3.3 KiB
How to use cron on Linux
The cron system is a method to automatically run commands on a schedule.
The cron system is a method to automatically run commands on a schedule. A scheduled job is called a cronjob, and it’s created in a file called a crontab. It’s the easiest and oldest way for a computer user to automate their computer.
Writing a cronjob
To create a cronjob, you edit your crontab using the -e option:
`$ crontab -e`
This opens your crontab your default text editor. To set the text editor explicitly, use the EDITOR environment variable:
`$ EDITOR=nano crontab -e`
Cron syntax
To schedule a cronjob, you provide the command you want your computer to execute, followed by a cron expression. The cron expression schedules when the command gets run:
-
minute (0 to 59)
-
hour (0 to 23, with 0 being midnight)
-
day of month (1 to 31)
-
month (1 to 12)
-
day of week (0 to 6, with Sunday being 0)
An asterisk (*) in a field translates to "every." For example, this expression runs a backup script at the 0th minute of every hour on every day of every month:
`/opt/backup.sh 0 * * * *`
This expression runs a backup script at 3:30 AM on Sunday:
`/opt/backup.sh 30 3 * * 0`
Simplified syntax
Modern cron implementations accept simplified macros instead of a cron expression:
-
@hourlyruns at the 0th minute of every hour of every day -
@dailyruns at the 0th minute of the 0th hour of every day -
@weeklyruns at the 0th minute of the 0th hour on Sunday -
@monthlyruns at the 0th minute of the 0th hour on the first day of the month
For example, this crontab line runs a backup script every day at midnight:
`/opt/backup.sh @daily`
How to stop a cronjob
Once you've started a cronjob, it's designed to run on schedule forever. To stop a cronjob once you've started it, you must edit your crontab, remove the line that triggers the job, and then save the file.
`$ EDITOR=nano crontab -e`
To stop a job that's actively running, use standard Linux process commands to stop a running process.
It’s automated
Once you’ve written your crontab, save the file and exit your editor. Your cronjob has been scheduled, so cron does the rest.
via: https://opensource.com/article/21/7/cron-linux
作者：Seth Kenlon 选题：lujun9972 译者：译者ID 校对：校对者ID