🧩 cronlint — Validate system crontab files

cronlint.sh is a small POSIX shell script that checks whether a file follows the syntax rules of a system crontab (e.g. /etc/crontab or files in /etc/cron.d/).

It validates that each line has the correct number of fields, uses valid time specifications, contains a valid user field, and ends properly with a newline.
It also detects CRLF (Windows-style) line endings and misformatted entries.

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🧭 Features

✅ Validates field count (min hr dom mon dow USER COMMAND)
✅ Supports @special keywords (@reboot, @daily, @weekly, etc.)
✅ Checks time field syntax (lists, ranges, steps, month/day names)
✅ Validates value ranges for all time fields (minute, hour, day, month, weekday)
✅ Verifies USER field

• must match a valid username pattern (^[A-Za-z_][A-Za-z0-9_-]*[$]?$)
• must exist on the system (getent passwd or /etc/passwd)

✅ Detects Windows line endings (CRLF)
✅ Warns if file does not end with newline
✅ Ignores blank lines, comments, and environment variable assignments

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📦 Installation

Clone the repository and make the script executable:

git clone https://github.com/alphapialpha/cronlint.git
cd cronlint
chmod +x cronlint.sh

Optionally move it into your $PATH:

sudo mv cronlint.sh /usr/local/bin/cronlint

Now you can run it anywhere as cronlint.

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🚀 Usage

cronlint /path/to/crontab-file

Example:

cronlint /etc/crontab

✅ Valid Example

Valid Complex Crontab:

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Run daily backup as root
0 3 * * * root /usr/local/bin/backup.sh

# Every 10 minutes between 08:00–18:00, Monday–Friday
*/10 8-18 * * mon-fri  root      /usr/local/bin/test.sh

# At 03:15 on the 1st–10th of each month
15 3 1-10 * *           dbadmin   /usr/local/bin/db_maintenance.sh

# Every 5 minutes on weekends (Saturday and Sunday)
*/5 * * * sat,sun       www-data  /usr/local/bin/refresh-cache

# Every 30 minutes overnight (22:00–23:59 and 00:00–06:59)
*/30 22-23,0-6 * * *    sysop     /usr/local/bin/nightly_task.sh

# Every hour on the 1st and 15th during March–June
0 * 1,15 mar-jun *      report       /usr/local/bin/reportgen --monthly

Output:

OK: no errors.

❌ Invalid Examples

Example 1: Erroneous Schedule Fields

43 24 20 13 7 root cat /etc/crontab

Output:

ERROR: hour field invalid: 24
ERROR: month field invalid: 13
Line 22: ERROR: invalid schedule fields.
Found 1 error(s).

Example 2: No User Name or Invalid User Name

5 4 23 11 5 cat /etc/crontab

Output:

Line 1: ERROR: user not found: cat
Found 1 error(s).

⚠️ Warnings Example

File doesn’t end with a newline:

WARNING: File does not end with a newline character.
Note: 1 warning(s) reported.

File contains Windows CRLF endings:

ERROR: File contains Windows CRLF line endings (\r). Convert to LF only.

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⚙️ Exit Codes

Code	Meaning
0	OK (no errors)
1	Errors found
2	Invalid usage

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🧠 Notes

• cronlint is designed for system crontabs, which require a USER field.
• User crontabs created with crontab -e do not include a user field and will fail validation.
• The script is POSIX-compliant and runs on most UNIX-like systems (Linux, BSD, macOS).
• It uses only standard utilities: awk, grep, tail, and od.

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🧾 License

MIT License © 2025 André Pierre Appel

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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💡 Contributing

Pull requests are welcome!
If you find edge cases (e.g. unusual cron syntax or OS-specific differences), feel free to open an issue or submit a fix.

Example contribution workflow

git checkout -b fix-user-validation
# edit cronlint.sh
git commit -am "Improve user validation and field detection"
git push origin fix-user-validation

Then open a Pull Request on GitHub.

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🧩 Credits

Created with ☕ and 🐧 by André Pierre Appel

“Linting is just removing the fluff — even from your crontabs.”