crontab

Run a script every night at 2:00 or every 15 minutes: the crontab line explained, how to install it and check it ran.

>_ Try it: a simulated Linux, right herenothing runs on your computer

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Examples

echo "0 2 * * * /home/deploy/backup.sh" | crontab - every night at 02:00
crontab -l there it is
echo "*/15 * * * * /home/deploy/backup.sh" | crontab - every 15 minutes instead (this replaces the list)
crontab -l only the new line is left
grep CRON /var/log/syslog cron's runs in the system log
crontab -r remove all your jobs

How it works

cron runs commands on a schedule. Each line of your crontab is five time fields — minute, hour, day of month, month, weekday — then the command. * means "every": 0 2 * * * is 02:00 every day.

crontab -l lists your jobs. crontab FILE (or crontab - with a pipe) installs a whole list — it REPLACES the old one. Every run leaves a CRON line in /var/log/syslog.

Syntax

crontab [-u USER] { FILE | - | -l | -r | -e }

Options

Option	What it does
`m h dom mon dow`	minute hour day-of-month month weekday, then the command
`*/N`	every N: */15 in the minute field = every 15 minutes
`FILE or -`	install the jobs from a file, or from a pipe
`-l`	list your jobs
`-r`	remove all your jobs
`-e`	edit them in a text editor

Good to know

Test the script by hand first: a job that fails at 2 a.m. fails quietly.
Weekday 0 and 7 are both Sunday; 30 6 * * 1 is 06:30 every Monday. Server clocks often run on UTC (date).