Running a cron job at 2:30 AM everyday

You would have to run this as root, but:

for user in $(cut -f1 -d: /etc/passwd); do crontab -u $user -l; done

will loop over each user name listing out their crontab. The crontabs are owned by the respective users so you won't be able to see another user's crontab w/o being them or root.

Edit if you want to know which user a crontab belongs to, use echo $user

for user in $(cut -f1 -d: /etc/passwd); do echo $user; crontab -u $user -l; done

Use flock. It's new. It's better.

Now you don't have to write the code yourself. Check out more reasons here: https://serverfault.com/a/82863

/usr/bin/flock -n /tmp/my.lockfile /usr/local/bin/my_script