Would like help with trap command in Terminal

Question

I'm learning about the trap command from my bash book. I tried out the little script they gave:
Code: 
trap "echo 'You hit control-C!' " INTwhile true; do     sleep 60done
But when I type control-c, the script just stops and the message is not displayed. I checked trap -l and saw that control-c is intr, but the same thing happened whet I tried that. Can anyone tell me what I'm missing?
I'm using terminal (BSD) in OS X Snow Leopard

Answer 1

I'm assuming that you are typing these commands in the terminal window and not initiating them thru a shell script. I'd like to suggest that you avoid infinite loops. Since you are bound to orphan processes. I'm going the use the external sleep command in my examples because it affords us some time to type CONTROL-C.
[bash-4.2.8]~ $ trap "echo '......You hit control-C'" INT; sleep 15
^C  
[bash-4.2.8]~ $
[bash-4.2.8]~ $ ^C......You hit control-C
The trap is set but Bash does not intercept the trap because sleep is running in the foreground. The interrupt signal is sent to the command sleep.
Let's try running sleep in the background ->
[bash-4.2.8]~ $ trap 2 #Set SIGINT to it's default
[bash-4.2.8]~ $ trap "echo '......You hit control-C'" INT; sleep 15&
[1] 368
[bash-4.2.8]~ $ ^C......You hit control-C
[bash-4.2.8]~ $ ^C......You hit control-C
[1]+  Done                    sleep 15
[bash-4.2.8]~ $ ^C......You hit control-C
It works! Well, almost. You can't send the interrupt signal to a background process. In order for Bash to intercept the trap it must stay in the foreground. This happens when you initiate a sub-shell via a shell script or by rapping a command in paretheses ( ) ->
[bash-4.2.8]~ $ trap 2  #Set SIGINT to it's default
[bash-4.2.8]~ $ (trap "echo '......You hit control-C'" INT; sleep 15) ^C......You hit control-C
[bash-4.2.8]~ $
[bash-4.2.8]~ $ ^C
The trap works and the interrupt signal was sent to sleep. Notice, the trap is only set in the sub-shell.
Bash behaves differently when you use the built-in commands since you are working within the shell. I'll use an example similar to one of yours, except that you can break out of the loop by typing the number 10 or higher.
[bash-4.2.8]~ $ trap "echo '.......You hit control-C'" INT ; x=0; while (( $x < 10 )); do echo "Enter a number"; read x; done
Enter a number
3
Enter a number
^C.......You hit control-C
3
Enter a number
10
[bash-4.2.8]~ $
[bash-4.2.8]~ $ ^C......You hit control-C
[bash-4.2.8]~ $ trap 2  #Set SIGINT to it's default
As you can see the trap works. I'm going to let you sort out what happens after I typed  CONTROL -C in the last example.....:-)  Hope this helps.
Message was edited by: Mark Jalbert