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I need to a crontab execute each 12 hours, so I have the following:
0 */12 * * * . /X.sh
then Linux is installing new crontab. after that, I am waiting for 2 hours and still the cron job didn't start?!
When has it to start the job?
The crontab entry you've written is equivalent to
0 0-23/12 * * * . /X.sh
This requests execution on the hour between midnight and 11pm, using 12-hour steps — so cron will run the job every day at midnight and noon.
0 10,22 * * *
Required, but never shown