#!/bin/bash
# This is a sample program by Bryan Lee ( leebc11 at acm dot org ) that runs mul
tiple commands in parallel in bash.
# Distributed under GPL version 2
# Let me know if you find it useful

# Arguments should be specified on the initial command line.  They will be used
as the first argument for the parallel command one at a time.


#This changes how bash handles process notification.  It MUST be set.
set -bm

startprocess () {
        if [ $1 ]
        then
                echo Processing file $1
                xv -quit $1 1>/dev/null &       # This is the parallel command
                # OR
                #xview $1 &                     # Or this without IO redirection
        else
                echo "Out of files"
                echo "Waiting for remaining processes to finish"
                wait                    # This wait waits for everything else
                echo "All done...Bye, bye."
                exit 255
        fi
}

#This starts a new process each time a child exits
trap 'startprocess $1; shift'   SIGCHLD



echo Starting with $# arguments

#Have one of these for each parallel process you want running
startprocess $1;        shift
startprocess $1;        shift
startprocess $1;        shift
startprocess $1;        shift

wait

exit

###############
# How it works
#
# When a child process exits, it sends a signal back to the parent, SIGCHLD.
# The "trap" command catches this signal and calls the startProcessing
# function with the next filename.  The function checks to see that it
# was passed anything, and waits, then exits if it was not passed anything.
# I use built in shell functionality to process the arguments.  The first
# argument to the script is $1, then $2, then $3....  The "shift" command
# shifts these arguments, $2 becomes $1, $3 becomes $2, $4 becomes $3, etc.
# This is the typical method for processing arguments in a script.  Each
# time the startProcessing function is called, you have to shift.
# The "set -bm" changes shell behavior.
# The "wait" command waits for child processes to finish before
# proceeding.  The wait in the function before the exit (which exits the
# entire script) makes sure the children finish before exiting.
#
# If you don't give it enough arguments initially, you'll get two "Out of
# files" messages, and the whole thing will probably fail if there's any
# whitespace in the arguments.