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

command -v crontab

Debian and Ubuntu ship crontab in the cron package. The daemon has to be running or the table never fires.

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:

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.

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.

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:

crontab -n jobs.cron

Install, then list. Confirm the old lines are still there and the new line was added.

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.

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