Si usted necesita algunas veces o siempre ciertos programas o scripts, este código comprueba si el programa o script se ha muerto! Momento en que volverá a ejecutarlo de nuevo.

Para estar seguros de esta función, basta con añadir el handler de bloqueo siguiente en la parte superior de la secuencia de comandos antes de la funcionalidad del script principal):

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#!/bin/bash
######### Lock checker ###########
myName="`echo $0 | awk '{print $NF}' FS='/'`"
lockDir="/var/lock/"
lockFile=$lockDir$myName.pid
currentPID=$$
oldPID="`cat $lockFile`"
oldderExist=` kill -0 $oldPID 2>/dev/null ; echo $? `
if [ "$oldderExist" == "0" ]; then echo "An Other Instance Is Running...! PID:$oldPID" ; exit 
 else echo $currentPID > $lockFile
fi
######### Lock checker ###########

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También, puedes ver este código...

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#!/bin/bash

# don't use pid's they can be doubled in theory
# Under *nix, process IDs are usually allocated on a sequential basis,
# beginning at 0 and rising to a maximum value which varies from system to system.
# -- see: cat /proc/sys/kernel/pid_max
# Once this limit is reached, allocation restarts at 300 and again increases.
# meanwhile if another proccess taken your stored PID-ID you get a double but 
# not the assumed proccess you where looking for!

# use the kernel random generator function instead, the odds of creating
# a few tens of trillions of UUIDs in a year and having one duplicate
uuidkey=`cat /proc/sys/kernel/random/uuid`

# better use /tmp for lockfiles, gets cleaned after reboot if tmpfs is used... 
lockfile=/tmp/${0##*/}.lock

if test -f "$lockfile"
 then
 olduuid=`cat $lockfile |awk '{print $2}'`
 if [ "$olduuid" == "$uuidkey" ]
 then
 # LoL, this can never be right, one chance in a ...
 echo "This can't be right!, the same UUID's twice"
 echo "new: $uuidkey"
 echo "old: $olduuid"
 else
 gettimestamp=`cat $lockfile |awk '{print $1}'`
 echo "There is a lockfile, created at: `date -d@$gettimestamp`"
 exit 1
 fi
 else
 echo "`date +%s` $uuidkey" > $lockfile 
fi
# got here, put the rest of the script below...

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