Cron Jobs Cheat Sheet

cron_jobs_cheatsheet.md

Cron Jobs Cheat Sheet

1. Cron Time Format (Crontab Syntax)

Field	Allowed Values	Special Characters	Meaning	Example
Minute	0–59	* , - /	Minute of the hour	0 = top of the hour
Hour	0–23	* , - /	Hour of the day	5 = 5 AM
Day	1–31	* , - /	Day of the month	15 = 15th
Month	1–12 or Jan–Dec	* , - /	Month of the year	1 = January
Weekday	0–6 or Sun–Sat	* , - /	Day of the week (0 = Sunday)	1 = Monday

Special Characters

• * : Every value
• , : Value list separator
• - : Range of values
• / : Step values (e.g. */5 means every 5th unit)

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2. Common Schedule Examples

Expression	Description	Example Command
* * * * *	Every minute	* * * * * /path/to/script.sh
0 * * * *	Every hour	0 * * * * /path/to/script.sh
0 0 * * *	Every day at midnight	0 0 * * * /path/to/script.sh
0 9 * * 1-5	Weekdays at 9 AM	0 9 * * 1-5 /path/to/backup.sh
0 0 1 * *	First day of every month	0 0 1 * * /path/to/monthly.sh
*/15 * * * *	Every 15 minutes	*/15 * * * * /path/to/checker.sh
0 12 * * 0	Every Sunday at noon	0 12 * * 0 /path/to/clean.sh

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3. Special Macros

Macro	Equivalent	Description
@reboot	(N/A)	Run at system startup
@yearly	0 0 1 1 *	Once a year
@annually	0 0 1 1 *	Once a year
@monthly	0 0 1 * *	Once a month
@weekly	0 0 * * 0	Once a week
@daily	0 0 * * *	Once a day
@midnight	0 0 * * *	Once a day
@hourly	0 * * * *	Once an hour

Example:

@reboot /path/to/startup_script.sh
@daily /usr/bin/freshclam

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4. Crontab Commands

Command	Description	Example
crontab -e	Edit current user’s crontab	crontab -e
crontab -l	List current user’s crontab	crontab -l
crontab -r	Remove current user’s crontab	crontab -r
crontab -u <user> -e	Edit another user’s crontab (as root)	crontab -u root -e
crontab -u <user> -l	List another user’s crontab (as root)	crontab -u alice -l

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5. Logging & Debugging

Method	Description	Example
Output redirection	Save stdout and stderr	/path/to/job.sh > /tmp/out.log 2>&1
Use logger	Log to syslog	logger "Job ran at $(date)"
Check cron log (if enabled)	/var/log/cron or /var/log/syslog	grep CRON /var/log/syslog

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6. Environment Variables in Crontab

• PATH, HOME, SHELL can be declared at the top

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

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7. Good Practices

Practice	Why
Use absolute paths	Cron uses a limited environment
Use full paths for scripts/binaries	Avoid relying on PATH
Log output for each job	Easier debugging
Test scripts manually first	Ensure correctness
Keep crontab clean and commented	Maintenance & clarity

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8. Example Full Crontab Entry

# Run backup script every day at 1am
0 1 * * * /home/melashri/scripts/backup.sh >> /home/melashri/logs/backup.log 2>&1

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9. Edit system-wide cron jobs

File	Purpose
/etc/crontab	System-wide cron table
/etc/cron.d/	Drop-in files with cron jobs
/etc/cron.hourly/ etc.	Periodic directories

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10. Run Cron Job as a Specific User

In /etc/crontab or /etc/cron.d/* files:

# Format: m h dom mon dow user command
0 5 * * * root /usr/local/bin/system_maintenance