Learning Cron Job - Crontab Syntax & Time Fields

#Introduction

In episode 2 we saw that cron works by matching five time fields. Now it's time to fully understand that scheduling language — because most chronic errors come from here: schedules that look right but run on the wrong day, or run twice.

Cron syntax is actually simple. Once you master the five fields and the *, ,, -, / characters, you can read any crontab on any server in the world.

#The Five Time Fields

Each crontab line starts with five fields that determine when the command runs:

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

The simplest example — every night at 02.30:

30 2 * * * /usr/local/bin/backup.sh

The day-of-month and month fields are marked *, meaning "every day" and "every month". The day-of-week field is also *, meaning "every day of the week".

#Special Characters

#* — All Values

* means "every possible value". * * * * * means every minute.

* * * * * /usr/local/bin/check.sh

#, — Lists

A comma creates a list of values. A schedule at 07:00, 12:00, and 18:00:

0 7,12,18 * * * /usr/local/bin/report.sh

#- — Ranges

A dash creates a range. Every working hour from 09.00-17.00:

0 9-17 * * 1-5 /usr/local/bin/ping.sh

#/ — Steps

A slash sets an interval. Every 5 minutes:

*/5 * * * * /usr/local/bin/healthcheck.sh

A step can be combined with a range — every 10 minutes between 9 and 11:

*/10 9-11 * * * /usr/local/bin/snapshot.sh

#Month and Day Names

Cron accepts the first three letters of month and day names, case-insensitively:

month:  jan feb mar apr may jun jul aug sep oct nov dec
day:    sun mon tue wed thu fri sat

0 22 * * mon-fri /usr/local/bin/nightly.sh

#The OR/AND Rules That Frequently Trap You

This is the part most often misunderstood. When both day-of-month and day-of-week are set to specific values (not *), cron combines them with OR, not AND:

0 0 1,15 * mon /usr/local/bin/task.sh

The line above runs on the 1st and 15th of every month OR every Monday — not "the 1st/15th that fall on a Monday". This means the job can run several times in a single month. This is cron's historical behavior and you should always keep it in mind.

#Complete Examples

# Setiap 5 menit
*/5 * * * * /usr/local/bin/check.sh
 
# Pukul 02.30 setiap malam
30 2 * * * /usr/local/bin/backup.sh
 
# Tiap Senin pukul 07.00
0 7 * * mon /usr/local/bin/weekly-report.sh
 
# Tanggal 1 setiap bulan pukul 00.00
0 0 1 * * /usr/local/bin/monthly-cleanup.sh
 
# Setiap jam kerja (09-17) tiap menit ke-0 dan ke-30
0,30 9-17 * * 1-5 /usr/local/bin/clock.sh

#Syntax Verification

Before saving, verify your schedule with crontab.guru (episode 7 covers it in more depth). As a quick test, count manually: 30 2 * * * means "minute 30, hour 2, all days" — clear, right? If you're unsure about a strange combination, this tool will explain it in plain language.

#Closing

Key takeaways:

• Format: minute hour day-of-month month day-of-week command.
• * all values, , lists, - ranges, / steps, three-letter month/day names.
• day-of-month and day-of-week when both are non-* combine with OR.
• Test complex schedules on crontab.guru before saving.
• Simplify schedules as much as possible.

In episode 4 we'll practice crontab management — crontab -e, crontab -l, crontab -r, crontab -u for other users, plus the structure of /etc/crontab, /etc/cron.d/, and the per-user spool /var/spool/cron/. Time to write your first real schedule!