we use Ctrl + C to end a process, but this can be trapped using a UNIX shell command “trap”. There are signal numbers for various signals as shown in table below:

 

Table 1. Signals In Unix

We can see a basic shell code which is used to trap the signals 1, 2, 3 (from Table 1).

Example:

trap "" 1 2 3

figlet terminal

figlet locked

                      while true

                      do

                                  echo "Enter Password:"

                                  stty -echo

                                  read password

                                  stty sane

                                  if [ "$password" = "dinesh" ]

                                  then

                                          break

                                  else

                                 echo "Wrong Password, Please Enter Again: "

                                 fi

                      done

OUTPUT:

Figure 1. Output

In the above program we can see various commands such as,

1. figlet: Display large characters made up of ordinary screen characters.

2. stty: Change and print terminal line settings.

For more information about the commands use the manual pages.