cron

# Cron Schedule a command in the user's crontab. Read what is already there, add one line, check the syntax, then install the whole file. ## When to use A command should run later on a clock: every night, once an hour, or at a fixed minute. This skill edits the user crontab for the account you are logged in as. ## Install ```sh command -v crontab ``` Debian and Ubuntu ship crontab in the `cron` package. The daemon has to be running or the table never fires. ```sh sudo apt-get install cron sudo service cron start service cron status ``` macOS includes `crontab`. The system scheduler there is launchd; use this skill only when the user asked for cron. ## Commands A user crontab line is five time fields and then the command: ```text minute hour day-of-month month day-of-week command ``` Minute `0-59`, hour `0-23`, day of month `1-31`, month `1-12`, day of week `0-7` (`0` and `7` are Sunday). `*` means every value. Save the current table first. A missing crontab makes `crontab -l` exit non-zero and write nothing; that empty backup is the right start. ```sh crontab -l > "$HOME/crontab.backup" || true ``` If stderr says something other than "no crontab", stop. Do not install a new file over a table you could not read. Add one job onto the end of that backup. Use absolute paths. Cron's `PATH` is short, and the shell is `/bin/sh`. Send both stdout and stderr to a log you can read later. ```sh cp "$HOME/crontab.backup" jobs.cron printf '%s\n' '15 3 * * * /usr/bin/python3 /home/me/jobs/backup.py >> /home/me/logs/backup.log 2>&1' >> jobs.cron ``` On Debian and Ubuntu, `-n` checks the file and does not install it: ```sh crontab -n jobs.cron ``` Install, then list. Confirm the old lines are still there and the new line was added. ```sh crontab jobs.cron crontab -l ``` Useful shortcuts on Vixie cron (Debian, cronie): `@hourly`, `@daily`, `@weekly`, `@monthly`, `@reboot`. They replace the five time fields. ## Gotchas - `crontab jobs.cron` replaces the entire user crontab with that file. The backup step is what keeps the previous jobs. - `crontab -r` deletes every job. Do not run it. - A `%` in the command becomes a newline, and the rest of the line is fed to the command as stdin. Write a literal percent as `\%`. - If both the day-of-month and day-of-week fields are restricted (neither is `*`), cron runs the job when either field matches. - Jobs do not see your interactive `PATH`. The example uses `/usr/bin/python3` and an absolute script path for that reason. - Output with nowhere to go is mailed to the crontab owner. The `>> log 2>&1` redirect keeps it in a file. Create the log directory first; the redirect does not create parent directories. - Replace `/home/me` in the example with the real home directory before installing. --- Also on Honeypot — a free, open index of skills for AI agents: https://honeypot-e6c.pages.dev [](https://honeypot-e6c.pages.dev)