Explain the syntax for writing a CRON expression in Salesforce Schedule job?

CRON is a software utility that is a time-based job scheduler in Unix-like computer operating systems. Developers who want to set up and maintain software environments, use this cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.

A CRON expression is basically a string of five or six fields separated by white spaces that represents a set of times, normally as a schedule to execute some routine.

Use schedule with an Apex class that implements the Schedulable interface to schedule the class to run at the time specified by a Cron expression.

System.Schedule(JobName, CronExpression, SchedulableClass);

The System.Schedule method takes three arguments: a name for the job, an expression used to represent the time and date the job is scheduled to run, and the name of the class.

Sample Code

TestSchedulableClass testobj = new TestSchedulableClass();

String cronexpression = ‘0 0 0 ? * * *’

System.schedule(‘Testing’, cronexpression, testobj);

The Above Code Executes TestSchedulableClass At 12:00 AM Every Day .

CRON expression has the following syntax:

0 0 5 ? * 1,2,3,4,5,6,7

{1} {2} {3} {4} {5} {6}

{1} Seconds – so 0 here i.e. start of the minute.

{2} Minutes – 0 again so start of the hour.

{3} Hours –  5 so 5 am. Uses 24 hour notation so 21 = 9pm

{4} Day_of_month – ? means no specific value, only available for day of the month and day of the week.

{5} Month – * indicates all values, i.e. every month. (if we only want to run on 1st Jan say, this would be 1)

{6} Day_of_week – 1,2,3,4,5,6,7 here specifies days 1,2,3,4,5,6,7 in the week. We could also write this string as MON-FRI or preferably as * to indicate all values.

So this job reads to run at “0 seconds past 0 minutes of the 5th hour on no specified day of the month for every month of the year for every day of the week”.