Difference between Cron and Crontab?

Question

I am not able to understand the answer for this question: "What's the difference between cron and crontab." Are they both schedulers with one executing the files once and the other executing the files on a regular interval OR does cron schedule a job and crontab stores them in a table or file for execution?

Wiki page for Cron mentions :

Cron is driven by a crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule.

But wiki.dreamhost for crontab mentiones :

The crontab command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically. It reads a series of commands from standard input and collects them into a file known as a "crontab" which is later read and whose instructions are carried out.

Specifically, When I schedule a job to be repeated : (Quoting from wiki)

1 0 * * *  printf > /var/log/apache/error_log

or executing a job only once

at -f myScripts/call_show_fn.sh 1:55 2014-10-14

Am I doing a cron function in both the commands which is pushed in crontab OR is the first one a crontab and the second a cron function?

Answer 1

cron is the general name for the service that runs scheduled actions. crond is the name of the daemon that runs in the background and reads crontab files. A crontab is a file containing jobs in the format

minute hour day-of-month month day-of-week  command

crontabs are normally stored by the system in /var/spool/<username>/crontab. These files are not meant to be edited directly. You can use the crontab command to invoke a text editor (what you have defined for the EDITOR env variable) to modify a crontab file.

There are various implementations of cron. Commonly there will be per-user crontab files (accessed with the command crontab -e) as well as system crontabs in /etc/cron.daily, /etc/cron.hourly, etc.

In your first example you are scheduling a job via a crontab. In your second example you're using the at command to queue a job for later execution.