cron

Format

Min  Hour Day  Mon  Weekday
┬    ┬    ┬    ┬    ┬
│    │    │    │    └─  Day of Week   (0=Sun .. 6=Sat)
│    │    │    └──────  Month         (1..12)
│    │    └───────────  Day of Month  (1..31)
│    └────────────────  Hour          (0..23)
└─────────────────────  Minute        (0..59)

~~Field~~Field	~~Range~~Range	Special ~~characters~~characters
Minute	0-59	`, - * /`
Hour	0-23	`, - * /`
Day of Month	1-31	`, - * ? / L WW`
Month	1-12	`, - * /`
Day of Week	0-6	`, - * ? / L ##`

Examples

~~Format~~Format	~~Result~~Result
`/15 * * *`	Every 15 mins
`0 * * * *`	Every hour
`0 /2 * *`	Every 2 hours
`15 2 * * *`	At 2:15AM of every day
`15 2 * * ?`	At 2:15AM of every day
`10 9 * * 55`	At 9:10AM of every Friday
`0 0 * * 00`	At midnight of every Sunday
`15 2 * * 1L1L`	At 2:15am on the last monday of every month
`15 0 * * 4#24#2`	At 00:15am on the second thursday of every month
`0 0 1 * *`	Every 1st of month (monthly)
`0 0 1 1 *`	Every 1st of january (yearly)
`@reboot`	Every reboot (non-standard)

Special strings

Format	Result
`@reboot`	~~Every~~Run ~~reboot~~once, at system startup (non-standard)
`@yearly`	Run once every year, "`0 0 1 1 *`" (non-standard)
`@annually`	(same as @yearly) (non-standard)
`@monthly`	Run once every month, "`0 0 1 * *`" (non-standard)
`@weekly`	Run once every week, "`0 0 * * 0`" (non-standard)
`@daily`	Run once each day, "`0 0 * * *`" (non-standard)
`@midnight`	(same as `@daily`) (non-standard)
`@hourly`	Run once an hour, "`0 * * * *`" (non-standard)

Crontab command

Command	Result
`crontab -e`	Edit or create a crontab file if doesn’t already exist.
`crontab -l`	Display the crontab file.
`crontab -r`	Remove the crontab file.
`crontab -v`	Display the last time you edited your crontab file. (non-standard)

Special stringscharacters

Character	Observations
`Asterik(*)`	Matches all values in the field or any possible value.
`Hyphen(-)`	Used to define a range.Ex: 1-5 in 5th field(Day Of Week) Every Weekday i.e., Monday to Friday
`Slash (/)`	1st field(Minute) /15 meaning every fifteen minute or increment of range.
`Comma (,)`	Used to separate items.Ex: 2,6,8 in 2nd fields(Hour) executes at 2am,6am and 8am
`L`	It is allowed only for Day of Month or Day Of Week field, 2L in Day of week indicates Last tuesday of every month
`Hash (#)`	It is allowed only for Day Of Week field, which must be followed within range of 1 to 5. For example, 4#1 means "The first Thursday" of given month.
`Question mark (?)`	Can be instead of '*' and allowed for Day of Month and Day Of Week. Usage is restricted to either Day of Month or Day Of Week in a cron expression.

Also see

Devhints (devhints.io)
Crontab Generator (crontab-generator.org)
Crontab guru (crontab.guru)