The next part describes a problem where you would want to do a search on a file system to find all directories in it, and to start a backup session per directory found, but not more than 20 backup sessions at once. Usually you would use the "find" command to find those directories, with the "-exec" parameter to execute the backup command. But in this case, it would result in possibly more than 20 active backup sessions at once, which might overload the system.

So, you can create a script that does a "find" and dumps the output to a file first, and then starts reading that file and initiating 20 backups in parallel. But then, the backup can't start, before the "find" command completes, which may take quite a long time, especially if run on a file system with a large number of files. So how do you do "find" commands and backups in parallel? Solve this problem with a pipeline.

Create a pipeline:

# rm -f /tmp/pipe
# mknod /tmp/pipe p

# find [/filesystem] -type d -exec echo {} \; > /tmp/pipe

cat /tmp/pipe | while read entry
do
   # Wait until less than 20 backup sessions are active
   while [ $(jobs -p|wc -l|awk '{print $1}') -ge 20 ]
   do
      sleep 5
   done

   # start backup session in the background
   [backup-command] &
   echo Started backup of $entry at `date`
done
# wait for all backup sessions to end
wait
echo `date`: Backup complete

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