If you have ever wanted an easy way to execute multiple jobs in parallel in bash, then this is the snippet for you. This was originally posted on Stack Exchange. It has been modified a bit.

<code>#!/bin/bash

#how many jobs to run at one time
JOBS_AT_ONCE=20

# The bgxupdate and bgxlimit functions below allow for
# running X jobs in parallel in bash.  They are taken from:
# http://stackoverflow.com/questions/1537956/bash-limit-the-number-of-concurrent-jobs/1685440#1685440

# bgxupdate - update active processes in a group.
#   Works by transferring each process to new group
#   if it is still active.
# in:  bgxgrp - current group of processes.
# out: bgxgrp - new group of processes.
# out: bgxcount - number of processes in new group.

bgxupdate() {
    bgxoldgrp=${bgxgrp}
    bgxgrp=""
    ((bgxcount = 0))
    bgxjobs=" $(jobs -pr | tr '\n' ' ')"
    for bgxpid in ${bgxoldgrp} ; do
        echo "${bgxjobs}" | grep " ${bgxpid} " >/dev/null 2>&1
        if [[ $? -eq 0 ]] ; then
            bgxgrp="${bgxgrp} ${bgxpid}"
            ((bgxcount = bgxcount + 1))
        fi
    done
}

# bgxlimit - start a sub-process with a limit.

#   Loops, calling bgxupdate until there is a free
#   slot to run another sub-process. Then runs it
#   an updates the process group.
# in:  $1     - the limit on processes.
# in:  $2+    - the command to run for new process.
# in:  bgxgrp - the current group of processes.
# out: bgxgrp - new group of processes

bgxlimit() {
    bgxmax=$1 ; shift
    bgxupdate
    while [[ ${bgxcount} -ge ${bgxmax} ]] ; do
        sleep 1
        bgxupdate
    done
    if [[ "$1" != "-" ]] ; then
        $* &
        bgxgrp="${bgxgrp} $!"
    fi
}

bgxgrp="process_group_1"
for LINE in `cat hosts`
do
    CHECK_SCRIPT='echo $(hostname),$(cat /etc/debian_version)'
    bgxlimit $JOBS_AT_ONCE ssh ${LINE} "${CHECK_SCRIPT}"
done
# Wait until all queued processes are done.

bgxupdate
while [[ ${bgxcount} -ne 0 ]] ; do
    oldcount=${bgxcount}
    while [[ ${oldcount} -eq ${bgxcount} ]] ; do
        sleep 1
        bgxupdate
    done
done
</code>

In this script the primary changes are defining the max number of simultaneous jobs, as well as doing somewhat useful work in returning the hostname and the debian version.