Per crontab man page:
Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly.
Files under /var/spool are considered temporary/working and it's always a good practice to back up your cron entries or keep them in a file in your home directory.
I assume you're using crontab -e to create crontab files on the fly. If so, you can get a "copy" of your crontab file by doing crontab -l. Pipe that to a file to get a "backup":
crontab -l > my-crontab
Then you can edit that my-crontab file to add or modify entries, and then "install" it by giving it to crontab:
crontab my-crontab
This does the same syntax checking as crontab -e.
To list all cron jobs from all users in your system:
for user in $(cut -f1 -d: /etc/passwd)
do
echo $user
crontab -u $user -l
done
or
for user in $(cut -f1 -d: /etc/passwd)
do
echo $user
crontab -l $user
done
