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Crontab

Cron Syntax

┌───────── minute (0–59)
│ ┌─────── hour (0–23)
│ │ ┌───── day of month (1–31)
│ │ │ ┌─── month (1–12)
│ │ │ │ ┌─ day of week (0–7, both 0 and 7 = Sunday)
│ │ │ │ │
* * * * *  command

Symbol	Meaning	Example
`*`	Every value	`* * * * *` = every minute
`,`	List of values	`1,15,30 * * * *` = at minute 1, 15, and 30
`-`	Range	`9-17 * * * *` = every minute from hour 9 to 17
`/`	Step	`/5 * * *` = every 5 minutes

Common Schedules

Schedule	Cron expression
Every minute	`* * * * *`
Every 5 minutes	`/5 * * *`
Every hour	`0 * * * *`
Every day at midnight	`0 0 * * *`
Every day at 3:30 AM	`30 3 * * *`
Every Monday at 9 AM	`0 9 * * 1`
Every 1st of the month	`0 0 1 * *`
Every weekday at 6 PM	`0 18 * * 1-5`

Shorthand (if supported):

Shorthand	Equivalent
`@reboot`	Run once at startup
`@hourly`	`0 * * * *`
`@daily`	`0 0 * * *`
`@weekly`	`0 0 * * 0`
`@monthly`	`0 0 1 * *`
`@yearly`	`0 0 1 1 *`

Managing Crontabs

crontab -e              # Edit current user's crontab
crontab -l              # List current user's crontab
crontab -r              # Remove current user's crontab
crontab -u krav -l      # List another user's crontab (root only)

System vs User Crontabs

Location	Owner	Format
`crontab -e`	Per-user	`* * * * * command`
`/etc/crontab`	System-wide	`* * * * * user command` (has username field)
`/etc/cron.d/`	System packages	Same format as `/etc/crontab`
`/etc/cron.daily/`	Daily scripts	Drop a script here — no cron syntax needed
`/etc/cron.hourly/`	Hourly scripts	Same — just an executable script
`/etc/cron.weekly/`	Weekly scripts	Same
`/etc/cron.monthly/`	Monthly scripts	Same
`/var/spool/cron/crontabs/`	User crontab storage	Don’t edit directly — use `crontab -e`

Practical Tips

Redirect output to avoid mail spam

*/5 * * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Without redirection, cron emails output to the user (or to /var/mail/).

Use full paths

Cron runs with a minimal PATH. Always use absolute paths:

# Bad — might not find the binary
*/5 * * * * backup.sh

# Good
*/5 * * * * /usr/local/bin/backup.sh

Set PATH explicitly

PATH=/usr/local/bin:/usr/bin:/bin
*/5 * * * * backup.sh

Debug a cron job

# Check if cron is running
systemctl status cron

# Check cron logs
grep CRON /var/log/syslog

# Test the exact command cron will run
/bin/sh -c '/usr/local/bin/backup.sh >> /var/log/backup.log 2>&1'

Security Notes

Crontabs are a common persistence mechanism — check them during incident response
find / -name "crontab" -type f 2>/dev/null and ls /etc/cron.* to enumerate
Writable cron scripts or directories = privilege escalation vector

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