UNIX for Advanced & Expert Users

#!/usr/bin/ksh

pid_line=$$_$1
queue_file=/tmp/queue_file

# sleep for no reason for a random (5) seconds
sleep $(echo $$%5|bc)

# make your entry into queue at the end
 echo $pid_line >> $queue_file

# while you are not the top most job loop
 while [[ $(grep -n $pid_line $queue_file | cut -d: -f1 ) -ne 1 ]]
 do

# if by any rare chance someone removed you from the queue, join back
 if [[ $(grep $pid_line $queue_file | wc -l) -eq 0 ]]
 then
     echo $pid_line >> $queue_file
 fi

# if the current top of the queue job is killed or terminated before it could remove itself, remove it
 curr_pid_line=$(head -1 $queue_file)
 if [ $(ps -ef | grep $(echo $curr_pid_line|cut -d_ -f1) | grep -v grep  | wc -l ) -eq 0 ]
 then
    sleep 1
    grep -Ev $curr_pid_line $queue_file | cat > $queue_file
 else
# if someother job is running, wait for sometime before attempting
  sleep 20
 fi
 done

# you are the first job now, run yourself now
 $1
 result=$?

# remove yourself from the queue
 grep -vE $pid_line $queue_file | cat > $queue_file

# exit with the exit code of the command that you ran
 exit $result

$ cp /etc/passwd testfile
$ grep -E abcd testfile | cat > testfile
$ cat testfile
abcd:x:100:100:ABCD,,,:/home/abcd:/bin/bash

echo $pid_line >> $queue_file

grep -vE $pid_line $queue_file | cat > $queue_file

#! /bin/ksh

typeset workdir=/path/to/dir
typeset myself=$$
typeset action="$1"    # the job gets passed from outside

touch ${workdir}/job.${myself}     # enqueue the job

while [ "$(ls -rt ${workdir} | head -1)" != "job.${myself}" ] ; do
     sleep 5     # wait until our job is the oldest in the directory,
                    # which means we are the first in queue
done

# now that we are the first one do the action:

$action   # do whatever the job is supposed to do

rm ${workdir}/job.${myself}  # remove ourselves from the queue

exit 0