Running a cron job on Linux every six hours

0 */6 * * * command

This will be the perfect way to say 6 hours a day.

Your command puts in for six minutes!

You should include a path to your command, since cron runs with an extensively cut-down environment. You won't have all the environment variables you have in your interactive shell session.

It's a good idea to specify an absolute path to your script/binary, or define PATH in the crontab itself. To help debug any issues I would also redirect stdout/err to a log file.

You forgot a *, and you've too many fields. It's the hour you need to care about

0 */6 * * * /path/to/mycommand

This means every sixth hour starting from 0, i.e. at hour 0, 6, 12 and 18 which you could write as

0 0,6,12,18 * * * /path/to/mycommand