#! /bin/sh

# Example to show signal handling

for (( i = 1; i < 32; ++i ))
do
    trap "echo caught signal $i; exit 1" $i
done

echo My process ID is $$
echo "Now copy and paste the following into another window:"
echo 'for ((i=0;i < 32; ++i));do if ((i==9 || i==19));then let ++i;fi;echo Sending signal $i:;kill -$i '$$';sleep 1;done'
echo Then try pasting this code, and understand why this process terminates:
echo 'for ((i=0;i < 32; ++i));do echo Sending signal $i:;kill -$i '$$';sleep 1;done'

# So how can we terminate it?

# Note:
# The colon `:' is the null statement that can be used wherever the
# syntax requires a statement.  It returns the value 0 (i.e., true).
# In this case, it gives us an infinite loop.

# Try:
# $ help :

while :
do
    sleep 1
done

# Try running this in one window, and in another window, type this:
# for ((i=0;i < 32; ++i));do if ((i==9 || i==19));then let ++i;fi;echo Sending signal $i:;kill -$i 21306;sleep 1;done
# Of course, instead of 21306, put the process ID of this process.

# Then try removing the "if ((i==9 || i==19));then let ++i;fi;" statement,
# and run it again.

# Question 6:

# Download the script
# http://ictlab.tyict.vtc.edu.hk/ossi/lectures/shell/bin/trapall.
# Make it executable and run it.  Send the executing process all
# signals in the range 1..31 as described in the existing comments
# in the file.  Modify it so that it will still print the message, but
# it will exit when it receives any signal.  In the modified program,
# add comments that:
# o explain how you could terminate the original program;
# o list all signals in the range 1..31 that the original program could
#   not trap; 
# o explain why the signals you list cannot be trapped by the original
#   program.

# Nick's answers:

# 1. I saw the process ID of the trapall process, and in another window,
#    typed:
#    $ kill -9 process_id
#    or
#    $ kill -KILL process_id
#    where process_id is the PID of the trapall process.

# 2. Only signals 9 (SIGKILL) and 19 (SIGSTOP) could not be handled

# 3. The operating system ensures that these two processes cannot be
#    handled by a process.  The KILL signal ensures that the system
#    administrator always has a way to terminate a process.  The STOP
#    signal ensures that debuggers can always single step through a
#    program, and that a system administrator may "freeze" a process and
#    examine the process at leisure.  This may be useful to examine a
#    process started by a cracker who has intruded into a system and has
#    run a program that needs to be examined to understand what damage
#    the cracker is trying to achieve.