# crontab — Schedule Recurring Commands with the cron Daemon > Practical guide to crontab — schedule recurring jobs with cron, master the five-field syntax, logging and environment, and avoid the crontab -r trap. Source: https://www.jpkc.com/db/en/cheatsheets/shell-system/crontab/ crontab is the classic job scheduler on Unix and Linux: you store commands alongside five time fields, and the cron daemon runs them reliably in the background – from a nightly backup to a health check every five minutes. The tricky part is rarely the schedule, it's the environment: cron starts jobs with a minimal `PATH` and without your login profile, a literal `%` in a command must be escaped, and `crontab -r` wipes your entire crontab with no confirmation – dangerously close to `crontab -e`. This guide walks you through the commands, the syntax and the pitfalls. ## Managing Crontabs `crontab -e` — Edit the current user's crontab file in the default editor. ```bash crontab -e ``` `crontab -l` — List (display) the current user's crontab entries. ```bash crontab -l ``` `crontab -r` — Remove (delete) the current user's entire crontab. Warning: no confirmation and easily confused with `crontab -e` – back it up first with `crontab -l > backup.txt`. ```bash crontab -r ``` `crontab -u -e` — Edit another user's crontab (requires root). ```bash sudo crontab -u www-data -e ``` `crontab -u -l` — List another user's crontab entries. ```bash sudo crontab -u deploy -l ``` `crontab ` — Install a crontab from a file (replaces existing crontab). ```bash crontab mycron.txt ``` `crontab -l > ` — Backup the current crontab to a file. ```bash crontab -l > crontab-backup.txt ``` ## Cron Expression Format `* * * * * ` — Format: minute (0-59) hour (0-23) day-of-month (1-31) month (1-12) day-of-week (0-7, 0 and 7 = Sunday). ```bash 30 2 * * * /usr/local/bin/backup.sh ``` `*/n` — Step value: run every n intervals. ```bash */5 * * * * (every 5 minutes) ``` `n,n,n` — List: run at specific values. ```bash 0 8,12,18 * * * (at 8:00, 12:00, 18:00) ``` `n-n` — Range: run for all values in the range. ```bash 0 9-17 * * 1-5 (every hour 9-17, Mon-Fri) ``` `n-n/s` — Range with step: run every s-th value within the range. ```bash 0-30/10 * * * * (at 0, 10, 20, 30 past) ``` ## Special Strings `@reboot ` — Run once at system startup. ```bash @reboot /home/user/start-service.sh ``` `@yearly ` — Run once a year (equivalent to 0 0 1 1 *). ```bash @yearly /usr/local/bin/annual-report.sh ``` `@monthly ` — Run once a month (equivalent to 0 0 1 * *). ```bash @monthly /usr/local/bin/monthly-cleanup.sh ``` `@weekly ` — Run once a week (equivalent to 0 0 * * 0). ```bash @weekly /usr/local/bin/weekly-backup.sh ``` `@daily ` — Run once a day (equivalent to 0 0 * * *). ```bash @daily /usr/local/bin/logrotate.sh ``` `@hourly ` — Run once an hour (equivalent to 0 * * * *). ```bash @hourly /usr/local/bin/check-health.sh ``` ## Common Schedules `* * * * *` — Every minute. ```bash * * * * * /usr/local/bin/monitor.sh ``` `*/5 * * * *` — Every 5 minutes. ```bash */5 * * * * /usr/local/bin/check-queue.sh ``` `*/15 * * * *` — Every 15 minutes. ```bash */15 * * * * /usr/local/bin/sync-data.sh ``` `0 * * * *` — Every hour (at minute 0). ```bash 0 * * * * /usr/local/bin/hourly-report.sh ``` `0 0 * * *` — Every day at midnight. ```bash 0 0 * * * /usr/local/bin/daily-backup.sh ``` `0 2 * * *` — Every day at 2:00 AM. ```bash 0 2 * * * /usr/local/bin/maintenance.sh ``` `0 9-17 * * 1-5` — Every hour during business hours (Mon-Fri, 9-17). ```bash 0 9-17 * * 1-5 /usr/local/bin/business-check.sh ``` `0 0 * * 0` — Every Sunday at midnight. ```bash 0 0 * * 0 /usr/local/bin/weekly-cleanup.sh ``` `0 0 1 * *` — First day of every month at midnight. ```bash 0 0 1 * * /usr/local/bin/monthly-report.sh ``` `30 4 1,15 * *` — At 4:30 AM on the 1st and 15th of every month. ```bash 30 4 1,15 * * /usr/local/bin/biweekly-task.sh ``` ## Output & Logging `* * * * * >> /var/log/cron.log 2>&1` — Redirect both stdout and stderr to a log file (append). ```bash 0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1 ``` `* * * * * > /dev/null 2>&1` — Suppress all output (no email, no log). ```bash */5 * * * * /usr/local/bin/check.sh > /dev/null 2>&1 ``` `MAILTO="user@example.com"` — Set the email recipient for cron output (place at top of crontab). ```bash MAILTO="admin@example.com" ``` `MAILTO=""` — Disable email notifications for all cron jobs. ```bash MAILTO="" ``` `* * * * * 2>&1 | logger -t mycron` — Send output to syslog with a custom tag. ```bash 0 * * * * /usr/local/bin/task.sh 2>&1 | logger -t hourly-task ``` ## Environment Variables `PATH=/usr/local/bin:/usr/bin:/bin` — Set the PATH for cron jobs (cron uses a minimal PATH by default). ```bash PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin ``` `SHELL=/bin/bash` — Set the shell used to run cron commands (default is /bin/sh). ```bash SHELL=/bin/bash ``` `HOME=/home/user` — Set the home directory for cron jobs. ```bash HOME=/home/deploy ``` `* * * * * . /home/user/.profile && ` — Source the user's profile to get the full environment before running. ```bash 0 * * * * . /home/deploy/.profile && /usr/local/bin/task.sh ``` `* * * * * env VAR=value ` — Set a variable inline for a specific cron job. ```bash 0 2 * * * env NODE_ENV=production /usr/local/bin/node /app/worker.js ``` ## System Cron Directories `/etc/crontab` — System-wide crontab file. Has an extra user field after the time fields. ```bash 0 * * * * root /usr/local/bin/system-check.sh ``` `/etc/cron.d/` — Directory for additional system crontab files (same format as /etc/crontab). ```bash ls /etc/cron.d/ ``` `/etc/cron.hourly/` — Scripts in this directory are run every hour by anacron/run-parts. ```bash cp myscript.sh /etc/cron.hourly/ ``` `/etc/cron.daily/` — Scripts run once a day. ```bash ls /etc/cron.daily/ ``` `/etc/cron.weekly/` — Scripts run once a week. ```bash ls /etc/cron.weekly/ ``` `/etc/cron.monthly/` — Scripts run once a month. ```bash ls /etc/cron.monthly/ ``` ## Troubleshooting `grep CRON /var/log/syslog` — Check the syslog for cron execution logs. ```bash grep CRON /var/log/syslog | tail -20 ``` `journalctl -u cron` — View cron logs via systemd journal. ```bash journalctl -u cron --since today ``` `systemctl status cron` — Check if the cron daemon is running. ```bash systemctl status cron ``` `run-parts --test /etc/cron.daily` — Test which scripts in a cron directory would be executed. ```bash run-parts --test /etc/cron.daily ``` `* * * * * /usr/bin/env > /tmp/cron-env.txt` — Debug cron environment by dumping all environment variables. ```bash * * * * * /usr/bin/env > /tmp/cron-env.txt 2>&1 ``` `cat /etc/cron.allow` — Check which users are allowed to use crontab (if file exists). ```bash cat /etc/cron.allow ``` `cat /etc/cron.deny` — Check which users are denied crontab access (if file exists). ```bash cat /etc/cron.deny ``` ## Conclusion crontab handles recurring tasks reliably in the background – for everyday work you rarely need more than `crontab -e` to edit, `crontab -l` to inspect, and the familiar five-field syntax. The most common problems come not from the schedule but from the environment: cron starts jobs with a minimal `PATH` and without your login profile, so use absolute paths or set `PATH` explicitly. A literal `%` in a command is interpreted as a newline by cron and must be escaped as `\%`. Log your output (`>> logfile 2>&1`) or set `MAILTO`, otherwise failures pass silently. And mind `crontab -r`: it deletes your entire crontab with no confirmation and sits right next to `crontab -e` on the keyboard – back it up first with `crontab -l > backup.txt`. ## Further Reading - [Debian Wiki: cron](https://wiki.debian.org/cron) – how cron is set up on Debian/Ubuntu, including the system cron directories - [crontab.guru](https://crontab.guru/) – interactive helper to build and decode cron expressions ## Related Commands - [at](https://www.jpkc.com/db/en/cheatsheets/shell-system/at/) – schedules one-off jobs at a specific time instead of recurring - [systemctl](https://www.jpkc.com/db/en/cheatsheets/shell-system/systemctl/) – manages systemd services and timers as a modern cron alternative - [journalctl](https://www.jpkc.com/db/en/cheatsheets/shell-system/journalctl/) – searches the systemd journal, e.g. for cron executions