On RedHat and family, crontab entries are best made as a separate file under any of the /etc/cron.* directories.
If it's enough that your program runs once every hour, day or month, just place a script or symlink in the respectively cron.hourly, cron.daily or cron.monthly directory.
If you need more control, create a file with a full crontab line in
/etc/cron.d.
A nice way to see whether your crontab entry was correct and was executed properly, is to use logger to write a message to the system log file afterwards. Your crontab entry would then look something like this:
30 15 * * * root thecommand && \
logger "thecommand executed successfully"This can be removed afterwards but it's a help for the first runs.
To run a crontab as a different user, use the su command. An example where the crontab entry is run as user apache, every five minutes:
*/5 * * * * root su -c '/usr/bin/some_command' \
-s /bin/sh apache