Watch file/folder for change and trigger an action

My use-case is that I have an upload folder which I'd like to reindex when something is uploaded/deleted. For this I watch the file size with du command.
It should be triggered via cron or systemd timer.

I run it with crond (on synology) twice a minute:

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1 2	* * * * * /volume1/bin/watch.sh /volume2/shared/upload * * * * * sleep 30; /volume1/bin/watch.sh /volume2/shared/upload

At the end of the script I run synoindex.

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#!/bin/sh ################################################## # Name:         watch.sh # Author:       krisko # Description:  Watch file/folder for change and trigger some action # Date:         20200109 ##################################################   usage() { cat << EOT Watch file/folder for change and trigger some action NOTE: this won't work if the change size is smaller than your blocksize (typically 4k)!   USAGE: $0 (OPTION) OPTIONS:     -h      Show this help     PATH    Absolute path to file/folder   EOT exit 0 }   # call usage if needed [[ ! -n $1 ]] || [[ $1 == "-h" ]] && usage   # file/folder to watch WATCH=$1 # logfile to store info LOG=/tmp/watch_${WATCH##*/}   # return current size getsize() {     du -s $1 | awk '{print $1}' }   [[ -f ${LOG}.pending ]] && { echo "INFO: another instance already waiting for change, exiting..."; exit 0; } # check if we have valid input [[ -f $WATCH ]] || [[ -d $WATCH ]] || { echo "ERROR: invalid file specified, use -h for help."; exit 1; } # create logfile if it does not exist [[ ! -f $LOG ]] && getsize $WATCH > $LOG || LOGGED=$(cat $LOG)   CURRENT=$(getsize $WATCH)   [[ $LOGGED -eq $CURRENT ]] && { echo "INFO: nothing changed"; exit 0; }   # when change detected, w8 for folder stabilization and proceed touch ${LOG}.pending while [[ $LOGGED -ne $CURRENT ]]; do     LOGGED=$CURRENT     sleep 2     CURRENT=$(getsize $WATCH) done rm ${LOG}.pending   # update log echo $CURRENT > $LOG   # run your favorite command synoindex -R $WATCH   exit 0   ################################################# # TODO/Changelog # Author # Description ################################################# # 1.0.0 20200109 # krisko # Genesis #################################################