Chapter 29. Debugging

#!/bin/bash
# parent.sh
# Running multiple processes on an SMP box.
# Author: Tedman Eng
# This is the first of two scripts,
#+ both of which must be present in the current working directory.
LIMIT=1ドル # Total number of process to start
NUMPROC=4 # Number of concurrent threads (forks?)
PROCID=1 # Starting Process ID
echo "My PID is $$"
function start_thread() {
 if [ $PROCID -le $LIMIT ] ; then
 ./child.sh $PROCID&
 let "PROCID++"
 else
 echo "Limit reached."
 wait
 exit
 fi
}
while [ "$NUMPROC" -gt 0 ]; do
 start_thread;
 let "NUMPROC--"
done
while true
do
trap "start_thread" SIGRTMIN
done
exit 0
# ======== Second script follows ========
#!/bin/bash
# child.sh
# Running multiple processes on an SMP box.
# This script is called by parent.sh.
# Author: Tedman Eng
temp=$RANDOM
index=1ドル
shift
let "temp %= 5"
let "temp += 4"
echo "Starting $index Time:$temp" "$@"
sleep ${temp}
echo "Ending $index"
kill -s SIGRTMIN $PPID
exit 0
# ======================= SCRIPT AUTHOR'S NOTES ======================= #
# It's not completely bug free.
# I ran it with limit = 500 and after the first few hundred iterations,
#+ one of the concurrent threads disappeared!
# Not sure if this is collisions from trap signals or something else.
# Once the trap is received, there's a brief moment while executing the
#+ trap handler but before the next trap is set. During this time, it may
#+ be possible to miss a trap signal, thus miss spawning a child process.
# No doubt someone may spot the bug and will be writing 
#+ . . . in the future.
# ===================================================================== #
# ----------------------------------------------------------------------#
#################################################################
# The following is the original script written by Vernia Damiano.
# Unfortunately, it doesn't work properly.
#################################################################
#!/bin/bash
# Must call script with at least one integer parameter
#+ (number of concurrent processes).
# All other parameters are passed through to the processes started.
INDICE=8 # Total number of process to start
TEMPO=5 # Maximum sleep time per process
E_BADARGS=65 # No arg(s) passed to script.
if [ $# -eq 0 ] # Check for at least one argument passed to script.
then
 echo "Usage: `basename 0ドル` number_of_processes [passed params]"
 exit $E_BADARGS
fi
NUMPROC=1ドル # Number of concurrent process
shift
PARAMETRI=( "$@" ) # Parameters of each process
function avvia() {
 local temp
 local index
 temp=$RANDOM
 index=1ドル
 shift
 let "temp %= $TEMPO"
 let "temp += 1"
 echo "Starting $index Time:$temp" "$@"
 sleep ${temp}
 echo "Ending $index"
 kill -s SIGRTMIN $$
}
function parti() {
 if [ $INDICE -gt 0 ] ; then
 avvia $INDICE "${PARAMETRI[@]}" &
 let "INDICE--"
 else
 trap : SIGRTMIN
 fi
}
trap parti SIGRTMIN
while [ "$NUMPROC" -gt 0 ]; do
 parti;
 let "NUMPROC--"
done
wait
trap - SIGRTMIN
exit $?
: <<SCRIPT_AUTHOR_COMMENTS
I had the need to run a program, with specified options, on a number of
different files, using a SMP machine. So I thought [I'd] keep running
a specified number of processes and start a new one each time . . . one
of these terminates.
The "wait" instruction does not help, since it waits for a given process
or *all* process started in background. So I wrote [this] bash script
that can do the job, using the "trap" instruction.
 --Vernia Damiano
SCRIPT_AUTHOR_COMMENTS

trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the remainder of the script. trap SIGNAL restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a script from an undesirable interrupt.