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Crontab - Linux jobs scheduler

2017年08月28日

Overview

In unix and unix-like operating systems, we can use crontab to schedule commands to be exectued periodically.

To see what crontabs are running:

$ crontab -l
* 11 * * 5 /bin/date >> /tmp/date_cron.log

To edit the list of cronjobs you can run:

$ crontab -e

1. Crontab Format

Cronjobs are written in the following format:

* * * * * /bin/execute/this/script.sh

The five stars represents different date parts in the following order:

  • minute (from 0 to 59)
  • hour (from 0 to 23)
  • day of month (from 1 to 31)
  • month (from 1 to 12)
  • day of week (from 0 to 6) (0=Sunday)

The above cronjob means the script is being executed every minute of every hour, every day of month, every month, every day of week.

2. Crontab Examples

  1. Execute every Friday 1AM 
    0 1 * * 5 /bin/execute/this/script.sh
  2. Execute on workdays 1AM 
    0 1 * * 1-5 /bin/execute/this/script.sh
  3. Execute 10 min past after every hour on the 1st of every month 
    10 * 1 * * /bin/execute/this/script.sh
  4. Execute every 10 min after every hour on the 1st of every month 
    0,10,20,30,40,50 * 1 * * /bin/execute/this/script.sh
or
*/10 * 1 * * /bin/execute/this/script.sh

3. Special words

For the first (minute) field, you can also put in a keyword instead of a number:

@reboot     Run once, at startup
@yearly     Run once  a year     "0 0 1 1 *"
@annually   (same as  @yearly)
@monthly    Run once  a month    "0 0 1 * *"
@weekly     Run once  a week     "0 0 * * 0"
@daily      Run once  a day      "0 0 * * *"
@midnight   (same as  @daily)
@hourly     Run once  an hour    "0 * * * *"

@daily /bin/execute/this/script.sh

4. Storing the crontab output

*/10 * * * * /bin/execute/this/script.sh >> /var/log/script_output.log 2>&1

Note. 2>&1 tells linux to store stderr in stdout.

5. Mailing the crontab output

MAILTO="yourname@yourdomain.com"

Need to install mailx first:

$ sudo apt-get install mailx

Just mail one cronjob output:

*/10 * * * * /bin/execute/this/script.sh 2>&1 | mail -s "Cronjob ouput" yourname@yourdomain.com

6. Special Character

  • % - % is a special character to the crontab, which gets translated to a newline
25 9 * * * mysqldump  > `date '+\%Y\%m\%d\%H\%M\%S'`.sql