Cron is a system-wide service to run tasks, so-called cronjobs, in user-specified intervals. For example, you might want to create a backup of your Uberspace every day. You could then create a cronjob that will copy all files on your account to another server.
Your cronjobs are stored in your
crontab. This is a table that contains all the information Cron needs to run your task. It looks like this:
* * * * * /path/to/your/job ┬ ┬ ┬ ┬ ┬ │ │ │ │ │ │ │ │ │ └──── Day of the week (0-7) (Sunday can be 0 or 7) │ │ │ └────── Month (1-12) │ │ └──────── Day of the month (1-31) │ └────────── Hour (0-23) └──────────── Minute (0-59)
An asterisk (
*) means that any value is valid, so if all columns contain an asterisk, the job will be started every minute, regardless of date, time, etc.
Please note that hours are always in 24-hour format, so 10 is 10 a.m., if you want 10 p.m., you need to enter 22 in the hour column.
To see your current crontab, run
15 * * * * /path/to/your/job/script.pl
The job /path/to/your/job/script.pl is started 15 minutes past every full hour.
30 10 * * * /path/to/your/job/script.pl
The job is started every day at 10:30 a.m.
* * * * * /path/to/your/job/script.pl
The job is started every minute.
/is used to divide a time.
,combines multiple times.
-specifies a range (such as 1-5).
*/5 * * * * /path/to/your/job/script.pl
The job is started every five minutes.
30 8-20 * * * /path/to/your/job/script.pl
The job is started at half past the hour between 8 a.m. and 8:30 p.m.
30 10 * * 1,2,3,4,5 /path/to/your/job/script.pl
The job is started on weekdays (Monday to Friday) at 10:30 a.m.
There are a couple of aliases that can be used instead of the numeric definitions:
@hourly: At every full hour (i.e.:
0 * * * *).
@midnight: Every day (i.e.:
0 0 * * *).
@weekly: Every week (i.e.:
0 0 * * 0).
@monthly: Once a month (i.e.:
0 0 1 * *).
@annually: Once a year (i.e.:
0 0 1 1 *).
@reboot: After every reboot.
Adding, Modifying, and Deleting a Cronjob¶
If you want to add or modify a new cronjob, you need to edit your
crontab. To do this, use the command
crontab -e. This will launch your standard editor and you can add a new job or modify an existing one.
To change your standard editor, set it in the
VISUAL variable. So to use
nano, add this line to your .bash_profile:
To remove a cronjob, delete the line. If you want to only temporarily disable a cronjob, put a
# at the beginning of the line. You can also use the
# to add comments to the file.
After you’ve saved the temporary file and exited the editor, the changes will be applied.
cron does not parse your .bash_profile or .bashrc, so
$PATH is different from your shell. You can define
$PATH in the
crontab. So if you want to include your
~/bin directory in
$PATH, you need to insert this line before your cronjob:
cron will email the result of your cronjob to your primary email address. To disable these emails, you can set an empty
MAILTO variable before your actual cronjob. You will still receive emails if there is an error.
MAILTO="" 15 * * * * /path/to/your/job/script.pl
Similarly, you can have the emails sent to a different email address:
MAILTO="email@example.com" 15 * * * * /path/to/your/job/script.pl
To completely disable any emails for a cronjob, you need to send both the standard output and standard error to
15 * * * * /path/to/your/job/script.pl > /dev/null 2>&1
If you want to save your cronjob’s output to a log file, you can do so by using the
>> operators. Please note that this also disables cron’s emails.
To save only the most recent output, use
15 * * * * /path/to/your/job/script.pl > /path/to/your/logfile 2>&1
To append the log file, use
15 * * * * /path/to/your/job/script.pl >> /path/to/your/logfile 2>&1