A signal is a kind of "message" that a program can send to another program; an example can be the hit of the CRTL + C keys combination, which produces an INT signal, which usually stops the script execution.
"Masking" a signal in a script means associating to this signal a function to be executed when the signal is received.
In the bash the system signals can be masked through the
trap signal, according to the following syntax:
trap function_name signal_name
Here there's an example:
#!/bin/bash
# The following script shows the usage of the trap function.
# In the function three countdowns from 10 seconds are present.
# You can try to stop the countdowns by pressing CRTL-C
# (INT signal) and verifying the script behaviour.
# During the first countdown, the INT signal is masked by an
# error function: the arrival of this signal causes the script
# to display this error message.
# Then during the second countdown the signal INT is disabled:
# its reception causes no effect.
# Finally durign the third countdown the mask is removed:
# it is now possible to stop the count.
function error
{
echo "Sorry, no interruption allowed"
sleep 3
}
function start_count
{
for i in 10 9 8 7 6 5 4 3 2 1;
do
echo "$i seconds have passed"
sleep 1
done
}
# Main program
####### Count 1
trap error INT # Masking of the signal with an errror function
echo "Count 1:"
start_count
echo " "
####### Count 2
trap '' INT # Signal disabling
echo "Count 2:"
start_count
echo " "
####### Count 3
trap - INT # Mask deletion
echo "Count 3:"
start_count
