1 · What Is Cron?

Purpose

Cron is a time-based job‑scheduler built into most Unix‑like operating systems. It launches processes at fixed dates or intervals, making it ideal for repetitive tasks such as backups, log rotation, data processing, and system maintenance.

The name derives from chronos, the Greek word for “time”.

2 · Anatomy of a Cron Expression

Field Layout

A standard entry contains five timing fields followed by the command to execute:

# ┌──────── min (0‑59)
# │ ┌────── hour (0‑23)
# │ │ ┌──── day‑of‑month (1‑31)
# │ │ │ ┌── month (1‑12 or JAN‑DEC)
# │ │ │ │ ┌─ day‑of‑week (0‑7 or SUN‑SAT; 0 & 7 = Sunday)
# │ │ │ │ │
# * * * * *  command_to_run

Field Behaviour

Symbol	Meaning	Example
`*`	Any value	Run every minute
`,`	Value list separator	`MON,WED,FRI`
`-`	Range	`1-5` (Mon–Fri)
`/`	Step values	`/15` (every 15 min*)

3 · Special Schedule Strings

Shortcut Keywords

@reboot — run once at startup
@hourly — top of every hour (0 * * * *)
@daily — midnight every day (0 0 * * *)
@weekly — midnight Sunday (0 0 * * 0)
@monthly — midnight 1st of month (0 0 1 * *)
@yearly/@annually — midnight Jan 1 (0 0 1 1 *)

Syntax extensions such as @every 3h45m exist in systemd timers and Go’s cron library but are not standard Vixie Cron.

4 · Editing & Managing Crontabs

User Crontab Commands

crontab ‑e — edit the current user’s crontab in $EDITOR
crontab ‑l — list the current user’s jobs
crontab ‑r — remove the current crontab
crontab ‑u user (file) — operate on another user (root only)

System‑Wide Crontabs

Besides per‑user crontabs stored in /var/spool/cron/, Unix distributions provide system files:

/etc/crontab (adds a user field: m h dom mon dow user command)
Hourly/Daily/Weekly/Monthly directories in /etc/cron.d or /etc/cron.*

5 · Environment & Execution Context

Key Points

Default $PATH is minimal (/usr/bin:/bin). Set explicit paths or export variables at top of crontab.
Jobs run non‑interactively with /bin/sh unless SHELL=/bin/bash is specified.
Relative paths resolve from the user’s home directory; prefer absolute paths.
Redirect stdout/stderr for logging (>/var/log/job.log 2>&1).

A missing environment variable (e.g., LANG for UTF‑8) is a frequent source of cron bugs.

6 · Logging & Troubleshooting

Where to Look

/var/log/cron or journalctl (journalctl -u cron)
MAILTO=you@example.com in crontab to receive job output
Use wrapper scripts that log start/stop times for long‑running tasks

Common Pitfalls

Mismatched shell features (e.g., bashisms in /bin/sh).
Permissions: scripts need execute bit (chmod +x) and readable paths.
Overlapping runs — use flock or test PID files to avoid duplicates.

7 · Security & Best Practices

Guidelines

Prefer dedicated service accounts rather than root.
Validate user input when cron triggers scripts processing external data.
Use absolute paths and verify checksums where integrity matters.
Version‑control cron scripts and maintain change history.
Document each job’s intent in comments (# Backup /var/www to S3).

8 · Quick Reference Examples

Typical Schedules

Schedule	Cron Syntax	Description
Every 5 minutes	`/5 * * *`	Frequent polling/health‑check
Every weekday at 08:00	`0 8 * * 1‑5`	Send daily report email
1st & 15th at 02:30	`30 2 1,15 * *`	Semi‑monthly payroll export
Last day of month 23:55	`55 23 28‑31 * * [ "$(date +\%d -d tomorrow)" == "01" ] && command`	End‑of‑month tasks (Bash test inside)

Inline Examples

# Backup /home at 01:00 daily
0 1 * * *  /usr/local/bin/backup.sh

# Clear system cache every Sunday at 03:15
15 3 * * 0  /usr/bin/sync ; /usr/bin/sysctl -w vm.drop_caches=3

# Run Python script at reboot
@reboot  /usr/bin/python3 /opt/app/startup.py >>/var/log/app.log 2>&1

Cron Jobs — Essential Guide