Crontab Expression Generator

Cron is a job scheduler utility program in Unix and similar operating systems.
It executes the given command at the specified time and date and specified frequency.
Cron runs as a daemon process.
Cron is derived from Chronos which means 'time' in Greek

crontab stands for cron table. It is a file with the cron schedule instructions.
crontab itself is a 'Command' used to manage a list of commands that have to be scheduled as a cron.
crontab -e: Create a new one, Edit if file exists
crontab -l: list of cronjobs , display crontab file contents.
crontab -r: Remove the crontab file.
crontab -v: Display the last edited time

A crontab job file consists of commands, one per line.
Each line has the following format.
[cron scheduler expression] [path to the script file] [command to execute]

The crontab expression consists of ths format
[minute] [hour] [day-of-month] [month] [day-of-week] [command].
From Left to Right.
The first * stands for Minute.
The second * stands for Hours.
The third * stands for date of the month (1-31).
The fourth * stands for Month (1-12).
The last * on the right side stands for Day of Week (0 for Sun, 6 for Sat)

A few crontab examples:
30 04 * * * ,
Run Daily at 04:30 am
Run the job every Friday at midnight
0 0 * * FRI

Cron schedule is a simple text field and is usually located in this path
/var/spool/cron/crontab .
The cron daemon looks for crontab file from the above path, reads the instructions and executes the commands as per the instruction

crontab -l

crontab -e

crontab -l

crontab has a time option called @reboot which allows to run a cron job once after reboot every time.

Cron expression does not support 1 sec. The setting starts with minimum of one minute (60 secs). To achieve this, we can have a script with sleep for 1 second and running in a loop to achieve this.