bash ctrl+c tarp and bash read with timeout !?!

bull81 · 01-24-2010, 08:38 AM

Hi all

I always find answers on this forum but this problem is just wrecking my head!

Please check this simple bash code:

Code:

#!/bin/bash
trap "echo 'you got me'" SIGINT SIGTERM        # to trap ctrl+c
echo "Press ctrl+c during 5 sec loop"
for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

read -t 1 -s tmp_with_timeout
echo "after timeout read, now press again ctrl+c during 5 sec loop"

for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

echo "end"

Output looks like this:

Code:

Press ctrl+c during 5 sec loop
0
1
you got me
2
3
4
after timeout read, now press again ctrl+c during 5 sec loop
0
1
2
you got me
end

And my question: How come code behaves normally and stops when ctrl+c signal is caught and resumes, but after I use at least one timeout read in the code it looks like, if signal is caught again it doesn't pause the execution but skips the loop.
If you remove -t (timeout) option from the read, both loops look the same!

Example is very simple to demonstrate the problem. It took me few hours to finally get it from over 2500 lines of code and guys you're my last hope!

Please help!

kbp · 01-24-2010, 06:05 PM

Try changing the line:

Code:

trap "echo 'you got me'" SIGINT SIGTERM        # to trap ctrl+c

to:

Code:

trap "echo 'you got me'; exit" SIGINT SIGTERM        # to trap ctrl+c

or if you need expand functionality:

Code:

function mytrap
{
echo 'you got me'
exit
}

trap mytrap SIGINT SIGTERM        # to trap ctrl+c

hth

bull81 · 01-25-2010, 05:44 AM

kbp: Thanks for the repla

If I just want to control exit all is good, but what I want to do is after ctrl +c ask user do you want to exit [yes/no]:
If user say no and I want to continue with script, I can't because as you can see from second loop, script doesn't pause on ctrl+c signal after timeout read. Everything works perfect till I use timeout read. For some reason after that read command script don't pause anymore on signals.
I am not that good with strace so I was hoping that one of the programming gurus can explain that to me.

catkin · 01-25-2010, 08:46 AM

The lm-sensors' fancontrol script's main loop may hold the solution to this issue:

Code:

# main loop calling the main function at specified intervals
while true
do
    UpdateFanSpeeds
    # Sleep while still handling signals
    sleep $INTERVAL &
    wait $!
done

kbp · 01-25-2010, 08:46 AM

Here's an updated function:

Code:

function mytrap
{
echo 'you got me'
read -p 'would you like to exit ? y/n ' ANS
if [ "$ANS" = "y" ]
then
exit
fi
}

.. seemed to test ok

Issue with timeout still occurs though... strange one

I was just reading the man page... the error may be occurring because 'read -t 1 ... ' actually returns with an error code > 128 if the timout is exceeded, I replaced the 'read -t 1' call with a 'sleep 1' and the handler works as expected

Code:

#!/bin/bash

function mytrap
{
echo 'you got me'
read -p 'would you like to exit ? y/n ' ANS
if [ "$ANS" = "y" ]
then
exit
else
return 0
fi
}

trap mytrap SIGINT SIGTERM        # to trap ctrl+c
echo "Press ctrl+c during 5 sec loop"
for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

#read -t 1 -s tmp_with_timeout
sleep 1
echo "after timeout read, now press again ctrl+c during 5 sec loop"

for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

echo "end"

cheers

bull81 · 01-25-2010, 08:55 AM

kbp:
Exactly what I have in my code! Now, with your function, try to ctrl+c in first loop answer n and then in the second loop and answer n! See the difference?! In first loop it pause and then resume after you answer n, in second loop, after timeout read it just exit the loop even if you don't answer 'y' to exit! ITS THE SAME CODE and yet it behaves differently if I use read with -t! I have no idea why!

For those who wants to try:

Code:

#!/bin/bash
trap mytrap SIGINT SIGTERM        # to trap ctrl+c

function mytrap
{
   echo 'you got me'
   read -p 'would you like to exit ? y/n ' ANS
   if [ "$ANS" = "y" ]
   then
   exit
   fi
}

echo "Press ctrl+c during 5 sec loop"
for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

read -t 1 -s tmp_with_timeout
echo "after timeout read, now press again ctrl+c during 5 sec loop"

for ((i=0;i<5;i++)); do
        echo $i
        sleep 1
done

kbp · 01-25-2010, 09:12 AM

Sorry... I edited my last post, see above

cheers

bull81 · 01-25-2010, 09:28 AM

kbp: Oh yes I know it works, because you comment out timeout read...now in your code please uncomment
#read -t 1 -s tmp_with_timeout
to have timeout read and check again, you will see what i mean

ps. I need timeout reads in code to avoid pauses if user is not watching.

kbp · 01-25-2010, 09:32 AM

I know it fails... that's what I was saying, if 'read -t 1 ...' doesn't receive any input before the timeout period, its return code is non-zero ( greater than 128 ) .... maybe this is the reason that the traps are handled differently

cheers

bull81 · 01-25-2010, 09:46 AM

Sorry I missed that. Let me dig a lttle....

Cheers!

catkin · 01-25-2010, 09:56 AM

Quote:

Originally Posted by bull81

Sorry I missed that. Let me dig a lttle....

Cheers!

And please try the technique in this post above which may have been overlooked in the flurry of replies.