crontab Command Cheat Sheet

crontab (cron table) is the command used to install, deinstall, or list the tables used to drive the cron daemon. The cron daemon checks time-based jobs and executes them in the background.

Synopsis

crontab [ -u user ] file
crontab [ -u user ] [ -i ] { -e | -l | -r }

Basic Operations

Edit Crontab

Opens your crontab file in the default editor (vi, nano, etc.).

crontab -e

List Crontab

Displays the current crontab content to standard output.

crontab -l

Remove Crontab

Deletes your current crontab file (be careful!).

crontab -r

Prompt Before Removal (-i)

Safely remove with confirmation.

crontab -i -r

Cron Syntax

A standard cron line consists of 5 time-and-date fields, followed by the command.

# Minute   Hour   Day of Month   Month   Day of Week   Command
# (0-59)  (0-23)     (1-31)      (1-12)     (0-7)     (Command to run)
  *        *          *           *          *        /path/to/command

Field Values

(Note: 0 and 7 are both Sunday).

Special Characters

• * : Every value (every minute, every hour, etc.)
• , : Value list separator (1,3,5 = 1st, 3rd, 5th)
• - : Range of values (1-5 = 1, 2, 3, 4, 5)
• / : Step values (*/5 = every 5th unit)

Common Examples

Every Minute

* * * * * /path/to/script.sh

Every Hour at Minute 0

0 * * * * /path/to/hourly_job.sh

Every Day at Midnight

0 0 * * * /path/to/daily_backup.sh

Every Monday at 5:30 AM

30 5 * * 1 /path/to/weekly_report.sh

Every 15 Minutes

*/15 * * * * /path/to/check_status.sh

Specific Months (e.g., January and June)

0 0 1 1,6 * /path/to/semi_annual.sh

Special Strings (Shortcuts)

Instead of the 5 fields, you can use these shortcuts (if supported by your cron daemon, which Vixie/Paul Vixie cron usually does).

Example: Run Script on Boot

@reboot /path/to/startup_script.sh

Environment Variables

Cron runs with a very minimal environment. It does not load your .bashrc or .profile.

Setting PATH

Always define PATH at the top of your crontab to ensure commands are found.

PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
SHELL=/bin/bash
MAILTO=user@example.com

* * * * * my_script.sh

MAILTO

If MAILTO is defined, cron sends the output (stdout/stderr) of jobs to that email address. If empty (MAILTO=""), no email is sent.

Logging and Troubleshooting

Redirect Output

Since you can't see the terminal output of a cron job, redirect it to a log file.

# Standard Output + Standard Error to log file
0 5 * * * /path/backup.sh > /var/log/backup.log 2>&1

# Ignore Output (discard stdout and stderr)
0 5 * * * /path/cleanup.sh > /dev/null 2>&1

Check Cron Logs

On Ubuntu/Debian:

grep CRON /var/log/syslog

On CentOS/RHEL:

tail -f /var/log/cron

Common "Silent Failure" Causes

1. Permissions: Does the script have +x?
2. Paths: Are you using absolute paths (/usr/bin/python3) instead of just python3?
3. Environment: Does the script rely on variables defined in your shell profile?
4. Newline: Does the crontab file end with a newline? (Some older crons fail without it).

System-Wide Cron

User crontabs are stored in /var/spool/cron/crontabs/. The system also has /etc/crontab and /etc/cron.d/.

Format of /etc/crontab: Includes a user field before the command.

# m h dom mon dow user  command
17 * * * * root    cd / && run-parts --report /etc/cron.hourly
25 6 * * * root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )

Managing Other Users

If you have root privileges (sudo), you can manage other users' crontabs.

# Edit alice's crontab
sudo crontab -u alice -e

# List bob's crontab
sudo crontab -u bob -l

Security (Cron Access Control)

You can allow or deny users from using crontab.

• /etc/cron.allow: If exists, ONLY users listed here can use crontab.
• /etc/cron.deny: If cron.allow doesn't exist, users listed here CANNOT use crontab.
• If neither exists, usually only root can use it (depending on distro).

Exit Status

Tools

• Crontab Guru: https://crontab.guru - Excellent visual editor for cron expressions.