# Crontab command explanation

The cron daemon (crond) is a background service on Unix-like operating systems that executes scheduled commands. It reads configuration from crontab (cron table) files, which define when and how often each command should run.

## Syntax

The crontab command manages per-user crontab files:

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

## Options

- **`-e`**: Edit the current user's crontab in the default editor ($VISUAL or $EDITOR). The new crontab is installed after the editor exits.
- **`-l`**: Display the current user's crontab to standard output.
- **`-r`**: Remove the current user's crontab entirely.
- **`-i`**: Prompt for confirmation before removing (used with -r).
- **`-u user`**: Operate on the specified user's crontab instead of your own. Requires root privileges.
- **`file`**: Install the crontab from the given file. Use - for standard input.

## Crontab file format

Each line in a crontab file is either an environment variable assignment, a comment (starting with #), or a cron job with this format:

```
┌───────────── minute (0–59)
│ ┌───────────── hour (0–23)
│ │ ┌───────────── day of month (1–31)
│ │ │ ┌───────────── month (1–12 or JAN–DEC)
│ │ │ │ ┌───────────── day of week (0–6 or SUN–SAT)
│ │ │ │ │
* * * * *  command to execute
```

## Field operators

- *** (asterisk)**: Matches every possible value for the field.
- **, (comma)**: Specifies a list of values. Example: 1,15 in the day-of-month field means the 1st and 15th.
- **- (hyphen)**: Defines an inclusive range. Example: 9-17 in the hour field means every hour from 9 through 17.
- **/ (slash)**: Defines a step. Example: */10 in the minute field means every 10 minutes. Can be combined with a range: 1-30/5.

## Special strings

Instead of the five time fields, you can use one of these shorthand strings:

| String | Equivalent |
|--------|-----------|
| `@yearly` / `@annually` | `0 0 1 1 *` |
| `@monthly` | `0 0 1 * *` |
| `@weekly` | `0 0 * * 0` |
| `@daily` / `@midnight` | `0 0 * * *` |
| `@hourly` | `0 * * * *` |
| `@reboot` | `Run once at startup` |

## Environment variables

You can set environment variables at the top of a crontab file. The most common ones:

- **SHELL**: The shell used to run commands (default: /bin/sh).
- **PATH**: The search path for commands. Cron's default PATH is minimal (usually /usr/bin:/bin), so always use absolute paths or set PATH explicitly.
- **MAILTO**: Where to send command output. Set to "" to suppress email. By default, output is mailed to the crontab owner.
- **CRON_TZ**: Set the timezone for the crontab (not supported on all systems). Without it, cron uses the system timezone.

```
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com

*/5 * * * * /home/user/scripts/backup.sh
```

## System crontab vs. user crontab

User crontabs (edited with crontab -e) have five time fields plus the command. The system crontab (/etc/crontab) and files in /etc/cron.d/ have an extra field between the time fields and the command: the username the command runs as.

System crontab example:

```
# /etc/crontab
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m  h  dom mon dow user    command
*/15 *  *   *   *   root    /usr/local/bin/system-check.sh
0    2  *   *   *   backup  /usr/local/bin/nightly-backup.sh
```

## Common pitfalls

### % (percent sign)

The percent sign (%) has special meaning in crontab: it is translated to a newline, and everything after the first % is sent as standard input to the command. To use a literal %, escape it as \%.

```
# Wrong — the % will be interpreted
0 0 * * * echo "Date: $(date +%Y-%m-%d)"

# Correct — escape the %
0 0 * * * echo "Date: $(date +\%Y-\%m-\%d)"
```

### PATH

Cron runs with a minimal PATH. Scripts that rely on commands in /usr/local/bin or other directories should either use absolute paths or set PATH at the top of the crontab.

### Day-of-month + day-of-week

When both day-of-month and day-of-week are restricted (not *), cron runs the command when either field matches (OR logic), not when both match. For example, 0 0 1 * 5 runs at midnight on the 1st of every month AND every Friday.

### Timezone

Cron uses the system timezone by default. If your server is in UTC but you want jobs in a local timezone, use CRON_TZ (where supported) or convert times manually.

### Output and logging

By default, cron mails any output (stdout and stderr) to the crontab owner. To silence a job, redirect output: command > /dev/null 2>&1. To log output, redirect to a file: command >> /var/log/myjob.log 2>&1.

```
# Discard all output
*/5 * * * * /path/to/script.sh > /dev/null 2>&1

# Log output to a file
*/5 * * * * /path/to/script.sh >> /var/log/myscript.log 2>&1
```

## Practical examples

- [`*/5 * * * *`](https://crontab.run/every-5-minutes) — run every 5 minutes
- [`0 0 * * *`](https://crontab.run/daily) — run once a day at midnight
- [`0 9 * * 1-5`](https://crontab.run/every-weekday-at-9am) — run at 9 AM on weekdays
- [`0 0 1 * *`](https://crontab.run/every-month) — run at midnight on the 1st of every month
- [`0 0 * * 0`](https://crontab.run/every-sunday) — run at midnight every Sunday

Canonical: https://crontab.run/command

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