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This has been the easiest way for me to create thumbnails from flv files with ffmpeg in a php envirnment

$ffmpegpath = "/usr/bin/ffmpeg";

	function make_jpg($input, $output, $fromdurasec="01") {
		global $ffmpegpath;
		if(!file_exists($input)) return false;

		$command = "$ffmpegpath -i $input -an -ss 00:00:
                        $fromdurasec -r 1 -vframes 1 -f mjpeg -y $output";

		@exec( $command, $ret );
		if(!file_exists($output)) return false;
		if(filesize($output)==0) return false;
		return true;
	}

Then use

#extract frame at 00:00:01 as jpeg
make_jpg("/test.flv", "/test.jpg", "01");

As a system admin you will run into times when you create a script that you want to run every so often and the best way to achive this is to set up a cron job.  A cron job is run the system and once you create the cron job, it will run until you remove it.

Today I needed a cron job to run every 6 hours to flush the mysql servers cache.

To achive this you can create a script like

#!/bin/bash
mysql -u root -pyour_root_password -e "flush query cache";

Then put that into the cron to run every so often, my case 6 hours

0 */6 * * * /location-to-script/script.sh

or you can just create a cron job like

0 */6 * * * mysql -u root -pyour_root_password -e "flush query cache";

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