May 18, 2010 19:40 · May 18, 2010
diff --git a/4view.sh b/4view.sh
@@ -0,0 +1,66 @@
+#!/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.