Crontab every 30 minutes is generated and shown below, please copy & paste it to your crontab

minute(s) hour(s) day(s) month(s) weekday(s)

Your next cron job scheduled at:
2017-10-24 00:30:00 UTC
2017-10-24 01:00:00 UTC
2017-10-24 01:30:00 UTC
2017-10-24 02:00:00 UTC
2017-10-24 02:30:00 UTC
.....

Generate New Crontab
minute(s) hour(s) day(s) month(s) weekday(s) command(s)
CategoryValueDescription
minute0-59This controls what minute of the hour the command will run on.
hour0-23This controls what hour the command will run on.
day1-31This is the Day of Month, that you want the command run on.
month1-12This is the month a specified command will run on.
weekday0-6This is the Day of Week that you want a command to be run on.
userrootThis is the user who runs the command.
command/cmdA valid shell syntax code, executables or combinations of both